parser — Access Python parse trees

The 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 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 Python Language Reference. The parser itself is created from a grammar specification defined in the 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 expr() or suite() functions, described below. The ST objects created by 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 st2list() or 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 Include/graminit.h and the Python module 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 if in an if_stmt, are included in the node tree without any special treatment. For example, the if keyword is represented by the tuple (1, 'if'), where 1 is the numeric value associated with all 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 if keyword above is representative. The various types of terminal symbols are defined in the C header file Include/token.h and the Python module 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 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.

1. 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.

parser.expr(source)

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

parser.suite(source)

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

parser.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 ParserError exception is raised. An ST object created this way should not be assumed to compile correctly; normal exceptions raised by compilation may still be initiated when the ST object is passed to compilest(). This may indicate problems not related to syntax (such as a 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.

parser.tuple2st(sequence)

This is the same function as sequence2st(). This entry point is maintained for backward compatibility.

2. 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.

parser.st2list(st, line_info=False, col_info=False)

This function accepts an ST object from the caller in st 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, 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.

parser.st2tuple(st, line_info=False, col_info=False)

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

parser.compilest(st, filename='<syntax-tree>')

The Python byte compiler can be invoked on an ST object to produce code objects which can be used as part of a call to the built-in exec() or eval() functions. This function provides the interface to the compiler, passing the internal parse tree from st 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 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 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 parser module. Most causes of compilation failure can be diagnosed programmatically by inspection of the parse tree.

3. 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 expr() or suite() or from a parse tree via sequence2st().

parser.isexpr(st)

When st 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 compilest() cannot be queried like this either, and are identical to those created by the built-in compile() function.

parser.issuite(st)

This function mirrors 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(st), as additional syntactic fragments may be supported in the future.

4. 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 parser.ParserError

Exception raised when a failure occurs within the parser module. This is generally produced for validation failures rather than the built-in SyntaxError raised during normal parsing. The exception argument is either a string describing the reason of the failure or a tuple containing a sequence causing the failure from a parse tree passed to sequence2st() and an explanatory string. Calls to 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 compilest(), expr(), and suite() may raise exceptions which are normally raised by the parsing and compilation process. These include the built in exceptions MemoryError, OverflowError, SyntaxError, and SystemError. In these cases, these exceptions carry all the meaning normally associated with them. Refer to the descriptions of each function for detailed information.

5. ST Objects

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

parser.STType

The type of the objects returned by expr(), suite() and sequence2st().

ST objects have the following methods:

ST.compile(filename='<syntax-tree>')

Same as compilest(st, filename).

ST.isexpr()

Same as isexpr(st).

ST.issuite()

Same as issuite(st).

ST.tolist(line_info=False, col_info=False)

Same as st2list(st, line_info, col_info).

ST.totuple(line_info=False, col_info=False)

Same as st2tuple(st, line_info, col_info).

6. Example: Emulation of 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 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 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()