source: python/trunk/Doc/library/parser.rst

:mod:`parser` --- Access Python parse trees
===========================================

.. module:: parser
   :synopsis: Access parse trees for Python source code.
.. moduleauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>


.. Copyright 1995 Virginia Polytechnic Institute and State University and Fred
   L. Drake, Jr.  This copyright notice must be distributed on all copies, but
   this document otherwise may be distributed as part of the Python
   distribution.  No fee may be charged for this document in any representation,
   either on paper or electronically.  This restriction does not affect other
   elements in a distributed package in any way.

.. index:: single: parsing; Python source code

The :mod:`parser` module provides an interface to Python's internal parser and
byte-code compiler.  The primary purpose for this interface is to allow Python
code to edit the parse tree of a Python expression and create executable code
from this.  This is better than trying to parse and modify an arbitrary Python
code fragment as a string because parsing is performed in a manner identical to
the code forming the application.  It is also faster.

.. note::

   From Python 2.5 onward, it's much more convenient to cut in at the Abstract
   Syntax Tree (AST) generation and compilation stage, using the :mod:`ast`
   module.

   The :mod:`parser` module exports the names documented here also with "st"
   replaced by "ast"; this is a legacy from the time when there was no other
   AST and has nothing to do with the AST found in Python 2.5.  This is also the
   reason for the functions' keyword arguments being called *ast*, not *st*.
   The "ast" functions have been removed in Python 3.

There are a few things to note about this module which are important to making
use of the data structures created.  This is not a tutorial on editing the parse
trees for Python code, but some examples of using the :mod:`parser` module are
presented.

Most importantly, a good understanding of the Python grammar processed by the
internal parser is required.  For full information on the language syntax, refer
to :ref:`reference-index`.  The parser
itself is created from a grammar specification defined in the file
:file:`Grammar/Grammar` in the standard Python distribution.  The parse trees
stored in the ST objects created by this module are the actual output from the
internal parser when created by the :func:`expr` or :func:`suite` functions,
described below.  The ST objects created by :func:`sequence2st` faithfully
simulate those structures.  Be aware that the values of the sequences which are
considered "correct" will vary from one version of Python to another as the
formal grammar for the language is revised.  However, transporting code from one
Python version to another as source text will always allow correct parse trees
to be created in the target version, with the only restriction being that
migrating to an older version of the interpreter will not support more recent
language constructs.  The parse trees are not typically compatible from one
version to another, whereas source code has always been forward-compatible.

Each element of the sequences returned by :func:`st2list` or :func:`st2tuple`
has a simple form.  Sequences representing non-terminal elements in the grammar
always have a length greater than one.  The first element is an integer which
identifies a production in the grammar.  These integers are given symbolic names
in the C header file :file:`Include/graminit.h` and the Python module
:mod:`symbol`.  Each additional element of the sequence represents a component
of the production as recognized in the input string: these are always sequences
which have the same form as the parent.  An important aspect of this structure
which should be noted is that keywords used to identify the parent node type,
such as the keyword :keyword:`if` in an :const:`if_stmt`, are included in the
node tree without any special treatment.  For example, the :keyword:`if` keyword
is represented by the tuple ``(1, 'if')``, where ``1`` is the numeric value
associated with all :const:`NAME` tokens, including variable and function names
defined by the user.  In an alternate form returned when line number information
is requested, the same token might be represented as ``(1, 'if', 12)``, where
the ``12`` represents the line number at which the terminal symbol was found.

Terminal elements are represented in much the same way, but without any child
elements and the addition of the source text which was identified.  The example
of the :keyword:`if` keyword above is representative.  The various types of
terminal symbols are defined in the C header file :file:`Include/token.h` and
the Python module :mod:`token`.

The ST objects are not required to support the functionality of this module,
but are provided for three purposes: to allow an application to amortize the
cost of processing complex parse trees, to provide a parse tree representation
which conserves memory space when compared to the Python list or tuple
representation, and to ease the creation of additional modules in C which
manipulate parse trees.  A simple "wrapper" class may be created in Python to
hide the use of ST objects.

The :mod:`parser` module defines functions for a few distinct purposes.  The
most important purposes are to create ST objects and to convert ST objects to
other representations such as parse trees and compiled code objects, but there
are also functions which serve to query the type of parse tree represented by an
ST object.


.. seealso::

   Module :mod:`symbol`
      Useful constants representing internal nodes of the parse tree.

   Module :mod:`token`
      Useful constants representing leaf nodes of the parse tree and functions for
      testing node values.


.. _creating-sts:

Creating ST Objects
-------------------

ST objects may be created from source code or from a parse tree. When creating
an ST object from source, different functions are used to create the ``'eval'``
and ``'exec'`` forms.


.. function:: expr(source)

   The :func:`expr` function parses the parameter *source* as if it were an input
   to ``compile(source, 'file.py', 'eval')``.  If the parse succeeds, an ST object
   is created to hold the internal parse tree representation, otherwise an
   appropriate exception is raised.


.. function:: suite(source)

   The :func:`suite` function parses the parameter *source* as if it were an input
   to ``compile(source, 'file.py', 'exec')``.  If the parse succeeds, an ST object
   is created to hold the internal parse tree representation, otherwise an
   appropriate exception is raised.


.. function:: sequence2st(sequence)

   This function accepts a parse tree represented as a sequence and builds an
   internal representation if possible.  If it can validate that the tree conforms
   to the Python grammar and all nodes are valid node types in the host version of
   Python, an ST object is created from the internal representation and returned
   to the called.  If there is a problem creating the internal representation, or
   if the tree cannot be validated, a :exc:`ParserError` exception is raised.  An
   ST object created this way should not be assumed to compile correctly; normal
   exceptions raised by compilation may still be initiated when the ST object is
   passed to :func:`compilest`.  This may indicate problems not related to syntax
   (such as a :exc:`MemoryError` exception), but may also be due to constructs such
   as the result of parsing ``del f(0)``, which escapes the Python parser but is
   checked by the bytecode compiler.

   Sequences representing terminal tokens may be represented as either two-element
   lists of the form ``(1, 'name')`` or as three-element lists of the form ``(1,
   'name', 56)``.  If the third element is present, it is assumed to be a valid
   line number.  The line number may be specified for any subset of the terminal
   symbols in the input tree.


.. function:: tuple2st(sequence)

   This is the same function as :func:`sequence2st`.  This entry point is
   maintained for backward compatibility.


.. _converting-sts:

Converting ST Objects
---------------------

ST objects, regardless of the input used to create them, may be converted to
parse trees represented as list- or tuple- trees, or may be compiled into
executable code objects.  Parse trees may be extracted with or without line
numbering information.


.. function:: st2list(ast[, line_info])

   This function accepts an ST object from the caller in *ast* and returns a
   Python list representing the equivalent parse tree.  The resulting list
   representation can be used for inspection or the creation of a new parse tree in
   list form.  This function does not fail so long as memory is available to build
   the list representation.  If the parse tree will only be used for inspection,
   :func:`st2tuple` should be used instead to reduce memory consumption and
   fragmentation.  When the list representation is required, this function is
   significantly faster than retrieving a tuple representation and converting that
   to nested lists.

   If *line_info* is true, line number information will be included for all
   terminal tokens as a third element of the list representing the token.  Note
   that the line number provided specifies the line on which the token *ends*.
   This information is omitted if the flag is false or omitted.


.. function:: st2tuple(ast[, line_info])

   This function accepts an ST object from the caller in *ast* and returns a
   Python tuple representing the equivalent parse tree.  Other than returning a
   tuple instead of a list, this function is identical to :func:`st2list`.

   If *line_info* is true, line number information will be included for all
   terminal tokens as a third element of the list representing the token.  This
   information is omitted if the flag is false or omitted.


.. function:: compilest(ast, filename='<syntax-tree>')

   .. index:: builtin: eval

   The Python byte compiler can be invoked on an ST object to produce code objects
   which can be used as part of an :keyword:`exec` statement or a call to the
   built-in :func:`eval` function. This function provides the interface to the
   compiler, passing the internal parse tree from *ast* to the parser, using the
   source file name specified by the *filename* parameter. The default value
   supplied for *filename* indicates that the source was an ST object.

   Compiling an ST object may result in exceptions related to compilation; an
   example would be a :exc:`SyntaxError` caused by the parse tree for ``del f(0)``:
   this statement is considered legal within the formal grammar for Python but is
   not a legal language construct.  The :exc:`SyntaxError` raised for this
   condition is actually generated by the Python byte-compiler normally, which is
   why it can be raised at this point by the :mod:`parser` module.  Most causes of
   compilation failure can be diagnosed programmatically by inspection of the parse
   tree.


.. _querying-sts:

Queries on ST Objects
---------------------

Two functions are provided which allow an application to determine if an ST was
created as an expression or a suite.  Neither of these functions can be used to
determine if an ST was created from source code via :func:`expr` or
:func:`suite` or from a parse tree via :func:`sequence2st`.


.. function:: isexpr(ast)

   .. index:: builtin: compile

   When *ast* represents an ``'eval'`` form, this function returns true, otherwise
   it returns false.  This is useful, since code objects normally cannot be queried
   for this information using existing built-in functions.  Note that the code
   objects created by :func:`compilest` cannot be queried like this either, and
   are identical to those created by the built-in :func:`compile` function.


.. function:: issuite(ast)

   This function mirrors :func:`isexpr` in that it reports whether an ST object
   represents an ``'exec'`` form, commonly known as a "suite."  It is not safe to
   assume that this function is equivalent to ``not isexpr(ast)``, as additional
   syntactic fragments may be supported in the future.


.. _st-errors:

Exceptions and Error Handling
-----------------------------

The parser module defines a single exception, but may also pass other built-in
exceptions from other portions of the Python runtime environment.  See each
function for information about the exceptions it can raise.


.. exception:: ParserError

   Exception raised when a failure occurs within the parser module.  This is
   generally produced for validation failures rather than the built-in
   :exc:`SyntaxError` raised during normal parsing. The exception argument is
   either a string describing the reason of the failure or a tuple containing a
   sequence causing the failure from a parse tree passed to :func:`sequence2st`
   and an explanatory string.  Calls to :func:`sequence2st` need to be able to
   handle either type of exception, while calls to other functions in the module
   will only need to be aware of the simple string values.

Note that the functions :func:`compilest`, :func:`expr`, and :func:`suite` may
raise exceptions which are normally raised by the parsing and compilation
process.  These include the built in exceptions :exc:`MemoryError`,
:exc:`OverflowError`, :exc:`SyntaxError`, and :exc:`SystemError`.  In these
cases, these exceptions carry all the meaning normally associated with them.
Refer to the descriptions of each function for detailed information.


.. _st-objects:

ST Objects
----------

Ordered and equality comparisons are supported between ST objects. Pickling of
ST objects (using the :mod:`pickle` module) is also supported.


.. data:: STType

   The type of the objects returned by :func:`expr`, :func:`suite` and
   :func:`sequence2st`.

ST objects have the following methods:


.. method:: ST.compile([filename])

   Same as ``compilest(st, filename)``.


.. method:: ST.isexpr()

   Same as ``isexpr(st)``.


.. method:: ST.issuite()

   Same as ``issuite(st)``.


.. method:: ST.tolist([line_info])

   Same as ``st2list(st, line_info)``.


.. method:: ST.totuple([line_info])

   Same as ``st2tuple(st, line_info)``.


Example: Emulation of :func:`compile`
-------------------------------------

While many useful operations may take place between parsing and bytecode
generation, the simplest operation is to do nothing.  For this purpose, using
the :mod:`parser` module to produce an intermediate data structure is equivalent
to the code ::

   >>> code = compile('a + 5', 'file.py', 'eval')
   >>> a = 5
   >>> eval(code)
   10

The equivalent operation using the :mod:`parser` module is somewhat longer, and
allows the intermediate internal parse tree to be retained as an ST object::

   >>> import parser
   >>> st = parser.expr('a + 5')
   >>> code = st.compile('file.py')
   >>> a = 5
   >>> eval(code)
   10

An application which needs both ST and code objects can package this code into
readily available functions::

   import parser

   def load_suite(source_string):
       st = parser.suite(source_string)
       return st, st.compile()

   def load_expression(source_string):
       st = parser.expr(source_string)
       return st, st.compile()