ast
— 抽象句法树
¶
New in version 2.5:
The low-level
_ast
module containing only the node classes.
2.6 版新增：
The high-level
ast
module containing all helpers.
源代码： Lib/ast.py
The
ast
模块帮助 Python 应用程序处理 Python 抽象句法语法树。抽象句法本身可以随着每 Python 发行而改变；此模块以编程方式帮助找出当前语法看起来像什么。
可以生成抽象句法树通过传递
ast.PyCF_ONLY_AST
as a flag to the
compile()
built-in function, or using the
parse()
helper provided in this module. The result will be a tree of objects whose classes all inherit from
ast.AST
. An abstract syntax tree can be compiled into a Python code object using the built-in
compile()
函数。
ast.
AST
¶
This is the base of all AST node classes. The actual node classes are derived from the
Parser/Python.asdl
file, which is reproduced
below
. They are defined in the
_ast
C module and re-exported in
ast
.
There is one class defined for each left-hand side symbol in the abstract grammar (for example,
ast.stmt
or
ast.expr
). In addition, there is one class defined for each constructor on the right-hand side; these classes inherit from the classes for the left-hand side trees. For example,
ast.BinOp
继承自
ast.expr
. For production rules with alternatives (aka “sums”), the left-hand side class is abstract: only instances of specific constructor nodes are ever created.
_fields
¶
Each concrete class has an attribute
_fields
which gives the names of all child nodes.
Each instance of a concrete class has one attribute for each child node, of the type as defined in the grammar. For example,
ast.BinOp
instances have an attribute
left
类型
ast.expr
.
If these attributes are marked as optional in the grammar (using a question mark), the value might be
None
. If the attributes can have zero-or-more values (marked with an asterisk), the values are represented as Python lists. All possible attributes must be present and have valid values when compiling an AST with
compile()
.
lineno
¶
col_offset
¶
实例化的
ast.expr
and
ast.stmt
subclasses have
lineno
and
col_offset
属性。
lineno
is the line number of source text (1-indexed so the first line is line 1) and the
col_offset
is the UTF-8 byte offset of the first token that generated the node. The UTF-8 offset is recorded because the parser uses UTF-8 internally.
The constructor of a class
ast.T
parses its arguments as follows:
If there are positional arguments, there must be as many as there are items in
T._fields
; they will be assigned as attributes of these names.
If there are keyword arguments, they will set the attributes of the same names to the given values.
例如，要创建和填充
ast.UnaryOp
节点，可以使用
node = ast.UnaryOp() node.op = ast.USub() node.operand = ast.Num() node.operand.n = 5 node.operand.lineno = 0 node.operand.col_offset = 0 node.lineno = 0 node.col_offset = 0
or the more compact
node = ast.UnaryOp(ast.USub(), ast.Num(5, lineno=0, col_offset=0), lineno=0, col_offset=0)
2.6 版新增： The constructor as explained above was added. In Python 2.5 nodes had to be created by calling the class constructor without arguments and setting the attributes afterwards.
The module defines a string constant
__version__
which is the decimal Subversion revision number of the file shown below.
The abstract grammar is currently defined as follows:
-- ASDL's five builtin types are identifier, int, string, object, bool
module Python version "$Revision$"
{
mod = Module(stmt* body)
| Interactive(stmt* body)
| Expression(expr body)
-- not really an actual node but useful in Jython's typesystem.
| Suite(stmt* body)
stmt = FunctionDef(identifier name, arguments args,
stmt* body, expr* decorator_list)
| ClassDef(identifier name, expr* bases, stmt* body, expr* decorator_list)
| Return(expr? value)
| Delete(expr* targets)
| Assign(expr* targets, expr value)
| AugAssign(expr target, operator op, expr value)
-- not sure if bool is allowed, can always use int
| Print(expr? dest, expr* values, bool nl)
-- use 'orelse' because else is a keyword in target languages
| For(expr target, expr iter, stmt* body, stmt* orelse)
| While(expr test, stmt* body, stmt* orelse)
| If(expr test, stmt* body, stmt* orelse)
| With(expr context_expr, expr? optional_vars, stmt* body)
-- 'type' is a bad name
| Raise(expr? type, expr? inst, expr? tback)
| TryExcept(stmt* body, excepthandler* handlers, stmt* orelse)
| TryFinally(stmt* body, stmt* finalbody)
| Assert(expr test, expr? msg)
| Import(alias* names)
| ImportFrom(identifier? module, alias* names, int? level)
-- Doesn't capture requirement that locals must be
-- defined if globals is
-- still supports use as a function!
| Exec(expr body, expr? globals, expr? locals)
| Global(identifier* names)
| Expr(expr value)
| Pass | Break | Continue
-- XXX Jython will be different
-- col_offset is the byte offset in the utf8 string the parser uses
attributes (int lineno, int col_offset)
-- BoolOp() can use left & right?
expr = BoolOp(boolop op, expr* values)
| BinOp(expr left, operator op, expr right)
| UnaryOp(unaryop op, expr operand)
| Lambda(arguments args, expr body)
| IfExp(expr test, expr body, expr orelse)
| Dict(expr* keys, expr* values)
| Set(expr* elts)
| ListComp(expr elt, comprehension* generators)
| SetComp(expr elt, comprehension* generators)
| DictComp(expr key, expr value, comprehension* generators)
| GeneratorExp(expr elt, comprehension* generators)
-- the grammar constrains where yield expressions can occur
| Yield(expr? value)
-- need sequences for compare to distinguish between
-- x < 4 < 3 and (x < 4) < 3
| Compare(expr left, cmpop* ops, expr* comparators)
| Call(expr func, expr* args, keyword* keywords,
expr? starargs, expr? kwargs)
| Repr(expr value)
| Num(object n) -- a number as a PyObject.
| Str(string s) -- need to specify raw, unicode, etc?
-- other literals? bools?
-- the following expression can appear in assignment context
| Attribute(expr value, identifier attr, expr_context ctx)
| Subscript(expr value, slice slice, expr_context ctx)
| Name(identifier id, expr_context ctx)
| List(expr* elts, expr_context ctx)
| Tuple(expr* elts, expr_context ctx)
-- col_offset is the byte offset in the utf8 string the parser uses
attributes (int lineno, int col_offset)
expr_context = Load | Store | Del | AugLoad | AugStore | Param
slice = Ellipsis | Slice(expr? lower, expr? upper, expr? step)
| ExtSlice(slice* dims)
| Index(expr value)
boolop = And | Or
operator = Add | Sub | Mult | Div | Mod | Pow | LShift
| RShift | BitOr | BitXor | BitAnd | FloorDiv
unaryop = Invert | Not | UAdd | USub
cmpop = Eq | NotEq | Lt | LtE | Gt | GtE | Is | IsNot | In | NotIn
comprehension = (expr target, expr iter, expr* ifs)
-- not sure what to call the first argument for raise and except
excepthandler = ExceptHandler(expr? type, expr? name, stmt* body)
attributes (int lineno, int col_offset)
arguments = (expr* args, identifier? vararg,
identifier? kwarg, expr* defaults)
-- keyword arguments supplied to call
keyword = (identifier arg, expr value)
-- import name with optional 'as' alias.
alias = (identifier name, identifier? asname)
}
ast
帮手
¶
2.6 版新增。
Apart from the node classes,
ast
module defines these utility functions and classes for traversing abstract syntax trees:
ast.
parse
(
source
,
filename='<unknown>'
,
mode='exec'
)
¶
Parse the source into an AST node. Equivalent to
compile(source,
filename, mode, ast.PyCF_ONLY_AST)
.
警告
It is possible to crash the Python interpreter with a sufficiently large/complex string due to stack depth limitations in Python’s AST compiler.
ast.
literal_eval
(
node_or_string
)
¶
Safely evaluate an expression node or a Unicode or
Latin-1
encoded string containing a Python literal or container display. The string or node provided may only consist of the following Python literal structures: strings, numbers, tuples, lists, dicts, booleans, and
None
.
This can be used for safely evaluating strings containing Python values from untrusted sources without the need to parse the values oneself. It is not capable of evaluating arbitrarily complex expressions, for example involving operators or indexing.
警告
It is possible to crash the Python interpreter with a sufficiently large/complex string due to stack depth limitations in Python’s AST compiler.
ast.
get_docstring
(
node
,
clean=True
)
¶
Return the docstring of the given
node
(which must be a
FunctionDef
,
ClassDef
or
Module
node), or
None
if it has no docstring. If
clean
is true, clean up the docstring’s indentation with
inspect.cleandoc()
.
ast.
fix_missing_locations
(
node
)
¶
When you compile a node tree with
compile()
, the compiler expects
lineno
and
col_offset
attributes for every node that supports them. This is rather tedious to fill in for generated nodes, so this helper adds these attributes recursively where not already set, by setting them to the values of the parent node. It works recursively starting at
node
.
ast.
increment_lineno
(
node
,
n=1
)
¶
Increment the line number of each node in the tree starting at node by n . This is useful to “move code” to a different location in a file.
ast.
copy_location
(
new_node
,
old_node
)
¶
Copy source location (
lineno
and
col_offset
) from
old_node
to
new_node
if possible, and return
new_node
.
ast.
iter_fields
(
node
)
¶
Yield a tuple of
(fieldname, value)
for each field in
node._fields
that is present on
node
.
ast.
iter_child_nodes
(
node
)
¶
Yield all direct child nodes of node , that is, all fields that are nodes and all items of fields that are lists of nodes.
ast.
walk
(
node
)
¶
Recursively yield all descendant nodes in the tree starting at node (including node itself), in no specified order. This is useful if you only want to modify nodes in place and don’t care about the context.
ast.
NodeVisitor
¶
A node visitor base class that walks the abstract syntax tree and calls a visitor function for every node found. This function may return a value which is forwarded by the
visit()
方法。
This class is meant to be subclassed, with the subclass adding visitor methods.
visit
(
node
)
¶
Visit a node. The default implementation calls the method called
self.visit_classname
where
classname
is the name of the node class, or
generic_visit()
if that method doesn’t exist.
generic_visit
(
node
)
¶
This visitor calls
visit()
on all children of the node.
Note that child nodes of nodes that have a custom visitor method won’t be visited unless the visitor calls
generic_visit()
or visits them itself.
不使用
NodeVisitor
if you want to apply changes to nodes during traversal. For this a special visitor exists (
NodeTransformer
) that allows modifications.
ast.
NodeTransformer
¶
A
NodeVisitor
subclass that walks the abstract syntax tree and allows modification of nodes.
The
NodeTransformer
will walk the AST and use the return value of the visitor methods to replace or remove the old node. If the return value of the visitor method is
None
, the node will be removed from its location, otherwise it is replaced with the return value. The return value may be the original node in which case no replacement takes place.
Here is an example transformer that rewrites all occurrences of name lookups (
foo
) 到
data['foo']
:
class RewriteName(NodeTransformer): def visit_Name(self, node): return copy_location(Subscript( value=Name(id='data', ctx=Load()), slice=Index(value=Str(s=node.id)), ctx=node.ctx ), node)
Keep in mind that if the node you’re operating on has child nodes you must either transform the child nodes yourself or call the
generic_visit()
method for the node first.
For nodes that were part of a collection of statements (that applies to all statement nodes), the visitor may also return a list of nodes rather than just a single node.
Usually you use the transformer like this:
node = YourTransformer().visit(node)
ast.
dump
(
node
,
annotate_fields=True
,
include_attributes=False
)
¶
Return a formatted dump of the tree in
node
. This is mainly useful for debugging purposes. The returned string will show the names and the values for fields. This makes the code impossible to evaluate, so if evaluation is wanted
annotate_fields
must be set to
False
. Attributes such as line numbers and column offsets are not dumped by default. If this is wanted,
include_attributes
can be set to
True
.