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 and allows modifications of it.
An abstract syntax tree can be generated by passing `ast.PyCF_ONLY_AST` as
a flag to the `compile()` builtin function or by using the `parse()`
function from this module. The result will be a tree of objects whose
classes all inherit from `ast.AST`.
A modified abstract syntax tree can be compiled into a Python code object
using the built-in `compile()` function.
Additionally various helper functions are provided that make working with
the trees simpler. The main intention of the helper functions and this
module in general is to provide an easy to use interface for libraries
that work tightly with the python syntax (template engines for example).
:copyright: Copyright 2008 by Armin Ronacher.
:license: Python License.
from _ast import __version__
def parse(source, filename='<unknown>', mode='exec'):
Parse the source into an AST node.
Equivalent to compile(source, filename, mode, PyCF_ONLY_AST).
return compile(source, filename, mode, PyCF_ONLY_AST)
def 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,
_safe_names = {'None': None, 'True': True, 'False': False}
if isinstance(node_or_string, basestring):
node_or_string = parse(node_or_string, mode='eval')
if isinstance(node_or_string, Expression):
node_or_string = node_or_string.body
if isinstance(node, Str):
elif isinstance(node, Num):
elif isinstance(node, Tuple):
return tuple(map(_convert, node.elts))
elif isinstance(node, List):
return list(map(_convert, node.elts))
elif isinstance(node, Dict):
return dict((_convert(k), _convert(v)) for k, v
in zip(node.keys, node.values))
elif isinstance(node, Name):
if node.id in _safe_names:
return _safe_names[node.id]
elif isinstance(node, BinOp) and \
isinstance(node.op, (Add, Sub)) and \
isinstance(node.right, Num) and \
isinstance(node.right.n, complex) and \
isinstance(node.left, Num) and \
isinstance(node.left.n, (int, long, float)):
if isinstance(node.op, Add):
raise ValueError('malformed string')
return _convert(node_or_string)
def 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.
if isinstance(node, AST):
fields = [(a, _format(b)) for a, b in iter_fields(node)]
rv = '%s(%s' % (node.__class__.__name__, ', '.join(
('%s=%s' % field for field in fields)
if include_attributes and node._attributes:
rv += fields and ', ' or ' '
rv += ', '.join('%s=%s' % (a, _format(getattr(node, a)))
for a in node._attributes)
elif isinstance(node, list):
return '[%s]' % ', '.join(_format(x) for x in node)
if not isinstance(node, AST):
raise TypeError('expected AST, got %r' % node.__class__.__name__)
def copy_location(new_node, old_node):
Copy source location (`lineno` and `col_offset` attributes) from
*old_node* to *new_node* if possible, and return *new_node*.
for attr in 'lineno', 'col_offset':
if attr in old_node._attributes and attr in new_node._attributes \
and hasattr(old_node, attr):
setattr(new_node, attr, getattr(old_node, attr))
def 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*.
def _fix(node, lineno, col_offset):
if 'lineno' in node._attributes:
if not hasattr(node, 'lineno'):
if 'col_offset' in node._attributes:
if not hasattr(node, 'col_offset'):
node.col_offset = col_offset
col_offset = node.col_offset
for child in iter_child_nodes(node):
_fix(child, lineno, col_offset)
def 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.
if 'lineno' in child._attributes:
child.lineno = getattr(child, 'lineno', 0) + n
Yield a tuple of ``(fieldname, value)`` for each field in ``node._fields``
that is present on *node*.
for field in node._fields:
yield field, getattr(node, field)
def 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.
for name, field in iter_fields(node):
if isinstance(field, AST):
elif isinstance(field, list):
if isinstance(item, AST):
def get_docstring(node, clean=True):
Return the docstring for the given node or None if no docstring can
be found. If the node provided does not have docstrings a TypeError
if not isinstance(node, (FunctionDef, ClassDef, Module)):
raise TypeError("%r can't have docstrings" % node.__class__.__name__)
if node.body and isinstance(node.body[0], Expr) and \
isinstance(node.body[0].value, Str):
return inspect.cleandoc(node.body[0].value.s)
return node.body[0].value.s
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.
from collections import deque
todo.extend(iter_child_nodes(node))
class NodeVisitor(object):
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
Per default the visitor functions for the nodes are ``'visit_'`` +
class name of the node. So a `TryFinally` node visit function would
be `visit_TryFinally`. This behavior can be changed by overriding
the `visit` method. If no visitor function exists for a node
(return value `None`) the `generic_visit` visitor is used instead.
Don't use the `NodeVisitor` if you want to apply changes to nodes during
traversing. For this a special visitor exists (`NodeTransformer`) that
method = 'visit_' + node.__class__.__name__
visitor = getattr(self, method, self.generic_visit)
def generic_visit(self, node):
"""Called if no explicit visitor function exists for a node."""
for field, value in iter_fields(node):
if isinstance(value, list):
if isinstance(item, AST):
elif isinstance(value, AST):
class NodeTransformer(NodeVisitor):
A :class:`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)),
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 :meth:`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
Usually you use the transformer like this::
node = YourTransformer().visit(node)
def generic_visit(self, node):
for field, old_value in iter_fields(node):
old_value = getattr(node, field, None)
if isinstance(old_value, list):
if isinstance(value, AST):
value = self.visit(value)
elif not isinstance(value, AST):
old_value[:] = new_values
elif isinstance(old_value, AST):
new_node = self.visit(old_value)
setattr(node, field, new_node)
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