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318 lines
12 KiB
Python
318 lines
12 KiB
Python
"""
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ast
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~~~
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The `ast` module helps Python applications to process trees of the Python
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abstract syntax grammar. The abstract syntax itself might change with
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each Python release; this module helps to find out programmatically what
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the current grammar looks like and allows modifications of it.
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An abstract syntax tree can be generated by passing `ast.PyCF_ONLY_AST` as
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a flag to the `compile()` builtin function or by using the `parse()`
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function from this module. The result will be a tree of objects whose
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classes all inherit from `ast.AST`.
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A modified abstract syntax tree can be compiled into a Python code object
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using the built-in `compile()` function.
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Additionally various helper functions are provided that make working with
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the trees simpler. The main intention of the helper functions and this
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module in general is to provide an easy to use interface for libraries
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that work tightly with the python syntax (template engines for example).
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:copyright: Copyright 2008 by Armin Ronacher.
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:license: Python License.
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"""
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from _ast import *
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from _ast import __version__
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def parse(source, filename='<unknown>', mode='exec'):
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"""
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Parse the source into an AST node.
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Equivalent to compile(source, filename, mode, PyCF_ONLY_AST).
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"""
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return compile(source, filename, mode, PyCF_ONLY_AST)
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def literal_eval(node_or_string):
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"""
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Safely evaluate an expression node or a string containing a Python
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expression. The string or node provided may only consist of the following
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Python literal structures: strings, numbers, tuples, lists, dicts, booleans,
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and None.
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"""
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_safe_names = {'None': None, 'True': True, 'False': False}
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if isinstance(node_or_string, str):
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node_or_string = parse(node_or_string, mode='eval')
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if isinstance(node_or_string, Expression):
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node_or_string = node_or_string.body
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def _convert(node):
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if isinstance(node, (Str, Bytes)):
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return node.s
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elif isinstance(node, Num):
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return node.n
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elif isinstance(node, Tuple):
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return tuple(map(_convert, node.elts))
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elif isinstance(node, List):
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return list(map(_convert, node.elts))
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elif isinstance(node, Set):
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return set(map(_convert, node.elts))
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elif isinstance(node, Dict):
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return dict((_convert(k), _convert(v)) for k, v
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in zip(node.keys, node.values))
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elif isinstance(node, Name):
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if node.id in _safe_names:
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return _safe_names[node.id]
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elif isinstance(node, UnaryOp) and \
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isinstance(node.op, (UAdd, USub)) and \
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isinstance(node.operand, (Num, UnaryOp, BinOp)):
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operand = _convert(node.operand)
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if isinstance(node.op, UAdd):
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return + operand
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else:
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return - operand
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elif isinstance(node, BinOp) and \
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isinstance(node.op, (Add, Sub)) and \
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isinstance(node.right, (Num, UnaryOp, BinOp)) and \
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isinstance(node.left, (Num, UnaryOp, BinOp)):
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left = _convert(node.left)
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right = _convert(node.right)
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if isinstance(node.op, Add):
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return left + right
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else:
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return left - right
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raise ValueError('malformed node or string: ' + repr(node))
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return _convert(node_or_string)
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def dump(node, annotate_fields=True, include_attributes=False):
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"""
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Return a formatted dump of the tree in *node*. This is mainly useful for
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debugging purposes. The returned string will show the names and the values
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for fields. This makes the code impossible to evaluate, so if evaluation is
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wanted *annotate_fields* must be set to False. Attributes such as line
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numbers and column offsets are not dumped by default. If this is wanted,
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*include_attributes* can be set to True.
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"""
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def _format(node):
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if isinstance(node, AST):
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fields = [(a, _format(b)) for a, b in iter_fields(node)]
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rv = '%s(%s' % (node.__class__.__name__, ', '.join(
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('%s=%s' % field for field in fields)
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if annotate_fields else
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(b for a, b in fields)
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))
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if include_attributes and node._attributes:
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rv += fields and ', ' or ' '
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rv += ', '.join('%s=%s' % (a, _format(getattr(node, a)))
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for a in node._attributes)
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return rv + ')'
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elif isinstance(node, list):
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return '[%s]' % ', '.join(_format(x) for x in node)
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return repr(node)
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if not isinstance(node, AST):
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raise TypeError('expected AST, got %r' % node.__class__.__name__)
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return _format(node)
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def copy_location(new_node, old_node):
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"""
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Copy source location (`lineno` and `col_offset` attributes) from
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*old_node* to *new_node* if possible, and return *new_node*.
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"""
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for attr in 'lineno', 'col_offset':
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if attr in old_node._attributes and attr in new_node._attributes \
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and hasattr(old_node, attr):
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setattr(new_node, attr, getattr(old_node, attr))
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return new_node
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def fix_missing_locations(node):
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"""
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When you compile a node tree with compile(), the compiler expects lineno and
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col_offset attributes for every node that supports them. This is rather
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tedious to fill in for generated nodes, so this helper adds these attributes
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recursively where not already set, by setting them to the values of the
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parent node. It works recursively starting at *node*.
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"""
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def _fix(node, lineno, col_offset):
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if 'lineno' in node._attributes:
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if not hasattr(node, 'lineno'):
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node.lineno = lineno
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else:
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lineno = node.lineno
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if 'col_offset' in node._attributes:
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if not hasattr(node, 'col_offset'):
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node.col_offset = col_offset
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else:
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col_offset = node.col_offset
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for child in iter_child_nodes(node):
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_fix(child, lineno, col_offset)
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_fix(node, 1, 0)
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return node
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def increment_lineno(node, n=1):
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"""
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Increment the line number of each node in the tree starting at *node* by *n*.
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This is useful to "move code" to a different location in a file.
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"""
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for child in walk(node):
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if 'lineno' in child._attributes:
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child.lineno = getattr(child, 'lineno', 0) + n
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return node
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def iter_fields(node):
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"""
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Yield a tuple of ``(fieldname, value)`` for each field in ``node._fields``
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that is present on *node*.
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"""
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for field in node._fields:
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try:
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yield field, getattr(node, field)
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except AttributeError:
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pass
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def iter_child_nodes(node):
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"""
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Yield all direct child nodes of *node*, that is, all fields that are nodes
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and all items of fields that are lists of nodes.
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"""
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for name, field in iter_fields(node):
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if isinstance(field, AST):
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yield field
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elif isinstance(field, list):
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for item in field:
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if isinstance(item, AST):
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yield item
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def get_docstring(node, clean=True):
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"""
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Return the docstring for the given node or None if no docstring can
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be found. If the node provided does not have docstrings a TypeError
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will be raised.
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"""
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if not isinstance(node, (FunctionDef, ClassDef, Module)):
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raise TypeError("%r can't have docstrings" % node.__class__.__name__)
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if node.body and isinstance(node.body[0], Expr) and \
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isinstance(node.body[0].value, Str):
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if clean:
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import inspect
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return inspect.cleandoc(node.body[0].value.s)
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return node.body[0].value.s
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def walk(node):
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"""
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Recursively yield all descendant nodes in the tree starting at *node*
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(including *node* itself), in no specified order. This is useful if you
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only want to modify nodes in place and don't care about the context.
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"""
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from collections import deque
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todo = deque([node])
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while todo:
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node = todo.popleft()
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todo.extend(iter_child_nodes(node))
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yield node
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class NodeVisitor(object):
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"""
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A node visitor base class that walks the abstract syntax tree and calls a
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visitor function for every node found. This function may return a value
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which is forwarded by the `visit` method.
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This class is meant to be subclassed, with the subclass adding visitor
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methods.
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Per default the visitor functions for the nodes are ``'visit_'`` +
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class name of the node. So a `TryFinally` node visit function would
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be `visit_TryFinally`. This behavior can be changed by overriding
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the `visit` method. If no visitor function exists for a node
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(return value `None`) the `generic_visit` visitor is used instead.
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Don't use the `NodeVisitor` if you want to apply changes to nodes during
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traversing. For this a special visitor exists (`NodeTransformer`) that
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allows modifications.
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"""
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def visit(self, node):
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"""Visit a node."""
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method = 'visit_' + node.__class__.__name__
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visitor = getattr(self, method, self.generic_visit)
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return visitor(node)
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def generic_visit(self, node):
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"""Called if no explicit visitor function exists for a node."""
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for field, value in iter_fields(node):
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if isinstance(value, list):
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for item in value:
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if isinstance(item, AST):
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self.visit(item)
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elif isinstance(value, AST):
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self.visit(value)
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class NodeTransformer(NodeVisitor):
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"""
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A :class:`NodeVisitor` subclass that walks the abstract syntax tree and
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allows modification of nodes.
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The `NodeTransformer` will walk the AST and use the return value of the
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visitor methods to replace or remove the old node. If the return value of
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the visitor method is ``None``, the node will be removed from its location,
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otherwise it is replaced with the return value. The return value may be the
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original node in which case no replacement takes place.
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Here is an example transformer that rewrites all occurrences of name lookups
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(``foo``) to ``data['foo']``::
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class RewriteName(NodeTransformer):
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def visit_Name(self, node):
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return copy_location(Subscript(
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value=Name(id='data', ctx=Load()),
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slice=Index(value=Str(s=node.id)),
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ctx=node.ctx
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), node)
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Keep in mind that if the node you're operating on has child nodes you must
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either transform the child nodes yourself or call the :meth:`generic_visit`
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method for the node first.
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For nodes that were part of a collection of statements (that applies to all
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statement nodes), the visitor may also return a list of nodes rather than
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just a single node.
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Usually you use the transformer like this::
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node = YourTransformer().visit(node)
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"""
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def generic_visit(self, node):
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for field, old_value in iter_fields(node):
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old_value = getattr(node, field, None)
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if isinstance(old_value, list):
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new_values = []
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for value in old_value:
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if isinstance(value, AST):
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value = self.visit(value)
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if value is None:
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continue
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elif not isinstance(value, AST):
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new_values.extend(value)
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continue
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new_values.append(value)
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old_value[:] = new_values
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elif isinstance(old_value, AST):
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new_node = self.visit(old_value)
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if new_node is None:
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delattr(node, field)
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else:
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setattr(node, field, new_node)
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return node
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