.. _compiler:

***********************

Python compiler package

***********************

.. deprecated:: 2.6

   The :mod:`compiler` package has been removed in Python 3.0.

.. sectionauthor:: Jeremy Hylton <jeremy@zope.com>

The Python compiler package is a tool for analyzing Python source code and

generating Python bytecode.  The compiler contains libraries to generate an

abstract syntax tree from Python source code and to generate Python

:term:`bytecode` from the tree.

The :mod:`compiler` package is a Python source to bytecode translator written in

Python.  It uses the built-in parser and standard :mod:`parser` module to

generated a concrete syntax tree.  This tree is used to generate an abstract

syntax tree (AST) and then Python bytecode.

The full functionality of the package duplicates the builtin compiler provided

with the Python interpreter.  It is intended to match its behavior almost

exactly.  Why implement another compiler that does the same thing?  The package

is useful for a variety of purposes.  It can be modified more easily than the

builtin compiler.  The AST it generates is useful for analyzing Python source

code.

This chapter explains how the various components of the :mod:`compiler` package

work.  It blends reference material with a tutorial.

The basic interface

===================

.. module:: compiler

   :synopsis: Python code compiler written in Python.

   :deprecated:

The top-level of the package defines four functions.  If you import

:mod:`compiler`, you will get these functions and a collection of modules

contained in the package.

.. function:: parse(buf)

   Returns an abstract syntax tree for the Python source code in *buf*. The

   function raises :exc:`SyntaxError` if there is an error in the source code.  The

   return value is a :class:`compiler.ast.Module` instance that contains the tree.

.. function:: parseFile(path)

   Return an abstract syntax tree for the Python source code in the file specified

   by *path*.  It is equivalent to ``parse(open(path).read())``.

.. function:: walk(ast, visitor[, verbose])

   Do a pre-order walk over the abstract syntax tree *ast*.  Call the appropriate

   method on the *visitor* instance for each node encountered.

.. function:: compile(source, filename, mode, flags=None,  dont_inherit=None)

   Compile the string *source*, a Python module, statement or expression, into a

   code object that can be executed by the exec statement or :func:`eval`. This

   function is a replacement for the built-in :func:`compile` function.

   The *filename* will be used for run-time error messages.

   The *mode* must be 'exec' to compile a module, 'single' to compile a single

   (interactive) statement, or 'eval' to compile an expression.

   The *flags* and *dont_inherit* arguments affect future-related statements, but

   are not supported yet.

.. function:: compileFile(source)

   Compiles the file *source* and generates a .pyc file.

The :mod:`compiler` package contains the following modules: :mod:`ast`,

:mod:`consts`, :mod:`future`, :mod:`misc`, :mod:`pyassem`, :mod:`pycodegen`,

:mod:`symbols`, :mod:`transformer`, and :mod:`visitor`.

Limitations

===========

There are some problems with the error checking of the compiler package.  The

interpreter detects syntax errors in two distinct phases.  One set of errors is

detected by the interpreter's parser, the other set by the compiler.  The

compiler package relies on the interpreter's parser, so it get the first phases

of error checking for free.  It implements the second phase itself, and that

implementation is incomplete.  For example, the compiler package does not raise

an error if a name appears more than once in an argument list:  ``def f(x, x):

...``

A future version of the compiler should fix these problems.

Python Abstract Syntax

======================

The :mod:`compiler.ast` module defines an abstract syntax for Python.  In the

abstract syntax tree, each node represents a syntactic construct.  The root of

the tree is :class:`Module` object.

The abstract syntax offers a higher level interface to parsed Python source

code.  The :mod:`parser` module and the compiler written in C for the Python

interpreter use a concrete syntax tree.  The concrete syntax is tied closely to

the grammar description used for the Python parser.  Instead of a single node

for a construct, there are often several levels of nested nodes that are

introduced by Python's precedence rules.

The abstract syntax tree is created by the :mod:`compiler.transformer` module.

The transformer relies on the builtin Python parser to generate a concrete

syntax tree.  It generates an abstract syntax tree from the concrete tree.

.. index::

   single: Stein, Greg

   single: Tutt, Bill

The :mod:`transformer` module was created by Greg Stein and Bill Tutt for an

experimental Python-to-C compiler.  The current version contains a number of

modifications and improvements, but the basic form of the abstract syntax and of

the transformer are due to Stein and Tutt.

AST Nodes

---------

.. module:: compiler.ast

The :mod:`compiler.ast` module is generated from a text file that describes each

node type and its elements.  Each node type is represented as a class that

inherits from the abstract base class :class:`compiler.ast.Node` and defines a

set of named attributes for child nodes.

.. class:: Node()

   The :class:`Node` instances are created automatically by the parser generator.

   The recommended interface for specific :class:`Node` instances is to use the

   public attributes to access child nodes.  A public attribute may be bound to a

   single node or to a sequence of nodes, depending on the :class:`Node` type.  For

   example, the :attr:`bases` attribute of the :class:`Class` node, is bound to a

   list of base class nodes, and the :attr:`doc` attribute is bound to a single

   node.

   Each :class:`Node` instance has a :attr:`lineno` attribute which may be

   ``None``.  XXX Not sure what the rules are for which nodes will have a useful

   lineno.

   All :class:`Node` objects offer the following methods:

   .. method:: getChildren()

      Returns a flattened list of the child nodes and objects in the order they

      occur.  Specifically, the order of the nodes is the order in which they

      appear in the Python grammar.  Not all of the children are :class:`Node`

      instances.  The names of functions and classes, for example, are plain

      strings.

   .. method:: getChildNodes()

      Returns a flattened list of the child nodes in the order they occur.  This

      method is like :meth:`getChildren`, except that it only returns those

      children that are :class:`Node` instances.

Two examples illustrate the general structure of :class:`Node` classes.  The

:keyword:`while` statement is defined by the following grammar production::

   while_stmt:     "while" expression ":" suite

                  ["else" ":" suite]

The :class:`While` node has three attributes: :attr:`test`, :attr:`body`, and

:attr:`else_`.  (If the natural name for an attribute is also a Python reserved

word, it can't be used as an attribute name.  An underscore is appended to the

word to make it a legal identifier, hence :attr:`else_` instead of

:keyword:`else`.)

The :keyword:`if` statement is more complicated because it can include several

tests.   ::

   if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]

The :class:`If` node only defines two attributes: :attr:`tests` and

:attr:`else_`.  The :attr:`tests` attribute is a sequence of test expression,

consequent body pairs.  There is one pair for each :keyword:`if`/:keyword:`elif`

clause.  The first element of the pair is the test expression.  The second

elements is a :class:`Stmt` node that contains the code to execute if the test

is true.

The :meth:`getChildren` method of :class:`If` returns a flat list of child

nodes.  If there are three :keyword:`if`/:keyword:`elif` clauses and no

:keyword:`else` clause, then :meth:`getChildren` will return a list of six

elements: the first test expression, the first :class:`Stmt`, the second text

expression, etc.

The following table lists each of the :class:`Node` subclasses defined in

:mod:`compiler.ast` and each of the public attributes available on their

instances.  The values of most of the attributes are themselves :class:`Node`

instances or sequences of instances.  When the value is something other than an

instance, the type is noted in the comment.  The attributes are listed in the

order in which they are returned by :meth:`getChildren` and

:meth:`getChildNodes`.

+-----------------------+--------------------+---------------------------------+

| Node type             | Attribute          | Value                           |

+=======================+====================+=================================+

| :class:`Add`          | :attr:`left`       | left operand                    |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`right`      | right operand                   |

+-----------------------+--------------------+---------------------------------+

| :class:`And`          | :attr:`nodes`      | list of operands                |

+-----------------------+--------------------+---------------------------------+

| :class:`AssAttr`      |                    | *attribute as target of         |

|                       |                    | assignment*                     |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`expr`       | expression on the left-hand     |

|                       |                    | side of the dot                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`attrname`   | the attribute name, a string    |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`flags`      | XXX                             |

+-----------------------+--------------------+---------------------------------+

| :class:`AssList`      | :attr:`nodes`      | list of list elements being     |

|                       |                    | assigned to                     |

+-----------------------+--------------------+---------------------------------+

| :class:`AssName`      | :attr:`name`       | name being assigned to          |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`flags`      | XXX                             |

+-----------------------+--------------------+---------------------------------+

| :class:`AssTuple`     | :attr:`nodes`      | list of tuple elements being    |

|                       |                    | assigned to                     |

+-----------------------+--------------------+---------------------------------+

| :class:`Assert`       | :attr:`test`       | the expression to be tested     |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`fail`       | the value of the                |

|                       |                    | :exc:`AssertionError`           |

+-----------------------+--------------------+---------------------------------+

| :class:`Assign`       | :attr:`nodes`      | a list of assignment targets,   |

|                       |                    | one per equal sign              |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`expr`       | the value being assigned        |

+-----------------------+--------------------+---------------------------------+

| :class:`AugAssign`    | :attr:`node`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`op`         |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Backquote`    | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Bitand`       | :attr:`nodes`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Bitor`        | :attr:`nodes`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Bitxor`       | :attr:`nodes`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Break`        |                    |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`CallFunc`     | :attr:`node`       | expression for the callee       |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`args`       | a list of arguments             |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`star_args`  | the extended \*-arg value       |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`dstar_args` | the extended \*\*-arg value     |

+-----------------------+--------------------+---------------------------------+

| :class:`Class`        | :attr:`name`       | the name of the class, a string |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`bases`      | a list of base classes          |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`doc`        | doc string, a string or         |

|                       |                    | ``None``                        |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`code`       | the body of the class statement |

+-----------------------+--------------------+---------------------------------+

| :class:`Compare`      | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`ops`        |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Const`        | :attr:`value`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Continue`     |                    |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Decorators`   | :attr:`nodes`      | List of function decorator      |

|                       |                    | expressions                     |

+-----------------------+--------------------+---------------------------------+

| :class:`Dict`         | :attr:`items`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Discard`      | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Div`          | :attr:`left`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`right`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Ellipsis`     |                    |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Expression`   | :attr:`node`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Exec`         | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`locals`     |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`globals`    |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`FloorDiv`     | :attr:`left`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`right`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`For`          | :attr:`assign`     |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`list`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`body`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`else_`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`From`         | :attr:`modname`    |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`names`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Function`     | :attr:`decorators` | :class:`Decorators` or ``None`` |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`name`       | name used in def, a string      |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`argnames`   | list of argument names, as      |

|                       |                    | strings                         |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`defaults`   | list of default values          |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`flags`      | xxx                             |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`doc`        | doc string, a string or         |

|                       |                    | ``None``                        |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`code`       | the body of the function        |

+-----------------------+--------------------+---------------------------------+

| :class:`GenExpr`      | :attr:`code`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`GenExprFor`   | :attr:`assign`     |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`iter`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`ifs`        |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`GenExprIf`    | :attr:`test`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`GenExprInner` | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`quals`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Getattr`      | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`attrname`   |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Global`       | :attr:`names`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`If`           | :attr:`tests`      |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`else_`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Import`       | :attr:`names`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Invert`       | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Keyword`      | :attr:`name`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Lambda`       | :attr:`argnames`   |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`defaults`   |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`flags`      |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`code`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`LeftShift`    | :attr:`left`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`right`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`List`         | :attr:`nodes`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`ListComp`     | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`quals`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`ListCompFor`  | :attr:`assign`     |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`list`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`ifs`        |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`ListCompIf`   | :attr:`test`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Mod`          | :attr:`left`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`right`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Module`       | :attr:`doc`        | doc string, a string or         |

|                       |                    | ``None``                        |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`node`       | body of the module, a           |

|                       |                    | :class:`Stmt`                   |

+-----------------------+--------------------+---------------------------------+

| :class:`Mul`          | :attr:`left`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`right`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Name`         | :attr:`name`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Not`          | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Or`           | :attr:`nodes`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Pass`         |                    |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Power`        | :attr:`left`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`right`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Print`        | :attr:`nodes`      |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`dest`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Printnl`      | :attr:`nodes`      |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`dest`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Raise`        | :attr:`expr1`      |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`expr2`      |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`expr3`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Return`       | :attr:`value`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`RightShift`   | :attr:`left`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`right`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Slice`        | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`flags`      |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`lower`      |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`upper`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Sliceobj`     | :attr:`nodes`      | list of statements              |

+-----------------------+--------------------+---------------------------------+

| :class:`Stmt`         | :attr:`nodes`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Sub`          | :attr:`left`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`right`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Subscript`    | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`flags`      |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`subs`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`TryExcept`    | :attr:`body`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`handlers`   |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`else_`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`TryFinally`   | :attr:`body`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`final`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Tuple`        | :attr:`nodes`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`UnaryAdd`     | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`UnarySub`     | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`While`        | :attr:`test`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`body`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`else_`      |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`With`         | :attr:`expr`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`vars`       |                                 |

+-----------------------+--------------------+---------------------------------+

|                       | :attr:`body`       |                                 |

+-----------------------+--------------------+---------------------------------+

| :class:`Yield`        | :attr:`value`      |                                 |

+-----------------------+--------------------+---------------------------------+

Assignment nodes

----------------

There is a collection of nodes used to represent assignments.  Each assignment

statement in the source code becomes a single :class:`Assign` node in the AST.

The :attr:`nodes` attribute is a list that contains a node for each assignment

target.  This is necessary because assignment can be chained, e.g. ``a = b =

2``. Each :class:`Node` in the list will be one of the following classes:

:class:`AssAttr`, :class:`AssList`, :class:`AssName`, or :class:`AssTuple`.

Each target assignment node will describe the kind of object being assigned to:

:class:`AssName` for a simple name, e.g. ``a = 1``. :class:`AssAttr` for an

attribute assigned, e.g. ``a.x = 1``. :class:`AssList` and :class:`AssTuple` for

list and tuple expansion respectively, e.g. ``a, b, c = a_tuple``.

The target assignment nodes also have a :attr:`flags` attribute that indicates

whether the node is being used for assignment or in a delete statement.  The

:class:`AssName` is also used to represent a delete statement, e.g. :class:`del

x`.

When an expression contains several attribute references, an assignment or