32.1. parser — 访问 Python 解析树

注解

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 module.

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 parser itself is created from a grammar specification defined in the file 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 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 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 . 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 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 .

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.

参见

模块

代表解析树内部节点的有用常量。

模块 token

代表解析树叶子节点和测试节点值的函数的有用常量。

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 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.

(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)

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 . This entry point is maintained for backward compatibility.

32.1.2. 转换 ST 对象

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, 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 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 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 module. Most causes of compilation failure can be diagnosed programmatically by inspection of the parse tree.

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

(st)

This function mirrors 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.

32.1.4. 异常和错误处理

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 raised when a failure occurs within the parser module. This is generally produced for validation failures rather than the built-in 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 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(), , and suite() may raise exceptions which are normally raised by the parsing and compilation process. These include the built in exceptions , OverflowError, , 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.

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

parser.STType

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

ST 对象具有以下方法:

ST.compile(filename=’<syntax-tree>’)

和 compilest(st, filename) 相同.

ST.isexpr()

和 isexpr(st) 相同。

ST.issuite()

和 issuite(st) 相同。

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

和 st2list(st, line_info, col_info) 相同。

(line_info=False, col_info=False)

和 st2tuple(st, line_info, col_info) 相同。

32.1.6. 示例: compile() 的模拟

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 to produce an intermediate data structure is equivalent to the code

The equivalent operation using the parser module is somewhat longer, and allows the intermediate internal parse tree to be retained as an ST object:

An application which needs both ST and code objects can package this code into readily available functions: