symbian-qemu-0.9.1-12/python-2.6.1/Doc/c-api/intro.rst
author martin.trojer@nokia.com
Fri, 31 Jul 2009 15:01:17 +0100
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.. highlightlang:: c
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.. _api-intro:
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************
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Introduction
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************
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The Application Programmer's Interface to Python gives C and C++ programmers
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access to the Python interpreter at a variety of levels.  The API is equally
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usable from C++, but for brevity it is generally referred to as the Python/C
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API.  There are two fundamentally different reasons for using the Python/C API.
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The first reason is to write *extension modules* for specific purposes; these
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are C modules that extend the Python interpreter.  This is probably the most
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common use.  The second reason is to use Python as a component in a larger
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application; this technique is generally referred to as :dfn:`embedding` Python
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in an application.
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Writing an extension module is a relatively well-understood process,  where a
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"cookbook" approach works well.  There are several tools  that automate the
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process to some extent.  While people have embedded  Python in other
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applications since its early existence, the process of  embedding Python is less
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straightforward than writing an extension.
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Many API functions are useful independent of whether you're embedding  or
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extending Python; moreover, most applications that embed Python  will need to
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provide a custom extension as well, so it's probably a  good idea to become
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familiar with writing an extension before  attempting to embed Python in a real
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application.
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.. _api-includes:
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Include Files
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=============
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All function, type and macro definitions needed to use the Python/C API are
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included in your code by the following line::
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   #include "Python.h"
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This implies inclusion of the following standard headers: ``<stdio.h>``,
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``<string.h>``, ``<errno.h>``, ``<limits.h>``, and ``<stdlib.h>`` (if
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available).
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.. warning::
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   Since Python may define some pre-processor definitions which affect the standard
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   headers on some systems, you *must* include :file:`Python.h` before any standard
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   headers are included.
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All user visible names defined by Python.h (except those defined by the included
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standard headers) have one of the prefixes ``Py`` or ``_Py``.  Names beginning
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with ``_Py`` are for internal use by the Python implementation and should not be
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used by extension writers. Structure member names do not have a reserved prefix.
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**Important:** user code should never define names that begin with ``Py`` or
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``_Py``.  This confuses the reader, and jeopardizes the portability of the user
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code to future Python versions, which may define additional names beginning with
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one of these prefixes.
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The header files are typically installed with Python.  On Unix, these  are
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located in the directories :file:`{prefix}/include/pythonversion/` and
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:file:`{exec_prefix}/include/pythonversion/`, where :envvar:`prefix` and
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:envvar:`exec_prefix` are defined by the corresponding parameters to Python's
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:program:`configure` script and *version* is ``sys.version[:3]``.  On Windows,
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the headers are installed in :file:`{prefix}/include`, where :envvar:`prefix` is
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the installation directory specified to the installer.
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To include the headers, place both directories (if different) on your compiler's
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search path for includes.  Do *not* place the parent directories on the search
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path and then use ``#include <pythonX.Y/Python.h>``; this will break on
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multi-platform builds since the platform independent headers under
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:envvar:`prefix` include the platform specific headers from
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:envvar:`exec_prefix`.
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C++ users should note that though the API is defined entirely using C, the
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header files do properly declare the entry points to be ``extern "C"``, so there
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is no need to do anything special to use the API from C++.
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.. _api-objects:
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Objects, Types and Reference Counts
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===================================
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.. index:: object: type
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Most Python/C API functions have one or more arguments as well as a return value
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of type :ctype:`PyObject\*`.  This type is a pointer to an opaque data type
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representing an arbitrary Python object.  Since all Python object types are
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treated the same way by the Python language in most situations (e.g.,
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assignments, scope rules, and argument passing), it is only fitting that they
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should be represented by a single C type.  Almost all Python objects live on the
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heap: you never declare an automatic or static variable of type
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:ctype:`PyObject`, only pointer variables of type :ctype:`PyObject\*` can  be
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declared.  The sole exception are the type objects; since these must never be
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deallocated, they are typically static :ctype:`PyTypeObject` objects.
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All Python objects (even Python integers) have a :dfn:`type` and a
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:dfn:`reference count`.  An object's type determines what kind of object it is
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(e.g., an integer, a list, or a user-defined function; there are many more as
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explained in :ref:`types`).  For each of the well-known types there is a macro
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to check whether an object is of that type; for instance, ``PyList_Check(a)`` is
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true if (and only if) the object pointed to by *a* is a Python list.
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.. _api-refcounts:
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Reference Counts
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----------------
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The reference count is important because today's computers have a  finite (and
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often severely limited) memory size; it counts how many  different places there
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are that have a reference to an object.  Such a  place could be another object,
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or a global (or static) C variable, or  a local variable in some C function.
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When an object's reference count  becomes zero, the object is deallocated.  If
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it contains references to  other objects, their reference count is decremented.
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Those other  objects may be deallocated in turn, if this decrement makes their
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reference count become zero, and so on.  (There's an obvious problem  with
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objects that reference each other here; for now, the solution is  "don't do
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that.")
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.. index::
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   single: Py_INCREF()
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   single: Py_DECREF()
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Reference counts are always manipulated explicitly.  The normal way is  to use
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the macro :cfunc:`Py_INCREF` to increment an object's reference count by one,
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and :cfunc:`Py_DECREF` to decrement it by   one.  The :cfunc:`Py_DECREF` macro
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is considerably more complex than the incref one, since it must check whether
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the reference count becomes zero and then cause the object's deallocator to be
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called. The deallocator is a function pointer contained in the object's type
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structure.  The type-specific deallocator takes care of decrementing the
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reference counts for other objects contained in the object if this is a compound
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object type, such as a list, as well as performing any additional finalization
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that's needed.  There's no chance that the reference count can overflow; at
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least as many bits are used to hold the reference count as there are distinct
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memory locations in virtual memory (assuming ``sizeof(Py_ssize_t) >= sizeof(void*)``).
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Thus, the reference count increment is a simple operation.
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It is not necessary to increment an object's reference count for every  local
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variable that contains a pointer to an object.  In theory, the  object's
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reference count goes up by one when the variable is made to  point to it and it
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goes down by one when the variable goes out of  scope.  However, these two
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cancel each other out, so at the end the  reference count hasn't changed.  The
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only real reason to use the  reference count is to prevent the object from being
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deallocated as  long as our variable is pointing to it.  If we know that there
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is at  least one other reference to the object that lives at least as long as
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our variable, there is no need to increment the reference count  temporarily.
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An important situation where this arises is in objects  that are passed as
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arguments to C functions in an extension module  that are called from Python;
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the call mechanism guarantees to hold a  reference to every argument for the
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duration of the call.
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However, a common pitfall is to extract an object from a list and hold on to it
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for a while without incrementing its reference count. Some other operation might
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conceivably remove the object from the list, decrementing its reference count
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and possible deallocating it. The real danger is that innocent-looking
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operations may invoke arbitrary Python code which could do this; there is a code
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path which allows control to flow back to the user from a :cfunc:`Py_DECREF`, so
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almost any operation is potentially dangerous.
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A safe approach is to always use the generic operations (functions  whose name
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begins with ``PyObject_``, ``PyNumber_``, ``PySequence_`` or ``PyMapping_``).
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These operations always increment the reference count of the object they return.
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This leaves the caller with the responsibility to call :cfunc:`Py_DECREF` when
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they are done with the result; this soon becomes second nature.
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.. _api-refcountdetails:
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Reference Count Details
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^^^^^^^^^^^^^^^^^^^^^^^
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The reference count behavior of functions in the Python/C API is best  explained
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in terms of *ownership of references*.  Ownership pertains to references, never
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to objects (objects are not owned: they are always shared).  "Owning a
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reference" means being responsible for calling Py_DECREF on it when the
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reference is no longer needed.  Ownership can also be transferred, meaning that
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the code that receives ownership of the reference then becomes responsible for
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eventually decref'ing it by calling :cfunc:`Py_DECREF` or :cfunc:`Py_XDECREF`
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when it's no longer needed---or passing on this responsibility (usually to its
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caller). When a function passes ownership of a reference on to its caller, the
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caller is said to receive a *new* reference.  When no ownership is transferred,
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the caller is said to *borrow* the reference. Nothing needs to be done for a
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borrowed reference.
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Conversely, when a calling function passes it a reference to an  object, there
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are two possibilities: the function *steals* a  reference to the object, or it
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does not.  *Stealing a reference* means that when you pass a reference to a
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function, that function assumes that it now owns that reference, and you are not
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responsible for it any longer.
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.. index::
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   single: PyList_SetItem()
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   single: PyTuple_SetItem()
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Few functions steal references; the two notable exceptions are
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:cfunc:`PyList_SetItem` and :cfunc:`PyTuple_SetItem`, which  steal a reference
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to the item (but not to the tuple or list into which the item is put!).  These
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functions were designed to steal a reference because of a common idiom for
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populating a tuple or list with newly created objects; for example, the code to
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create the tuple ``(1, 2, "three")`` could look like this (forgetting about
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error handling for the moment; a better way to code this is shown below)::
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   PyObject *t;
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   t = PyTuple_New(3);
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   PyTuple_SetItem(t, 0, PyInt_FromLong(1L));
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   PyTuple_SetItem(t, 1, PyInt_FromLong(2L));
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   PyTuple_SetItem(t, 2, PyString_FromString("three"));
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Here, :cfunc:`PyInt_FromLong` returns a new reference which is immediately
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stolen by :cfunc:`PyTuple_SetItem`.  When you want to keep using an object
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although the reference to it will be stolen, use :cfunc:`Py_INCREF` to grab
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another reference before calling the reference-stealing function.
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Incidentally, :cfunc:`PyTuple_SetItem` is the *only* way to set tuple items;
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:cfunc:`PySequence_SetItem` and :cfunc:`PyObject_SetItem` refuse to do this
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since tuples are an immutable data type.  You should only use
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:cfunc:`PyTuple_SetItem` for tuples that you are creating yourself.
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Equivalent code for populating a list can be written using :cfunc:`PyList_New`
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and :cfunc:`PyList_SetItem`.
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However, in practice, you will rarely use these ways of creating and populating
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a tuple or list.  There's a generic function, :cfunc:`Py_BuildValue`, that can
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create most common objects from C values, directed by a :dfn:`format string`.
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For example, the above two blocks of code could be replaced by the following
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(which also takes care of the error checking)::
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   PyObject *tuple, *list;
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   tuple = Py_BuildValue("(iis)", 1, 2, "three");
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   list = Py_BuildValue("[iis]", 1, 2, "three");
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It is much more common to use :cfunc:`PyObject_SetItem` and friends with items
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whose references you are only borrowing, like arguments that were passed in to
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the function you are writing.  In that case, their behaviour regarding reference
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counts is much saner, since you don't have to increment a reference count so you
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can give a reference away ("have it be stolen").  For example, this function
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sets all items of a list (actually, any mutable sequence) to a given item::
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   int
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   set_all(PyObject *target, PyObject *item)
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   {
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       int i, n;
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       n = PyObject_Length(target);
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       if (n < 0)
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           return -1;
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       for (i = 0; i < n; i++) {
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           PyObject *index = PyInt_FromLong(i);
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           if (!index)
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               return -1;
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           if (PyObject_SetItem(target, index, item) < 0)
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               return -1;
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           Py_DECREF(index);
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       }
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       return 0;
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   }
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.. index:: single: set_all()
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The situation is slightly different for function return values.   While passing
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a reference to most functions does not change your  ownership responsibilities
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for that reference, many functions that  return a reference to an object give
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you ownership of the reference. The reason is simple: in many cases, the
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returned object is created  on the fly, and the reference you get is the only
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reference to the  object.  Therefore, the generic functions that return object
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references, like :cfunc:`PyObject_GetItem` and  :cfunc:`PySequence_GetItem`,
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always return a new reference (the caller becomes the owner of the reference).
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It is important to realize that whether you own a reference returned  by a
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function depends on which function you call only --- *the plumage* (the type of
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the object passed as an argument to the function) *doesn't enter into it!*
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Thus, if you  extract an item from a list using :cfunc:`PyList_GetItem`, you
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don't own the reference --- but if you obtain the same item from the same list
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using :cfunc:`PySequence_GetItem` (which happens to take exactly the same
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arguments), you do own a reference to the returned object.
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.. index::
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   single: PyList_GetItem()
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   single: PySequence_GetItem()
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Here is an example of how you could write a function that computes the sum of
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the items in a list of integers; once using  :cfunc:`PyList_GetItem`, and once
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using :cfunc:`PySequence_GetItem`. ::
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   long
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   sum_list(PyObject *list)
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   {
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       int i, n;
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       long total = 0;
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       PyObject *item;
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       n = PyList_Size(list);
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       if (n < 0)
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           return -1; /* Not a list */
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       for (i = 0; i < n; i++) {
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           item = PyList_GetItem(list, i); /* Can't fail */
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           if (!PyInt_Check(item)) continue; /* Skip non-integers */
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           total += PyInt_AsLong(item);
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       }
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       return total;
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   }
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.. index:: single: sum_list()
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::
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   long
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   sum_sequence(PyObject *sequence)
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   {
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       int i, n;
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       long total = 0;
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       PyObject *item;
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       n = PySequence_Length(sequence);
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       if (n < 0)
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           return -1; /* Has no length */
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       for (i = 0; i < n; i++) {
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           item = PySequence_GetItem(sequence, i);
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           if (item == NULL)
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               return -1; /* Not a sequence, or other failure */
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           if (PyInt_Check(item))
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               total += PyInt_AsLong(item);
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           Py_DECREF(item); /* Discard reference ownership */
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diff changeset
   330
       }
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   331
       return total;
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   332
   }
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.. index:: single: sum_sequence()
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   335
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.. _api-types:
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Types
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-----
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There are few other data types that play a significant role in  the Python/C
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API; most are simple C types such as :ctype:`int`,  :ctype:`long`,
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   344
:ctype:`double` and :ctype:`char\*`.  A few structure types  are used to
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   345
describe static tables used to list the functions exported  by a module or the
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   346
data attributes of a new object type, and another is used to describe the value
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of a complex number.  These will  be discussed together with the functions that
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   348
use them.
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   349
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   351
.. _api-exceptions:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   352
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   353
Exceptions
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   354
==========
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   355
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   356
The Python programmer only needs to deal with exceptions if specific  error
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   357
handling is required; unhandled exceptions are automatically  propagated to the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   358
caller, then to the caller's caller, and so on, until they reach the top-level
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   359
interpreter, where they are reported to the  user accompanied by a stack
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   360
traceback.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   361
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   362
.. index:: single: PyErr_Occurred()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   363
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   364
For C programmers, however, error checking always has to be explicit.   All
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   365
functions in the Python/C API can raise exceptions, unless an  explicit claim is
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   366
made otherwise in a function's documentation.  In  general, when a function
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   367
encounters an error, it sets an exception,  discards any object references that
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   368
it owns, and returns an  error indicator --- usually *NULL* or ``-1``.  A few
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   369
functions  return a Boolean true/false result, with false indicating an error.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   370
Very few functions return no explicit error indicator or have an  ambiguous
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   371
return value, and require explicit testing for errors with
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   372
:cfunc:`PyErr_Occurred`.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   373
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   374
.. index::
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   375
   single: PyErr_SetString()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   376
   single: PyErr_Clear()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   377
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   378
Exception state is maintained in per-thread storage (this is  equivalent to
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   379
using global storage in an unthreaded application).  A  thread can be in one of
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   380
two states: an exception has occurred, or not. The function
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   381
:cfunc:`PyErr_Occurred` can be used to check for this: it returns a borrowed
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   382
reference to the exception type object when an exception has occurred, and
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   383
*NULL* otherwise.  There are a number of functions to set the exception state:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   384
:cfunc:`PyErr_SetString` is the most common (though not the most general)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   385
function to set the exception state, and :cfunc:`PyErr_Clear` clears the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   386
exception state.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   387
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   388
.. index::
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   389
   single: exc_type (in module sys)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   390
   single: exc_value (in module sys)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   391
   single: exc_traceback (in module sys)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   392
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   393
The full exception state consists of three objects (all of which can  be
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   394
*NULL*): the exception type, the corresponding exception  value, and the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   395
traceback.  These have the same meanings as the Python   objects
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   396
``sys.exc_type``, ``sys.exc_value``, and ``sys.exc_traceback``; however, they
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   397
are not the same: the Python objects represent the last exception being handled
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   398
by a Python  :keyword:`try` ... :keyword:`except` statement, while the C level
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   399
exception state only exists while an exception is being passed on between C
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   400
functions until it reaches the Python bytecode interpreter's  main loop, which
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   401
takes care of transferring it to ``sys.exc_type`` and friends.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   402
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   403
.. index:: single: exc_info() (in module sys)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   404
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   405
Note that starting with Python 1.5, the preferred, thread-safe way to access the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   406
exception state from Python code is to call the function :func:`sys.exc_info`,
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   407
which returns the per-thread exception state for Python code.  Also, the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   408
semantics of both ways to access the exception state have changed so that a
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   409
function which catches an exception will save and restore its thread's exception
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   410
state so as to preserve the exception state of its caller.  This prevents common
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   411
bugs in exception handling code caused by an innocent-looking function
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   412
overwriting the exception being handled; it also reduces the often unwanted
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   413
lifetime extension for objects that are referenced by the stack frames in the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   414
traceback.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   415
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   416
As a general principle, a function that calls another function to  perform some
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   417
task should check whether the called function raised an  exception, and if so,
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   418
pass the exception state on to its caller.  It  should discard any object
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   419
references that it owns, and return an  error indicator, but it should *not* set
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   420
another exception --- that would overwrite the exception that was just raised,
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   421
and lose important information about the exact cause of the error.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   422
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   423
.. index:: single: sum_sequence()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   424
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   425
A simple example of detecting exceptions and passing them on is shown in the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   426
:cfunc:`sum_sequence` example above.  It so happens that that example doesn't
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   427
need to clean up any owned references when it detects an error.  The following
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   428
example function shows some error cleanup.  First, to remind you why you like
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   429
Python, we show the equivalent Python code::
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   430
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   431
   def incr_item(dict, key):
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   432
       try:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   433
           item = dict[key]
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   434
       except KeyError:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   435
           item = 0
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   436
       dict[key] = item + 1
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   437
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   438
.. index:: single: incr_item()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   439
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   440
Here is the corresponding C code, in all its glory::
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   441
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   442
   int
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   443
   incr_item(PyObject *dict, PyObject *key)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   444
   {
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   445
       /* Objects all initialized to NULL for Py_XDECREF */
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   446
       PyObject *item = NULL, *const_one = NULL, *incremented_item = NULL;
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   447
       int rv = -1; /* Return value initialized to -1 (failure) */
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   448
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   449
       item = PyObject_GetItem(dict, key);
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   450
       if (item == NULL) {
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   451
           /* Handle KeyError only: */
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   452
           if (!PyErr_ExceptionMatches(PyExc_KeyError))
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   453
               goto error;
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   454
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   455
           /* Clear the error and use zero: */
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   456
           PyErr_Clear();
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   457
           item = PyInt_FromLong(0L);
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   458
           if (item == NULL)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   459
               goto error;
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   460
       }
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   461
       const_one = PyInt_FromLong(1L);
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   462
       if (const_one == NULL)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   463
           goto error;
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   464
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   465
       incremented_item = PyNumber_Add(item, const_one);
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   466
       if (incremented_item == NULL)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   467
           goto error;
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   468
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   469
       if (PyObject_SetItem(dict, key, incremented_item) < 0)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   470
           goto error;
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   471
       rv = 0; /* Success */
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   472
       /* Continue with cleanup code */
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   473
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   474
    error:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   475
       /* Cleanup code, shared by success and failure path */
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   476
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   477
       /* Use Py_XDECREF() to ignore NULL references */
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   478
       Py_XDECREF(item);
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   479
       Py_XDECREF(const_one);
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   480
       Py_XDECREF(incremented_item);
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   481
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   482
       return rv; /* -1 for error, 0 for success */
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   483
   }
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   484
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   485
.. index:: single: incr_item()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   486
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   487
.. index::
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   488
   single: PyErr_ExceptionMatches()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   489
   single: PyErr_Clear()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   490
   single: Py_XDECREF()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   491
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   492
This example represents an endorsed use of the ``goto`` statement  in C!
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   493
It illustrates the use of :cfunc:`PyErr_ExceptionMatches` and
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   494
:cfunc:`PyErr_Clear` to handle specific exceptions, and the use of
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   495
:cfunc:`Py_XDECREF` to dispose of owned references that may be *NULL* (note the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   496
``'X'`` in the name; :cfunc:`Py_DECREF` would crash when confronted with a
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   497
*NULL* reference).  It is important that the variables used to hold owned
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   498
references are initialized to *NULL* for this to work; likewise, the proposed
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   499
return value is initialized to ``-1`` (failure) and only set to success after
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   500
the final call made is successful.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   501
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   502
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   503
.. _api-embedding:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   504
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   505
Embedding Python
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   506
================
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   507
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   508
The one important task that only embedders (as opposed to extension writers) of
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   509
the Python interpreter have to worry about is the initialization, and possibly
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   510
the finalization, of the Python interpreter.  Most functionality of the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   511
interpreter can only be used after the interpreter has been initialized.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   512
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   513
.. index::
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   514
   single: Py_Initialize()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   515
   module: __builtin__
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   516
   module: __main__
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   517
   module: sys
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   518
   module: exceptions
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   519
   triple: module; search; path
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   520
   single: path (in module sys)
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   521
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   522
The basic initialization function is :cfunc:`Py_Initialize`. This initializes
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   523
the table of loaded modules, and creates the fundamental modules
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   524
:mod:`__builtin__`, :mod:`__main__`, :mod:`sys`, and :mod:`exceptions`.  It also
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   525
initializes the module search path (``sys.path``).
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   526
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   527
.. index:: single: PySys_SetArgv()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   528
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   529
:cfunc:`Py_Initialize` does not set the "script argument list"  (``sys.argv``).
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   530
If this variable is needed by Python code that  will be executed later, it must
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   531
be set explicitly with a call to  ``PySys_SetArgv(argc, argv)`` subsequent to
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   532
the call to :cfunc:`Py_Initialize`.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   533
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   534
On most systems (in particular, on Unix and Windows, although the details are
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   535
slightly different), :cfunc:`Py_Initialize` calculates the module search path
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   536
based upon its best guess for the location of the standard Python interpreter
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   537
executable, assuming that the Python library is found in a fixed location
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   538
relative to the Python interpreter executable.  In particular, it looks for a
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   539
directory named :file:`lib/python{X.Y}` relative to the parent directory
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   540
where the executable named :file:`python` is found on the shell command search
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   541
path (the environment variable :envvar:`PATH`).
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   542
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   543
For instance, if the Python executable is found in
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   544
:file:`/usr/local/bin/python`, it will assume that the libraries are in
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   545
:file:`/usr/local/lib/python{X.Y}`.  (In fact, this particular path is also
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   546
the "fallback" location, used when no executable file named :file:`python` is
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   547
found along :envvar:`PATH`.)  The user can override this behavior by setting the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   548
environment variable :envvar:`PYTHONHOME`, or insert additional directories in
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   549
front of the standard path by setting :envvar:`PYTHONPATH`.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   550
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   551
.. index::
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   552
   single: Py_SetProgramName()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   553
   single: Py_GetPath()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   554
   single: Py_GetPrefix()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   555
   single: Py_GetExecPrefix()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   556
   single: Py_GetProgramFullPath()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   557
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   558
The embedding application can steer the search by calling
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   559
``Py_SetProgramName(file)`` *before* calling  :cfunc:`Py_Initialize`.  Note that
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   560
:envvar:`PYTHONHOME` still overrides this and :envvar:`PYTHONPATH` is still
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   561
inserted in front of the standard path.  An application that requires total
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   562
control has to provide its own implementation of :cfunc:`Py_GetPath`,
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   563
:cfunc:`Py_GetPrefix`, :cfunc:`Py_GetExecPrefix`, and
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   564
:cfunc:`Py_GetProgramFullPath` (all defined in :file:`Modules/getpath.c`).
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   565
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   566
.. index:: single: Py_IsInitialized()
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   567
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   568
Sometimes, it is desirable to "uninitialize" Python.  For instance,  the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   569
application may want to start over (make another call to
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   570
:cfunc:`Py_Initialize`) or the application is simply done with its  use of
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   571
Python and wants to free memory allocated by Python.  This can be accomplished
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   572
by calling :cfunc:`Py_Finalize`.  The function :cfunc:`Py_IsInitialized` returns
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   573
true if Python is currently in the initialized state.  More information about
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   574
these functions is given in a later chapter. Notice that :cfunc:`Py_Finalize`
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   575
does *not* free all memory allocated by the Python interpreter, e.g. memory
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   576
allocated by extension modules currently cannot be released.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   577
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   578
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   579
.. _api-debugging:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   580
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   581
Debugging Builds
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   582
================
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   583
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   584
Python can be built with several macros to enable extra checks of the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   585
interpreter and extension modules.  These checks tend to add a large amount of
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   586
overhead to the runtime so they are not enabled by default.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   587
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   588
A full list of the various types of debugging builds is in the file
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   589
:file:`Misc/SpecialBuilds.txt` in the Python source distribution. Builds are
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   590
available that support tracing of reference counts, debugging the memory
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   591
allocator, or low-level profiling of the main interpreter loop.  Only the most
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   592
frequently-used builds will be described in the remainder of this section.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   593
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   594
Compiling the interpreter with the :cmacro:`Py_DEBUG` macro defined produces
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   595
what is generally meant by "a debug build" of Python. :cmacro:`Py_DEBUG` is
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   596
enabled in the Unix build by adding :option:`--with-pydebug` to the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   597
:file:`configure` command.  It is also implied by the presence of the
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   598
not-Python-specific :cmacro:`_DEBUG` macro.  When :cmacro:`Py_DEBUG` is enabled
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   599
in the Unix build, compiler optimization is disabled.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   600
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   601
In addition to the reference count debugging described below, the following
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   602
extra checks are performed:
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   603
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   604
* Extra checks are added to the object allocator.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   605
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   606
* Extra checks are added to the parser and compiler.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   607
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   608
* Downcasts from wide types to narrow types are checked for loss of information.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   609
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   610
* A number of assertions are added to the dictionary and set implementations.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   611
  In addition, the set object acquires a :meth:`test_c_api` method.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   612
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   613
* Sanity checks of the input arguments are added to frame creation.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   614
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   615
* The storage for long ints is initialized with a known invalid pattern to catch
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   616
  reference to uninitialized digits.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   617
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   618
* Low-level tracing and extra exception checking are added to the runtime
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   619
  virtual machine.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   620
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   621
* Extra checks are added to the memory arena implementation.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   622
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   623
* Extra debugging is added to the thread module.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   624
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   625
There may be additional checks not mentioned here.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   626
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   627
Defining :cmacro:`Py_TRACE_REFS` enables reference tracing.  When defined, a
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   628
circular doubly linked list of active objects is maintained by adding two extra
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   629
fields to every :ctype:`PyObject`.  Total allocations are tracked as well.  Upon
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   630
exit, all existing references are printed.  (In interactive mode this happens
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   631
after every statement run by the interpreter.)  Implied by :cmacro:`Py_DEBUG`.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   632
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   633
Please refer to :file:`Misc/SpecialBuilds.txt` in the Python source distribution
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   634
for more detailed information.
2fb8b9db1c86 Initial QEMU (symbian-qemu-0.9.1-12) import
martin.trojer@nokia.com
parents:
diff changeset
   635