ast.py

"""
 ast
 ~~~

 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_astimport*


defparse(source, filename='<unknown>', mode='exec'):
"""
 Parse the source into an AST node.
 Equivalent to compile(source, filename, mode, PyCF_ONLY_AST).
 """
returncompile(source, filename, mode, PyCF_ONLY_AST)


defliteral_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, bytes, numbers, tuples, lists, dicts,
 sets, booleans, and None.
 """
ifisinstance(node_or_string, str):
node_or_string=parse(node_or_string, mode='eval')
ifisinstance(node_or_string, Expression):
node_or_string=node_or_string.body
def_convert_num(node):
ifisinstance(node, Constant):
ifisinstance(node.value, (int, float, complex)):
returnnode.value
elifisinstance(node, Num):
returnnode.n
raiseValueError('malformed node or string: '+repr(node))
def_convert_signed_num(node):
ifisinstance(node, UnaryOp) andisinstance(node.op, (UAdd, USub)):
operand=_convert_num(node.operand)
ifisinstance(node.op, UAdd):
return+operand
else:
return-operand
return_convert_num(node)
def_convert(node):
ifisinstance(node, Constant):
returnnode.value
elifisinstance(node, (Str, Bytes)):
returnnode.s
elifisinstance(node, Num):
returnnode.n
elifisinstance(node, Tuple):
returntuple(map(_convert, node.elts))
elifisinstance(node, List):
returnlist(map(_convert, node.elts))
elifisinstance(node, Set):
returnset(map(_convert, node.elts))
elifisinstance(node, Dict):
returndict(zip(map(_convert, node.keys),
map(_convert, node.values)))
elifisinstance(node, NameConstant):
returnnode.value
elifisinstance(node, BinOp) andisinstance(node.op, (Add, Sub)):
left=_convert_signed_num(node.left)
right=_convert_num(node.right)
ifisinstance(left, (int, float)) andisinstance(right, complex):
ifisinstance(node.op, Add):
returnleft+right
else:
returnleft-right
return_convert_signed_num(node)
return_convert(node_or_string)


defdump(node, annotate_fields=True, include_attributes=False):
"""
 Return a formatted dump of the tree in node. This is mainly useful for
 debugging purposes. If annotate_fields is true (by default),
 the returned string will show the names and the values for fields.
 If annotate_fields is false, the result string will be more compact by
 omitting unambiguous field names. Attributes such as line
 numbers and column offsets are not dumped by default. If this is wanted,
 include_attributes can be set to true.
 """
def_format(node):
ifisinstance(node, AST):
args= []
keywords=annotate_fields
forfieldinnode._fields:
try:
value=getattr(node, field)
exceptAttributeError:
keywords=True
else:
ifkeywords:
args.append('%s=%s'% (field, _format(value)))
else:
args.append(_format(value))
ifinclude_attributesandnode._attributes:
forainnode._attributes:
try:
args.append('%s=%s'% (a, _format(getattr(node, a))))
exceptAttributeError:
pass
return'%s(%s)'% (node.__class__.__name__, ', '.join(args))
elifisinstance(node, list):
return'[%s]'%', '.join(_format(x) forxinnode)
returnrepr(node)
ifnotisinstance(node, AST):
raiseTypeError('expected AST, got %r'%node.__class__.__name__)
return_format(node)


defcopy_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*.
 """
forattrin'lineno', 'col_offset':
ifattrinold_node._attributesandattrinnew_node._attributes \
andhasattr(old_node, attr):
setattr(new_node, attr, getattr(old_node, attr))
returnnew_node


deffix_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'innode._attributes:
ifnothasattr(node, 'lineno'):
node.lineno=lineno
else:
lineno=node.lineno
if'col_offset'innode._attributes:
ifnothasattr(node, 'col_offset'):
node.col_offset=col_offset
else:
col_offset=node.col_offset
forchildiniter_child_nodes(node):
_fix(child, lineno, col_offset)
_fix(node, 1, 0)
returnnode


defincrement_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.
 """
forchildinwalk(node):
if'lineno'inchild._attributes:
child.lineno=getattr(child, 'lineno', 0) +n
returnnode


defiter_fields(node):
"""
 Yield a tuple of ``(fieldname, value)`` for each field in ``node._fields``
 that is present on *node*.
 """
forfieldinnode._fields:
try:
yieldfield, getattr(node, field)
exceptAttributeError:
pass


defiter_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.
 """
forname, fieldiniter_fields(node):
ifisinstance(field, AST):
yieldfield
elifisinstance(field, list):
foriteminfield:
ifisinstance(item, AST):
yielditem


defget_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
 will be raised.

 If *clean* is `True`, all tabs are expanded to spaces and any whitespace
 that can be uniformly removed from the second line onwards is removed.
 """
ifnotisinstance(node, (AsyncFunctionDef, FunctionDef, ClassDef, Module)):
raiseTypeError("%r can't have docstrings"%node.__class__.__name__)
ifnot(node.bodyandisinstance(node.body[0], Expr)):
returnNone
node=node.body[0].value
ifisinstance(node, Str):
text=node.s
elifisinstance(node, Constant) andisinstance(node.value, str):
text=node.value
else:
returnNone
ifclean:
importinspect
text=inspect.cleandoc(text)
returntext


defwalk(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.
 """
fromcollectionsimportdeque
todo=deque([node])
whiletodo:
node=todo.popleft()
todo.extend(iter_child_nodes(node))
yieldnode


classNodeVisitor(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
 methods.

 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
 allows modifications.
 """

defvisit(self, node):
"""Visit a node."""
method='visit_'+node.__class__.__name__
visitor=getattr(self, method, self.generic_visit)
returnvisitor(node)

defgeneric_visit(self, node):
"""Called if no explicit visitor function exists for a node."""
forfield, valueiniter_fields(node):
ifisinstance(value, list):
foriteminvalue:
ifisinstance(item, AST):
self.visit(item)
elifisinstance(value, AST):
self.visit(value)


classNodeTransformer(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 Subscript(
 value=Name(id='data', ctx=Load()),
 slice=Index(value=Str(s=node.id)),
 ctx=node.ctx
 )

 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
 just a single node.

 Usually you use the transformer like this::

 node = YourTransformer().visit(node)
 """

defgeneric_visit(self, node):
forfield, old_valueiniter_fields(node):
ifisinstance(old_value, list):
new_values= []
forvalueinold_value:
ifisinstance(value, AST):
value=self.visit(value)
ifvalueisNone:
continue
elifnotisinstance(value, AST):
new_values.extend(value)
continue
new_values.append(value)
old_value[:] =new_values
elifisinstance(old_value, AST):
new_node=self.visit(old_value)
ifnew_nodeisNone:
delattr(node, field)
else:
setattr(node, field, new_node)
returnnode