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(python2.1-lib.info)Information Discovery
Information Discovery
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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
`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 `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 `def'
statement at column zero of a module, but not a function defined within
a branch of an `if' ... `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 `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 `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
>>> 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, ''))
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 `symbol' and `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 `file_input'. Each of these inner tuples
contains an integer as the first element; these integers, `264', `4',
and `0', represent the node types `stmt', `NEWLINE', and `ENDMARKER',
respectively. Note that these values may change depending on the
version of Python you are using; consult `symbol.py' and `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 `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 `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 `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,
(token.STRING, ['docstring'])
)))))))))))))))),
(token.NEWLINE, '')
))
Using the `match()' function with this pattern, extracting the module
docstring from the parse tree created previously is easy:
>>> found, vars = match(DOCSTRING_STMT_PATTERN, tup[1])
>>> found
1
>>> vars
{'docstring': '"""Some documentation.\n"""'}
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 `stmt' node in a code block (`file_input' or
`suite' node types). A module consists of a single `file_input' node,
and class and function definitions each contain exactly one `suite'
node. Classes and functions are readily identified as subtrees of code
block nodes which start with `(stmt, (compound_stmt, (classdef, ...' or
`(stmt, (compound_stmt, (funcdef, ...'. Note that these subtrees
cannot be matched by `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 `ModuleInfo' constructor accepts an
optional NAME parameter since it cannot otherwise determine the name of
the module.
The public classes include `ClassInfo', `FunctionInfo', and
`ModuleInfo'. All objects provide the methods `get_name()',
`get_docstring()', `get_class_names()', and `get_class_info()'. The
`ClassInfo' objects support `get_method_names()' and
`get_method_info()' while the other classes provide
`get_function_names()' and `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,
`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 `def'
statement to define both types of elements.
Most of the accessor functions are declared in `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 `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
`SuiteInfoBase' definition from `example.py':
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)
After initializing some internal state, the constructor calls the
`_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 `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
def square(x): "Square an argument."; return x ** 2
while the long form uses an indented block and allows nested
definitions:
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
When the short form is used, the code block may contain a docstring as
the first, and possibly only, `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 `small_stmt' node in
the `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
`match()' function as described above, and the value of the docstring
is stored as an attribute of the `SuiteInfoBase' object.
After docstring extraction, a simple definition discovery algorithm
operates on the `stmt' nodes of the `suite' node. The special case of
the short form is not tested; since there are no `stmt' nodes in the
short form, the algorithm will silently skip the single `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 `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 `example.py'.)
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)
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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