31.2. Abstract Syntax Trees¶
New in version 2.5:
New in version 2.5: The low-level _ast module containing only the node classes.
New in version 2.6:
New in version 2.6: The high-level ast module containing all helpers.
New in version 2.5:
New in version 2.5: The low-level _ast module containing only the node classes.
New in version 2.6:
New in version 2.6: The high-level ast module containing all helpers.
The ast module helps Python applications to process trees of the Python
abstract syntax grammar. The abstract syntax itself might change with each
Python release; this module helps to find out programmatically what the current
grammar looks like.
An abstract syntax tree can be generated by passing 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() function.
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 inherits from 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 of type 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¶Instances of ast.expr and ast.stmt subclasses have
lineno and col_offset attributes. The 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:
T._fields; they will be assigned as attributes of these names.For example, to create and populate an ast.UnaryOp node, you could
use
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)
New in version 2.6:
New in version 2.6: 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)
| ListComp(expr elt, 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)
}
Apart from the node classes, ast module defines these utility functions
and classes for traversing abstract syntax trees:
ast.parse(expr, filename='<unknown>', mode='exec')¶Parse an expression into an AST node. Equivalent to compile(expr,
filename, mode, ast.PyCF_ONLY_AST).
ast.literal_eval(node_or_string)¶Safely evaluate an expression node or a string containing a Python
expression. 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 expressions from untrusted sources without the need to parse the values oneself.
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 child nodes of node, 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() method.
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.
Don’t use the 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) to 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.