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diff --git a/Doc/libparser.tex b/Doc/libparser.tex index 2264c89..859226a 100644 --- a/Doc/libparser.tex +++ b/Doc/libparser.tex @@ -17,20 +17,21 @@ The \code{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 can be better than trying -to parse and modify an arbitrary Python code fragment as a string, and -ensures that parsing is performed in a manner identical to the code -forming the application. It's also faster. +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. +on editing the parse trees for Python code, but some examples of using +the \code{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 Language Reference. The parser itself is created from a grammar specification defined in the file -\code{Grammar/Grammar} in the standard Python distribution. The parse +\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 \code{expr()} or \code{suite()} functions, described below. The AST @@ -51,16 +52,16 @@ Each element of the sequences returned by \code{ast2list} or 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 \code{Include/graminit.h} and the Python module -\code{Lib/symbol.py}. Each additional element of the sequence represents +file \file{Include/graminit.h} and the Python module +\file{Lib/symbol.py}. 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 -\code{if} in an \emph{if\_stmt}, are included in the node tree without +\code{if} in an \code{if_stmt}, are included in the node tree without any special treatment. For example, the \code{if} keyword is represented by the tuple \code{(1, 'if')}, where \code{1} is the -numeric value associated with all \code{NAME} elements, including +numeric value associated with all \code{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} @@ -70,51 +71,115 @@ 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 \code{if} keyword above is representative. The various types of terminal symbols are defined in -the C header file \code{Include/token.h} and the Python module -\code{Lib/token.py}. +the C header file \file{Include/token.h} and the Python module +\file{Lib/token.py}. + +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 \code{AST} library module provides a variety of such +classes. + +The \code{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. -The AST objects are not actually 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'' module may be created in Python to hide the use of -AST objects. +\renewcommand{\indexsubitem}{(in module parser)} -The \code{parser} module defines the following functions: +\subsection{Creating AST Objects} + +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}{string} +The \code{expr()} function parses the parameter \code{\var{string}} +as if it were an input to \code{compile(\var{string}, '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}{string} +The \code{suite()} function parses the parameter \code{\var{string}} +as if it were an input to \code{compile(\var{string}, '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 \code{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 \code{compileast()}. This may indicate +problems not related to syntax (such as a \code{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 \code{sequence2ast()}. This entry point +is maintained for backward compatibility. +\end{funcdesc} -\renewcommand{\indexsubitem}{(in module parser)} -\begin{funcdesc}{ast2list}{ast\optional{\, line\_info\code{ = 0}}} +\subsection{Converting AST Objects} + +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\code{ = 0}}} This function accepts an AST object from the caller in \code{\var{ast}} and returns a Python list representing the equivelent 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 a parse tree will only be used for +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, \code{ast2tuple()} should be used instead to reduce memory -consumption and fragmentation. When modifications are to be made to -the parse tree, this function is significantly faster than retrieving -a tuple representation and converting that to nested lists. - -If the \code{line\_info} flag is given true value, 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. +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 \code{\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}{ast2tuple}{ast\optional{\, line\_info\code{ = 0}}} +\begin{funcdesc}{ast2tuple}{ast\optional{\, line_info\code{ = 0}}} This function accepts an AST object from the caller in \code{\var{ast}} and returns a Python tuple representing the equivelent parse tree. Other than returning a tuple instead of a list, this function is identical to \code{ast2list()}. -If the \code{line\_info} flag is given true value, 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. +If \code{\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>'}}} @@ -128,7 +193,7 @@ for \code{\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 \code{SyntaxError} caused by the -parse tree for \code{del f(0)}; this statement is considered legal +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 \code{SyntaxError} raised for this condition is actually generated by the Python byte-compiler normally, which is why @@ -138,14 +203,13 @@ inspection of the parse tree. \end{funcdesc} -\begin{funcdesc}{expr}{string} -The \code{expr()} function parses the parameter \code{\var{string}} -as if it were an input to \code{compile(\var{string}, '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} +\subsection{Queries on AST Objects} +Two functions are provided which allow an application to determine if +an AST was create as an expression or a suite. Neither of these +functions can be used to determine if an AST was created from source +code via \code{expr()} or \code{suite()} or from a parse tree via +\code{sequence2ast()}. \begin{funcdesc}{isexpr}{ast} When \code{\var{ast}} represents an \code{'eval'} form, this function @@ -160,48 +224,10 @@ like this either, and are identical to those created by the built-in \begin{funcdesc}{issuite}{ast} This function mirrors \code{isexpr()} in that it reports whether an -AST object represents a suite of statements. It is not safe to assume -that this function is equivelent to \code{not isexpr(\var{ast})}, as -additional syntactic fragments may be supported in the future. -\end{funcdesc} - - -\begin{funcdesc}{suite}{string} -The \code{suite()} function parses the parameter \code{\var{string}} -as if it were an input to \code{compile(\var{string}, '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 \code{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 \code{compileast()}. This will normally -indicate problems not related to syntax (such as a \code{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 \code{sequence2ast}. This entry point is -maintained for backward compatibility. +AST object represents an \code{'exec'} form, commonly known as a +``suite.'' It is not safe to assume that this function is equivelent +to \code{not isexpr(\var{ast})}, as additional syntactic fragments may +be supported in the future. \end{funcdesc} @@ -235,10 +261,11 @@ to the descriptions of each function for detailed information. \subsection{AST Objects} -AST objects (returned by \code{expr()}, \code{suite()}, and -\code{sequence2ast()}, described above) have no methods of their own. +AST objects returned by \code{expr()}, \code{suite()}, and +\code{sequence2ast()} have no methods of their own. Some of the functions defined which accept an AST object as their -first argument may change to object methods in the future. +first argument may change to object methods in the future. The type +of these objects is available as \code{ASTType} in the module. Ordered and equality comparisons are supported between AST objects. @@ -247,12 +274,12 @@ Ordered and equality comparisons are supported between AST objects. 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 as -well. Two examples are presented. The simple example demonstrates -emulation of the \code{compile()} built-in function and the complex -example shows the use of a parse tree for information discovery. +for inspection of the parse tree for information gathering purposes. +Two examples are presented. The simple example demonstrates emulation +of the \code{compile()} built-in function and the complex example +shows the use of a parse tree for information discovery. -\subsubsection{Emulation of {\tt compile()}} +\subsubsection{Emulation of \sectcode{compile()}} While many useful operations may take place between parsing and bytecode generation, the simplest operation is to do nothing. For @@ -298,17 +325,16 @@ def load_expression(source_string): \subsubsection{Information Discovery} -Some applications can benfit from access to the parse tree itself, and -can take advantage of the intermediate data structure provided by the -\code{parser} module. The remainder of this section of examples will -demonstrate how the intermediate data structure can provide access to -module documentation defined in docstrings without requiring that the -code being examined be imported into a running interpreter. This can -be very useful for performing analyses of untrusted code. +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 +\code{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 is developed which provide programmatic access to high +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 @@ -316,7 +342,7 @@ 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. +\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 @@ -324,13 +350,13 @@ 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 \code{def} statement at column zero of a module, but not a function defined -within a branch of an \code{if} ... \code{else} construct, thought +within a branch of an \code{if} ... \code{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, +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. @@ -345,7 +371,7 @@ a module consisting of a docstring and nothing else. (See file 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 the nested tuples: +buried deep in nested tuples. \begin{verbatim} >>> import parser @@ -405,12 +431,12 @@ 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 may -be taken to checking any given subtree for equivelence 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 +component of the tree, we allow a simple pattern matching approach to +check any given subtree for equivelence 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}.) @@ -434,7 +460,7 @@ def match(pattern, data, vars=None): return same, vars \end{verbatim} -Using this simple recursive pattern matching function and the symbolic +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}.) @@ -518,17 +544,17 @@ methods \code{get_name()}, \code{get_docstring()}, 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 -access in the same way, with classes having the distinction that +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, our -implementation needs to maintain the same measure of distinction. +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, \code{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 mirrors the use of the \code{def} statement to -define both types of functions. +information; this paralels the use of the \code{def} statement to +define both types of elements. Most of the accessor functions are declared in \code{SuiteInfoBase} and do not need to be overriden by subclasses. More importantly, the @@ -602,25 +628,25 @@ When the short form is used, the code block may contain a docstring as the first, and possibly only, \code{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 -given in the code, the docstring will only be found if there is only +implemented, the docstring will only be found if there is only one \code{small_stmt} node in the \code{simple_stmt} node. Since most -functions and methods which use the short form do not provide +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 \code{match()} function as described above, and the value of the docstring is stored as an attribute of the \code{SuiteInfoBase} object. -After docstring extraction, the operates a simple definition discovery -algorithm on the \code{stmt} nodes of the \code{suite} node. The +After docstring extraction, a simple definition discovery +algorithm operates on the \code{stmt} nodes of the \code{suite} node. The special case of the short form is not tested; since there are no \code{stmt} nodes in the short form, the algorithm will silently skip the single \code{simple_stmt} node and correctly not discover any nested definitions. -Each statement in the code block bing examined is categorized as being -a class definition, function definition (including methods), or +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 being defined is extracted and representation object +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 repesentation objects are stored in instance variables and may be retrieved by name using @@ -630,7 +656,7 @@ The public classes provide any accessors required which are more specific than those provided by the \code{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: +of information from a source file. (See file \file{example.py}.) \begin{verbatim} def get_docs(fileName): |