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class ClauseVisitor(object):
"""Traverses and visits ``ClauseElement`` structures.
Calls visit_XXX() methods dynamically generated for each particular
``ClauseElement`` subclass encountered. Traversal of a
hierarchy of ``ClauseElements`` is achieved via the
``traverse()`` method, which is passed the lead
``ClauseElement``.
By default, ``ClauseVisitor`` traverses all elements
fully. Options can be specified at the class level via the
``__traverse_options__`` dictionary which will be passed
to the ``get_children()`` method of each ``ClauseElement``;
these options can indicate modifications to the set of
elements returned, such as to not return column collections
(column_collections=False) or to return Schema-level items
(schema_visitor=True).
``ClauseVisitor`` also supports a simultaneous copy-and-traverse
operation, which will produce a copy of a given ``ClauseElement``
structure while at the same time allowing ``ClauseVisitor`` subclasses
to modify the new structure in-place.
"""
__traverse_options__ = {}
def traverse_single(self, obj, **kwargs):
meth = getattr(self, "visit_%s" % obj.__visit_name__, None)
if meth:
return meth(obj, **kwargs)
def traverse_chained(self, obj, **kwargs):
v = self
while v is not None:
meth = getattr(self, "visit_%s" % obj.__visit_name__, None)
if meth:
meth(obj, **kwargs)
v = getattr(v, '_next', None)
def iterate(self, obj):
stack = [obj]
traversal = []
while len(stack) > 0:
t = stack.pop()
yield t
traversal.insert(0, t)
for c in t.get_children(**self.__traverse_options__):
stack.append(c)
def traverse(self, obj, clone=False):
if clone:
cloned = {}
def do_clone(obj):
# the full traversal will only make a clone of a particular element
# once.
if obj not in cloned:
cloned[obj] = obj._clone()
return cloned[obj]
obj = do_clone(obj)
stack = [obj]
traversal = []
while len(stack) > 0:
t = stack.pop()
traversal.insert(0, t)
if clone:
t._copy_internals(clone=do_clone)
for c in t.get_children(**self.__traverse_options__):
stack.append(c)
for target in traversal:
v = self
while v is not None:
meth = getattr(v, "visit_%s" % target.__visit_name__, None)
if meth:
meth(target)
v = getattr(v, '_next', None)
return obj
def chain(self, visitor):
"""'chain' an additional ClauseVisitor onto this ClauseVisitor.
the chained visitor will receive all visit events after this one."""
tail = self
while getattr(tail, '_next', None) is not None:
tail = tail._next
tail._next = visitor
return self
class NoColumnVisitor(ClauseVisitor):
"""ClauseVisitor with 'column_collections' set to False; will not
traverse the front-facing Column collections on Table, Alias, Select,
and CompoundSelect objects.
"""
__traverse_options__ = {'column_collections':False}
def traverse(clause, **kwargs):
clone = kwargs.pop('clone', False)
class Vis(ClauseVisitor):
__traverse_options__ = kwargs.pop('traverse_options', {})
def __getattr__(self, key):
if key in kwargs:
return kwargs[key]
else:
return None
return Vis().traverse(clause, clone=clone)
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