author | martin.trojer@nokia.com |
Fri, 31 Jul 2009 15:01:17 +0100 | |
changeset 1 | 2fb8b9db1c86 |
permissions | -rw-r--r-- |
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.. highlightlang:: c |
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.. _embedding: |
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*************************************** |
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Embedding Python in Another Application |
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*************************************** |
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The previous chapters discussed how to extend Python, that is, how to extend the |
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functionality of Python by attaching a library of C functions to it. It is also |
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possible to do it the other way around: enrich your C/C++ application by |
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embedding Python in it. Embedding provides your application with the ability to |
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implement some of the functionality of your application in Python rather than C |
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or C++. This can be used for many purposes; one example would be to allow users |
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to tailor the application to their needs by writing some scripts in Python. You |
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can also use it yourself if some of the functionality can be written in Python |
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more easily. |
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Embedding Python is similar to extending it, but not quite. The difference is |
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that when you extend Python, the main program of the application is still the |
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Python interpreter, while if you embed Python, the main program may have nothing |
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to do with Python --- instead, some parts of the application occasionally call |
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the Python interpreter to run some Python code. |
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So if you are embedding Python, you are providing your own main program. One of |
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the things this main program has to do is initialize the Python interpreter. At |
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the very least, you have to call the function :cfunc:`Py_Initialize`. There are |
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optional calls to pass command line arguments to Python. Then later you can |
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call the interpreter from any part of the application. |
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There are several different ways to call the interpreter: you can pass a string |
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containing Python statements to :cfunc:`PyRun_SimpleString`, or you can pass a |
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stdio file pointer and a file name (for identification in error messages only) |
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to :cfunc:`PyRun_SimpleFile`. You can also call the lower-level operations |
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described in the previous chapters to construct and use Python objects. |
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A simple demo of embedding Python can be found in the directory |
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:file:`Demo/embed/` of the source distribution. |
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.. seealso:: |
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:ref:`c-api-index` |
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The details of Python's C interface are given in this manual. A great deal of |
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necessary information can be found here. |
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.. _high-level-embedding: |
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Very High Level Embedding |
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========================= |
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The simplest form of embedding Python is the use of the very high level |
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interface. This interface is intended to execute a Python script without needing |
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to interact with the application directly. This can for example be used to |
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perform some operation on a file. :: |
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#include <Python.h> |
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int |
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main(int argc, char *argv[]) |
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{ |
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Py_Initialize(); |
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PyRun_SimpleString("from time import time,ctime\n" |
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"print 'Today is',ctime(time())\n"); |
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Py_Finalize(); |
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return 0; |
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} |
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The above code first initializes the Python interpreter with |
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:cfunc:`Py_Initialize`, followed by the execution of a hard-coded Python script |
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that print the date and time. Afterwards, the :cfunc:`Py_Finalize` call shuts |
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the interpreter down, followed by the end of the program. In a real program, |
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you may want to get the Python script from another source, perhaps a text-editor |
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routine, a file, or a database. Getting the Python code from a file can better |
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be done by using the :cfunc:`PyRun_SimpleFile` function, which saves you the |
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trouble of allocating memory space and loading the file contents. |
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.. _lower-level-embedding: |
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Beyond Very High Level Embedding: An overview |
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============================================= |
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The high level interface gives you the ability to execute arbitrary pieces of |
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Python code from your application, but exchanging data values is quite |
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cumbersome to say the least. If you want that, you should use lower level calls. |
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At the cost of having to write more C code, you can achieve almost anything. |
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It should be noted that extending Python and embedding Python is quite the same |
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activity, despite the different intent. Most topics discussed in the previous |
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chapters are still valid. To show this, consider what the extension code from |
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Python to C really does: |
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#. Convert data values from Python to C, |
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#. Perform a function call to a C routine using the converted values, and |
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#. Convert the data values from the call from C to Python. |
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When embedding Python, the interface code does: |
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#. Convert data values from C to Python, |
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#. Perform a function call to a Python interface routine using the converted |
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values, and |
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108 |
|
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#. Convert the data values from the call from Python to C. |
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110 |
|
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As you can see, the data conversion steps are simply swapped to accommodate the |
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112 |
different direction of the cross-language transfer. The only difference is the |
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113 |
routine that you call between both data conversions. When extending, you call a |
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C routine, when embedding, you call a Python routine. |
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115 |
|
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This chapter will not discuss how to convert data from Python to C and vice |
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versa. Also, proper use of references and dealing with errors is assumed to be |
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118 |
understood. Since these aspects do not differ from extending the interpreter, |
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you can refer to earlier chapters for the required information. |
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120 |
|
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121 |
|
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.. _pure-embedding: |
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123 |
|
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124 |
Pure Embedding |
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125 |
============== |
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126 |
|
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The first program aims to execute a function in a Python script. Like in the |
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section about the very high level interface, the Python interpreter does not |
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directly interact with the application (but that will change in the next |
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130 |
section). |
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131 |
|
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The code to run a function defined in a Python script is: |
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133 |
|
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.. literalinclude:: ../includes/run-func.c |
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135 |
|
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136 |
|
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This code loads a Python script using ``argv[1]``, and calls the function named |
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in ``argv[2]``. Its integer arguments are the other values of the ``argv`` |
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array. If you compile and link this program (let's call the finished executable |
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:program:`call`), and use it to execute a Python script, such as:: |
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141 |
|
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def multiply(a,b): |
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print "Will compute", a, "times", b |
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c = 0 |
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for i in range(0, a): |
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c = c + b |
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return c |
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148 |
|
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149 |
then the result should be:: |
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150 |
|
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$ call multiply multiply 3 2 |
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Will compute 3 times 2 |
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Result of call: 6 |
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154 |
|
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Although the program is quite large for its functionality, most of the code is |
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156 |
for data conversion between Python and C, and for error reporting. The |
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interesting part with respect to embedding Python starts with :: |
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158 |
|
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159 |
Py_Initialize(); |
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160 |
pName = PyString_FromString(argv[1]); |
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/* Error checking of pName left out */ |
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pModule = PyImport_Import(pName); |
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163 |
|
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After initializing the interpreter, the script is loaded using |
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:cfunc:`PyImport_Import`. This routine needs a Python string as its argument, |
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which is constructed using the :cfunc:`PyString_FromString` data conversion |
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routine. :: |
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168 |
|
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pFunc = PyObject_GetAttrString(pModule, argv[2]); |
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/* pFunc is a new reference */ |
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171 |
|
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if (pFunc && PyCallable_Check(pFunc)) { |
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173 |
... |
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174 |
} |
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Py_XDECREF(pFunc); |
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176 |
|
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177 |
Once the script is loaded, the name we're looking for is retrieved using |
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:cfunc:`PyObject_GetAttrString`. If the name exists, and the object returned is |
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179 |
callable, you can safely assume that it is a function. The program then |
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proceeds by constructing a tuple of arguments as normal. The call to the Python |
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function is then made with:: |
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182 |
|
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183 |
pValue = PyObject_CallObject(pFunc, pArgs); |
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184 |
|
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Upon return of the function, ``pValue`` is either *NULL* or it contains a |
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186 |
reference to the return value of the function. Be sure to release the reference |
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187 |
after examining the value. |
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188 |
|
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189 |
|
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.. _extending-with-embedding: |
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191 |
|
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192 |
Extending Embedded Python |
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193 |
========================= |
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194 |
|
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195 |
Until now, the embedded Python interpreter had no access to functionality from |
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196 |
the application itself. The Python API allows this by extending the embedded |
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197 |
interpreter. That is, the embedded interpreter gets extended with routines |
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198 |
provided by the application. While it sounds complex, it is not so bad. Simply |
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199 |
forget for a while that the application starts the Python interpreter. Instead, |
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200 |
consider the application to be a set of subroutines, and write some glue code |
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201 |
that gives Python access to those routines, just like you would write a normal |
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Python extension. For example:: |
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203 |
|
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static int numargs=0; |
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205 |
|
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/* Return the number of arguments of the application command line */ |
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207 |
static PyObject* |
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emb_numargs(PyObject *self, PyObject *args) |
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209 |
{ |
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if(!PyArg_ParseTuple(args, ":numargs")) |
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return NULL; |
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return Py_BuildValue("i", numargs); |
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} |
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214 |
|
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static PyMethodDef EmbMethods[] = { |
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{"numargs", emb_numargs, METH_VARARGS, |
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"Return the number of arguments received by the process."}, |
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{NULL, NULL, 0, NULL} |
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}; |
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Insert the above code just above the :cfunc:`main` function. Also, insert the |
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following two statements directly after :cfunc:`Py_Initialize`:: |
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numargs = argc; |
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Py_InitModule("emb", EmbMethods); |
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These two lines initialize the ``numargs`` variable, and make the |
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:func:`emb.numargs` function accessible to the embedded Python interpreter. |
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With these extensions, the Python script can do things like :: |
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|
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import emb |
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print "Number of arguments", emb.numargs() |
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In a real application, the methods will expose an API of the application to |
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Python. |
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|
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.. TODO: threads, code examples do not really behave well if errors happen |
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(what to watch out for) |
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|
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.. _embeddingincplusplus: |
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|
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Embedding Python in C++ |
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======================= |
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|
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It is also possible to embed Python in a C++ program; precisely how this is done |
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will depend on the details of the C++ system used; in general you will need to |
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write the main program in C++, and use the C++ compiler to compile and link your |
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program. There is no need to recompile Python itself using C++. |
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|
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|
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.. _link-reqs: |
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|
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Linking Requirements |
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==================== |
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|
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While the :program:`configure` script shipped with the Python sources will |
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correctly build Python to export the symbols needed by dynamically linked |
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extensions, this is not automatically inherited by applications which embed the |
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Python library statically, at least on Unix. This is an issue when the |
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application is linked to the static runtime library (:file:`libpython.a`) and |
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needs to load dynamic extensions (implemented as :file:`.so` files). |
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|
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The problem is that some entry points are defined by the Python runtime solely |
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for extension modules to use. If the embedding application does not use any of |
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these entry points, some linkers will not include those entries in the symbol |
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table of the finished executable. Some additional options are needed to inform |
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the linker not to remove these symbols. |
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|
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Determining the right options to use for any given platform can be quite |
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difficult, but fortunately the Python configuration already has those values. |
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To retrieve them from an installed Python interpreter, start an interactive |
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interpreter and have a short session like this:: |
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|
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>>> import distutils.sysconfig |
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>>> distutils.sysconfig.get_config_var('LINKFORSHARED') |
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'-Xlinker -export-dynamic' |
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|
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.. index:: module: distutils.sysconfig |
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The contents of the string presented will be the options that should be used. |
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If the string is empty, there's no need to add any additional options. The |
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:const:`LINKFORSHARED` definition corresponds to the variable of the same name |
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in Python's top-level :file:`Makefile`. |
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285 |