: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 will be removed in Python 3.0.

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 thrown.

.. 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 thrown.

.. 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 thrown.  An

   ST object created this way should not be assumed to compile correctly; normal

   exceptions thrown 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` thrown 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

throw exceptions which are normally thrown 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)``.

.. _st-examples:

Examples

--------

.. index:: builtin: compile

The parser modules allows operations to be performed on the parse tree of Python

source code before the :term:`bytecode` is generated, and provides for inspection of the

parse tree for information gathering purposes. Two examples are presented.  The

simple example demonstrates emulation of the :func:`compile` built-in function

and the complex example shows the use of a parse tree for information discovery.

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()

Information Discovery

^^^^^^^^^^^^^^^^^^^^^

.. index::

   single: string; documentation

   single: docstrings

Some applications benefit from direct access to the parse tree.  The remainder

of this section demonstrates how the parse tree provides access to module

documentation defined in docstrings without requiring that the code being

examined be loaded into a running interpreter via :keyword:`import`.  This can

be very useful for performing analyses of untrusted code.

Generally, the example will demonstrate how the parse tree may be traversed to

distill interesting information.  Two functions and a set of classes are

developed which provide programmatic access to high level function and class

definitions provided by a module.  The classes extract information from the

parse tree and provide access to the information at a useful semantic level, one

function provides a simple low-level pattern matching capability, and the other

function defines a high-level interface to the classes by handling file

operations on behalf of the caller.  All source files mentioned here which are

not part of the Python installation are located in the :file:`Demo/parser/`

directory of the distribution.

The dynamic nature of Python allows the programmer a great deal of flexibility,

but most modules need only a limited measure of this when defining classes,

functions, and methods.  In this example, the only definitions that will be

considered are those which are defined in the top level of their context, e.g.,

a function defined by a :keyword:`def` statement at column zero of a module, but

not a function defined within a branch of an :keyword:`if` ... :keyword:`else`

construct, though there are some good reasons for doing so in some situations.

Nesting of definitions will be handled by the code developed in the example.

To construct the upper-level extraction methods, we need to know what the parse

tree structure looks like and how much of it we actually need to be concerned

about.  Python uses a moderately deep parse tree so there are a large number of

intermediate nodes.  It is important to read and understand the formal grammar

used by Python.  This is specified in the file :file:`Grammar/Grammar` in the

distribution. Consider the simplest case of interest when searching for

docstrings: a module consisting of a docstring and nothing else.  (See file

:file:`docstring.py`.) ::

   """Some documentation.

   """

Using the interpreter to take a look at the parse tree, we find a bewildering

mass of numbers and parentheses, with the documentation buried deep in nested

tuples. ::

   >>> import parser

   >>> import pprint

   >>> st = parser.suite(open('docstring.py').read())

   >>> tup = st.totuple()

   >>> pprint.pprint(tup)

   (257,

    (264,

     (265,

      (266,

       (267,

        (307,

         (287,

          (288,

           (289,

            (290,

             (292,

              (293,

               (294,

                (295,

                 (296,

                  (297,

                   (298,

                    (299,

                     (300, (3, '"""Some documentation.\n"""'))))))))))))))))),

      (4, ''))),

    (4, ''),

    (0, ''))

The numbers at the first element of each node in the tree are the node types;

they map directly to terminal and non-terminal symbols in the grammar.

Unfortunately, they are represented as integers in the internal representation,

and the Python structures generated do not change that.  However, the

:mod:`symbol` and :mod:`token` modules provide symbolic names for the node types

and dictionaries which map from the integers to the symbolic names for the node

types.

In the output presented above, the outermost tuple contains four elements: the

integer ``257`` and three additional tuples.  Node type ``257`` has the symbolic

name :const:`file_input`.  Each of these inner tuples contains an integer as the

first element; these integers, ``264``, ``4``, and ``0``, represent the node

types :const:`stmt`, :const:`NEWLINE`, and :const:`ENDMARKER`, respectively.

Note that these values may change depending on the version of Python you are

using; consult :file:`symbol.py` and :file:`token.py` for details of the

mapping.  It should be fairly clear that the outermost node is related primarily

to the input source rather than the contents of the file, and may be disregarded

for the moment.  The :const:`stmt` node is much more interesting.  In

particular, all docstrings are found in subtrees which are formed exactly as

this node is formed, with the only difference being the string itself.  The

association between the docstring in a similar tree and the defined entity

(class, function, or module) which it describes is given by the position of the

docstring subtree within the tree defining the described structure.

By replacing the actual docstring with something to signify a variable component

of the tree, we allow a simple pattern matching approach to check any given

subtree for equivalence to the general pattern for docstrings.  Since the

example demonstrates information extraction, we can safely require that the tree

be in tuple form rather than list form, allowing a simple variable

representation to be ``['variable_name']``.  A simple recursive function can

implement the pattern matching, returning a Boolean and a dictionary of variable

name to value mappings.  (See file :file:`example.py`.) ::

   from types import ListType, TupleType

   def match(pattern, data, vars=None):

       if vars is None:

           vars = {}

       if type(pattern) is ListType:

           vars[pattern[0]] = data

           return 1, vars

       if type(pattern) is not TupleType:

           return (pattern == data), vars

       if len(data) != len(pattern):

           return 0, vars

       for pattern, data in map(None, pattern, data):

           same, vars = match(pattern, data, vars)

           if not same:

               break

       return same, vars

Using this simple representation for syntactic variables and the symbolic node

types, the pattern for the candidate docstring subtrees becomes fairly readable.

(See file :file:`example.py`.) ::

   import symbol

   import token

   DOCSTRING_STMT_PATTERN = (

       symbol.stmt,

       (symbol.simple_stmt,

        (symbol.small_stmt,

         (symbol.expr_stmt,

          (symbol.testlist,

           (symbol.test,

            (symbol.and_test,

             (symbol.not_test,

              (symbol.comparison,

               (symbol.expr,

                (symbol.xor_expr,

                 (symbol.and_expr,

                  (symbol.shift_expr,

                   (symbol.arith_expr,

                    (symbol.term,

                     (symbol.factor,

                      (symbol.power,

                       (symbol.atom,