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"""Generic visitor pattern implementation for Python objects."""
import enum
class Visitor(object):
defaultStop = False
@classmethod
def _register(celf, clazzes_attrs):
assert celf != Visitor, "Subclass Visitor instead."
if "_visitors" not in celf.__dict__:
celf._visitors = {}
def wrapper(method):
assert method.__name__ == "visit"
for clazzes, attrs in clazzes_attrs:
if type(clazzes) != tuple:
clazzes = (clazzes,)
if type(attrs) == str:
attrs = (attrs,)
for clazz in clazzes:
_visitors = celf._visitors.setdefault(clazz, {})
for attr in attrs:
assert attr not in _visitors, (
"Oops, class '%s' has visitor function for '%s' defined already."
% (clazz.__name__, attr)
)
_visitors[attr] = method
return None
return wrapper
@classmethod
def register(celf, clazzes):
if type(clazzes) != tuple:
clazzes = (clazzes,)
return celf._register([(clazzes, (None,))])
@classmethod
def register_attr(celf, clazzes, attrs):
clazzes_attrs = []
if type(clazzes) != tuple:
clazzes = (clazzes,)
if type(attrs) == str:
attrs = (attrs,)
for clazz in clazzes:
clazzes_attrs.append((clazz, attrs))
return celf._register(clazzes_attrs)
@classmethod
def register_attrs(celf, clazzes_attrs):
return celf._register(clazzes_attrs)
@classmethod
def _visitorsFor(celf, thing, _default={}):
typ = type(thing)
for celf in celf.mro():
_visitors = getattr(celf, "_visitors", None)
if _visitors is None:
break
m = celf._visitors.get(typ, None)
if m is not None:
return m
return _default
def visitObject(self, obj, *args, **kwargs):
"""Called to visit an object. This function loops over all non-private
attributes of the objects and calls any user-registered (via
@register_attr() or @register_attrs()) visit() functions.
If there is no user-registered visit function, of if there is and it
returns True, or it returns None (or doesn't return anything) and
visitor.defaultStop is False (default), then the visitor will proceed
to call self.visitAttr()"""
keys = sorted(vars(obj).keys())
_visitors = self._visitorsFor(obj)
defaultVisitor = _visitors.get("*", None)
for key in keys:
if key[0] == "_":
continue
value = getattr(obj, key)
visitorFunc = _visitors.get(key, defaultVisitor)
if visitorFunc is not None:
ret = visitorFunc(self, obj, key, value, *args, **kwargs)
if ret == False or (ret is None and self.defaultStop):
continue
self.visitAttr(obj, key, value, *args, **kwargs)
def visitAttr(self, obj, attr, value, *args, **kwargs):
"""Called to visit an attribute of an object."""
self.visit(value, *args, **kwargs)
def visitList(self, obj, *args, **kwargs):
"""Called to visit any value that is a list."""
for value in obj:
self.visit(value, *args, **kwargs)
def visitDict(self, obj, *args, **kwargs):
"""Called to visit any value that is a dictionary."""
for value in obj.values():
self.visit(value, *args, **kwargs)
def visitLeaf(self, obj, *args, **kwargs):
"""Called to visit any value that is not an object, list,
or dictionary."""
pass
def visit(self, obj, *args, **kwargs):
"""This is the main entry to the visitor. The visitor will visit object
obj.
The visitor will first determine if there is a registered (via
@register()) visit function for the type of object. If there is, it
will be called, and (visitor, obj, *args, **kwargs) will be passed to
the user visit function.
If there is no user-registered visit function, of if there is and it
returns True, or it returns None (or doesn't return anything) and
visitor.defaultStop is False (default), then the visitor will proceed
to dispatch to one of self.visitObject(), self.visitList(),
self.visitDict(), or self.visitLeaf() (any of which can be overriden in
a subclass)."""
visitorFunc = self._visitorsFor(obj).get(None, None)
if visitorFunc is not None:
ret = visitorFunc(self, obj, *args, **kwargs)
if ret == False or (ret is None and self.defaultStop):
return
if hasattr(obj, "__dict__") and not isinstance(obj, enum.Enum):
self.visitObject(obj, *args, **kwargs)
elif isinstance(obj, list):
self.visitList(obj, *args, **kwargs)
elif isinstance(obj, dict):
self.visitDict(obj, *args, **kwargs)
else:
self.visitLeaf(obj, *args, **kwargs)
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