From 458120dd40db6b4df55a4e96b650e16798ef06a0 Mon Sep 17 00:00:00 2001
From: cinap_lenrek <cinap_lenrek@localhost>
Date: Tue, 3 May 2011 11:25:13 +0000
Subject: add hg and python

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+\section{\module{parser} ---
+         Access Python parse trees}
+
+% 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.
+
+\declaremodule{builtin}{parser}
+\modulesynopsis{Access parse trees for Python source code.}
+\moduleauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
+\sectionauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
+
+
+\index{parsing!Python source code}
+
+The \module{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.
+
+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 \module{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 the \citetitle[../ref/ref.html]{Python
+Language Reference}.  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 AST
+objects created by this module are the actual output from the internal
+parser when created by the \function{expr()} or \function{suite()}
+functions, described below.  The AST objects created by
+\function{sequence2ast()} 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 \function{ast2list()} or
+\function{ast2tuple()} 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
+\refmodule{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 \constant{if_stmt}, are included in the node tree without
+any special treatment.  For example, the \keyword{if} keyword is
+represented by the tuple \code{(1, 'if')}, where \code{1} is the
+numeric value associated with all \constant{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 \code{(1, 'if', 12)}, where the \code{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
+\refmodule{token}.
+
+The AST 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 AST
+objects.
+
+The \module{parser} module defines functions for a few distinct
+purposes.  The most important purposes are to create AST objects and
+to convert AST 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 AST object.
+
+
+\begin{seealso}
+  \seemodule{symbol}{Useful constants representing internal nodes of
+                     the parse tree.}
+  \seemodule{token}{Useful constants representing leaf nodes of the
+                    parse tree and functions for testing node values.}
+\end{seealso}
+
+
+\subsection{Creating AST Objects \label{Creating ASTs}}
+
+AST objects may be created from source code or from a parse tree.
+When creating an AST object from source, different functions are used
+to create the \code{'eval'} and \code{'exec'} forms.
+
+\begin{funcdesc}{expr}{source}
+The \function{expr()} function parses the parameter \var{source}
+as if it were an input to \samp{compile(\var{source}, 'file.py',
+'eval')}.  If the parse succeeds, an AST object is created to hold the
+internal parse tree representation, otherwise an appropriate exception
+is thrown.
+\end{funcdesc}
+
+\begin{funcdesc}{suite}{source}
+The \function{suite()} function parses the parameter \var{source}
+as if it were an input to \samp{compile(\var{source}, 'file.py',
+'exec')}.  If the parse succeeds, an AST object is created to hold the
+internal parse tree representation, otherwise an appropriate exception
+is thrown.
+\end{funcdesc}
+
+\begin{funcdesc}{sequence2ast}{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 AST 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 \exception{ParserError} exception is thrown.  An AST
+object created this way should not be assumed to compile correctly;
+normal exceptions thrown by compilation may still be initiated when
+the AST object is passed to \function{compileast()}.  This may indicate
+problems not related to syntax (such as a \exception{MemoryError}
+exception), but may also be due to constructs such as the result of
+parsing \code{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 \code{(1, 'name')} or as three-element
+lists of the form \code{(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.
+\end{funcdesc}
+
+\begin{funcdesc}{tuple2ast}{sequence}
+This is the same function as \function{sequence2ast()}.  This entry point
+is maintained for backward compatibility.
+\end{funcdesc}
+
+
+\subsection{Converting AST Objects \label{Converting ASTs}}
+
+AST 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.
+
+\begin{funcdesc}{ast2list}{ast\optional{, line_info}}
+This function accepts an AST object from the caller in
+\var{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, \function{ast2tuple()} 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 \var{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 \emph{ends}.  This information is
+omitted if the flag is false or omitted.
+\end{funcdesc}
+
+\begin{funcdesc}{ast2tuple}{ast\optional{, line_info}}
+This function accepts an AST object from the caller in
+\var{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 \function{ast2list()}.
+
+If \var{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.
+\end{funcdesc}
+
+\begin{funcdesc}{compileast}{ast\optional{, filename\code{ = '<ast>'}}}
+The Python byte compiler can be invoked on an AST object to produce
+code objects which can be used as part of an \keyword{exec} statement or
+a call to the built-in \function{eval()}\bifuncindex{eval} function.
+This function provides the interface to the compiler, passing the
+internal parse tree from \var{ast} to the parser, using the
+source file name specified by the \var{filename} parameter.
+The default value supplied for \var{filename} indicates that
+the source was an AST object.
+
+Compiling an AST object may result in exceptions related to
+compilation; an example would be a \exception{SyntaxError} caused by the
+parse tree for \code{del f(0)}: this statement is considered legal
+within the formal grammar for Python but is not a legal language
+construct.  The \exception{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 \module{parser} module.  Most
+causes of compilation failure can be diagnosed programmatically by
+inspection of the parse tree.
+\end{funcdesc}
+
+
+\subsection{Queries on AST Objects \label{Querying ASTs}}
+
+Two functions are provided which allow an application to determine if
+an AST was created as an expression or a suite.  Neither of these
+functions can be used to determine if an AST was created from source
+code via \function{expr()} or \function{suite()} or from a parse tree
+via \function{sequence2ast()}.
+
+\begin{funcdesc}{isexpr}{ast}
+When \var{ast} represents an \code{'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
+\function{compileast()} cannot be queried like this either, and are
+identical to those created by the built-in
+\function{compile()}\bifuncindex{compile} function.
+\end{funcdesc}
+
+
+\begin{funcdesc}{issuite}{ast}
+This function mirrors \function{isexpr()} in that it reports whether an
+AST object represents an \code{'exec'} form, commonly known as a
+``suite.''  It is not safe to assume that this function is equivalent
+to \samp{not isexpr(\var{ast})}, as additional syntactic fragments may
+be supported in the future.
+\end{funcdesc}
+
+
+\subsection{Exceptions and Error Handling \label{AST Errors}}
+
+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.
+
+\begin{excdesc}{ParserError}
+Exception raised when a failure occurs within the parser module.  This
+is generally produced for validation failures rather than the built in
+\exception{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 \function{sequence2ast()} and an explanatory string.  Calls to
+\function{sequence2ast()} 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.
+\end{excdesc}
+
+Note that the functions \function{compileast()}, \function{expr()}, and
+\function{suite()} may throw exceptions which are normally thrown by the
+parsing and compilation process.  These include the built in
+exceptions \exception{MemoryError}, \exception{OverflowError},
+\exception{SyntaxError}, and \exception{SystemError}.  In these cases, these
+exceptions carry all the meaning normally associated with them.  Refer
+to the descriptions of each function for detailed information.
+
+
+\subsection{AST Objects \label{AST Objects}}
+
+Ordered and equality comparisons are supported between AST objects.
+Pickling of AST objects (using the \refmodule{pickle} module) is also
+supported.
+
+\begin{datadesc}{ASTType}
+The type of the objects returned by \function{expr()},
+\function{suite()} and \function{sequence2ast()}.
+\end{datadesc}
+
+
+AST objects have the following methods:
+
+
+\begin{methoddesc}[AST]{compile}{\optional{filename}}
+Same as \code{compileast(\var{ast}, \var{filename})}.
+\end{methoddesc}
+
+\begin{methoddesc}[AST]{isexpr}{}
+Same as \code{isexpr(\var{ast})}.
+\end{methoddesc}
+
+\begin{methoddesc}[AST]{issuite}{}
+Same as \code{issuite(\var{ast})}.
+\end{methoddesc}
+
+\begin{methoddesc}[AST]{tolist}{\optional{line_info}}
+Same as \code{ast2list(\var{ast}, \var{line_info})}.
+\end{methoddesc}
+
+\begin{methoddesc}[AST]{totuple}{\optional{line_info}}
+Same as \code{ast2tuple(\var{ast}, \var{line_info})}.
+\end{methoddesc}
+
+
+\subsection{Examples \label{AST Examples}}
+
+The parser modules allows operations to be performed on the parse tree
+of Python source code before the 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 \function{compile()}\bifuncindex{compile} built-in function and
+the complex example shows the use of a parse tree for information
+discovery.
+
+\subsubsection{Emulation of \function{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 \module{parser} module to produce an
+intermediate data structure is equivalent to the code
+
+\begin{verbatim}
+>>> code = compile('a + 5', 'file.py', 'eval')
+>>> a = 5
+>>> eval(code)
+10
+\end{verbatim}
+
+The equivalent operation using the \module{parser} module is somewhat
+longer, and allows the intermediate internal parse tree to be retained
+as an AST object:
+
+\begin{verbatim}
+>>> import parser
+>>> ast = parser.expr('a + 5')
+>>> code = ast.compile('file.py')
+>>> a = 5
+>>> eval(code)
+10
+\end{verbatim}
+
+An application which needs both AST and code objects can package this
+code into readily available functions:
+
+\begin{verbatim}
+import parser
+
+def load_suite(source_string):
+    ast = parser.suite(source_string)
+    return ast, ast.compile()
+
+def load_expression(source_string):
+    ast = parser.expr(source_string)
+    return ast, ast.compile()
+\end{verbatim}
+
+\subsubsection{Information Discovery}
+
+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\index{string!documentation}\index{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}.)
+
+\begin{verbatim}
+"""Some documentation.
+"""
+\end{verbatim}
+
+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.
+
+\begin{verbatim}
+>>> import parser
+>>> import pprint
+>>> ast = parser.suite(open('docstring.py').read())
+>>> tup = ast.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, ''))
+\end{verbatim}
+
+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 \refmodule{symbol} and \refmodule{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 \code{257} and three additional tuples.  Node
+type \code{257} has the symbolic name \constant{file_input}.  Each of
+these inner tuples contains an integer as the first element; these
+integers, \code{264}, \code{4}, and \code{0}, represent the node types
+\constant{stmt}, \constant{NEWLINE}, and \constant{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 \constant{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
+\code{['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}.)
+
+\begin{verbatim}
+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
+\end{verbatim}
+
+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}.)
+
+\begin{verbatim}
+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,
+                     (token.STRING, ['docstring'])
+                     )))))))))))))))),
+     (token.NEWLINE, '')
+     ))
+\end{verbatim}
+
+Using the \function{match()} function with this pattern, extracting the
+module docstring from the parse tree created previously is easy:
+
+\begin{verbatim}
+>>> found, vars = match(DOCSTRING_STMT_PATTERN, tup[1])
+>>> found
+1
+>>> vars
+{'docstring': '"""Some documentation.\n"""'}
+\end{verbatim}
+
+Once specific data can be extracted from a location where it is
+expected, the question of where information can be expected
+needs to be answered.  When dealing with docstrings, the answer is
+fairly simple: the docstring is the first \constant{stmt} node in a code
+block (\constant{file_input} or \constant{suite} node types).  A module
+consists of a single \constant{file_input} node, and class and function
+definitions each contain exactly one \constant{suite} node.  Classes and
+functions are readily identified as subtrees of code block nodes which
+start with \code{(stmt, (compound_stmt, (classdef, ...} or
+\code{(stmt, (compound_stmt, (funcdef, ...}.  Note that these subtrees
+cannot be matched by \function{match()} since it does not support multiple
+sibling nodes to match without regard to number.  A more elaborate
+matching function could be used to overcome this limitation, but this
+is sufficient for the example.
+
+Given the ability to determine whether a statement might be a
+docstring and extract the actual string from the statement, some work
+needs to be performed to walk the parse tree for an entire module and
+extract information about the names defined in each context of the
+module and associate any docstrings with the names.  The code to
+perform this work is not complicated, but bears some explanation.
+
+The public interface to the classes is straightforward and should
+probably be somewhat more flexible.  Each ``major'' block of the
+module is described by an object providing several methods for inquiry
+and a constructor which accepts at least the subtree of the complete
+parse tree which it represents.  The \class{ModuleInfo} constructor
+accepts an optional \var{name} parameter since it cannot
+otherwise determine the name of the module.
+
+The public classes include \class{ClassInfo}, \class{FunctionInfo},
+and \class{ModuleInfo}.  All objects provide the
+methods \method{get_name()}, \method{get_docstring()},
+\method{get_class_names()}, and \method{get_class_info()}.  The
+\class{ClassInfo} objects support \method{get_method_names()} and
+\method{get_method_info()} while the other classes provide
+\method{get_function_names()} and \method{get_function_info()}.
+
+Within each of the forms of code block that the public classes
+represent, most of the required information is in the same form and is
+accessed in the same way, with classes having the distinction that
+functions defined at the top level are referred to as ``methods.''
+Since the difference in nomenclature reflects a real semantic
+distinction from functions defined outside of a class, the
+implementation needs to maintain the distinction.
+Hence, most of the functionality of the public classes can be
+implemented in a common base class, \class{SuiteInfoBase}, with the
+accessors for function and method information provided elsewhere.
+Note that there is only one class which represents function and method
+information; this parallels the use of the \keyword{def} statement to
+define both types of elements.
+
+Most of the accessor functions are declared in \class{SuiteInfoBase}
+and do not need to be overridden by subclasses.  More importantly, the
+extraction of most information from a parse tree is handled through a
+method called by the \class{SuiteInfoBase} constructor.  The example
+code for most of the classes is clear when read alongside the formal
+grammar, but the method which recursively creates new information
+objects requires further examination.  Here is the relevant part of
+the \class{SuiteInfoBase} definition from \file{example.py}:
+
+\begin{verbatim}
+class SuiteInfoBase:
+    _docstring = ''
+    _name = ''
+
+    def __init__(self, tree = None):
+        self._class_info = {}
+        self._function_info = {}
+        if tree:
+            self._extract_info(tree)
+
+    def _extract_info(self, tree):
+        # extract docstring
+        if len(tree) == 2:
+            found, vars = match(DOCSTRING_STMT_PATTERN[1], tree[1])
+        else:
+            found, vars = match(DOCSTRING_STMT_PATTERN, tree[3])
+        if found:
+            self._docstring = eval(vars['docstring'])
+        # discover inner definitions
+        for node in tree[1:]:
+            found, vars = match(COMPOUND_STMT_PATTERN, node)
+            if found:
+                cstmt = vars['compound']
+                if cstmt[0] == symbol.funcdef:
+                    name = cstmt[2][1]
+                    self._function_info[name] = FunctionInfo(cstmt)
+                elif cstmt[0] == symbol.classdef:
+                    name = cstmt[2][1]
+                    self._class_info[name] = ClassInfo(cstmt)
+\end{verbatim}
+
+After initializing some internal state, the constructor calls the
+\method{_extract_info()} method.  This method performs the bulk of the
+information extraction which takes place in the entire example.  The
+extraction has two distinct phases: the location of the docstring for
+the parse tree passed in, and the discovery of additional definitions
+within the code block represented by the parse tree.
+
+The initial \keyword{if} test determines whether the nested suite is of
+the ``short form'' or the ``long form.''  The short form is used when
+the code block is on the same line as the definition of the code
+block, as in
+
+\begin{verbatim}
+def square(x): "Square an argument."; return x ** 2
+\end{verbatim}
+
+while the long form uses an indented block and allows nested
+definitions:
+
+\begin{verbatim}
+def make_power(exp):
+    "Make a function that raises an argument to the exponent `exp'."
+    def raiser(x, y=exp):
+        return x ** y
+    return raiser
+\end{verbatim}
+
+When the short form is used, the code block may contain a docstring as
+the first, and possibly only, \constant{small_stmt} element.  The
+extraction of such a docstring is slightly different and requires only
+a portion of the complete pattern used in the more common case.  As
+implemented, the docstring will only be found if there is only
+one \constant{small_stmt} node in the \constant{simple_stmt} node.
+Since most functions and methods which use the short form do not
+provide a docstring, this may be considered sufficient.  The
+extraction of the docstring proceeds using the \function{match()} function
+as described above, and the value of the docstring is stored as an
+attribute of the \class{SuiteInfoBase} object.
+
+After docstring extraction, a simple definition discovery
+algorithm operates on the \constant{stmt} nodes of the
+\constant{suite} node.  The special case of the short form is not
+tested; since there are no \constant{stmt} nodes in the short form,
+the algorithm will silently skip the single \constant{simple_stmt}
+node and correctly not discover any nested definitions.
+
+Each statement in the code block is categorized as
+a class definition, function or method definition, or
+something else.  For the definition statements, the name of the
+element defined is extracted and a representation object
+appropriate to the definition is created with the defining subtree
+passed as an argument to the constructor.  The representation objects
+are stored in instance variables and may be retrieved by name using
+the appropriate accessor methods.
+
+The public classes provide any accessors required which are more
+specific than those provided by the \class{SuiteInfoBase} class, but
+the real extraction algorithm remains common to all forms of code
+blocks.  A high-level function can be used to extract the complete set
+of information from a source file.  (See file \file{example.py}.)
+
+\begin{verbatim}
+def get_docs(fileName):
+    import os
+    import parser
+
+    source = open(fileName).read()
+    basename = os.path.basename(os.path.splitext(fileName)[0])
+    ast = parser.suite(source)
+    return ModuleInfo(ast.totuple(), basename)
+\end{verbatim}
+
+This provides an easy-to-use interface to the documentation of a
+module.  If information is required which is not extracted by the code
+of this example, the code may be extended at clearly defined points to
+provide additional capabilities.
-- 
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