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#!/usr/bin/env python
# -*- coding: utf-8 -*-
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals
__all__ = ['singledispatch']
from functools import update_wrapper
from weakref import WeakKeyDictionary
from singledispatch_helpers import MappingProxyType, get_cache_token
################################################################################
### singledispatch() - single-dispatch generic function decorator
################################################################################
def _compose_mro(cls, haystack):
"""Calculates the MRO for a given class `cls`, including relevant abstract
base classes from `haystack`.
"""
bases = set(cls.__mro__)
mro = list(cls.__mro__)
for needle in haystack:
if (needle in bases or not hasattr(needle, '__mro__')
or not issubclass(cls, needle)):
continue # either present in the __mro__ already or unrelated
for index, base in enumerate(mro):
if not issubclass(base, needle):
break
if base in bases and not issubclass(needle, base):
# Conflict resolution: put classes present in __mro__ and their
# subclasses first. See test_mro_conflicts() in test_functools.py
# for examples.
index += 1
mro.insert(index, needle)
return mro
def _find_impl(cls, registry):
"""Returns the best matching implementation for the given class `cls` in
`registry`. Where there is no registered implementation for a specific
type, its method resolution order is used to find a more generic
implementation.
Note: if `registry` does not contain an implementation for the base
`object` type, this function may return None.
"""
mro = _compose_mro(cls, registry.keys())
match = None
for t in mro:
if match is not None:
# If `match` is an ABC but there is another unrelated, equally
# matching ABC. Refuse the temptation to guess.
if (t in registry and not issubclass(match, t)
and match not in cls.__mro__):
raise RuntimeError("Ambiguous dispatch: {0} or {1}".format(
match, t))
break
if t in registry:
match = t
return registry.get(match)
def singledispatch(func):
"""Single-dispatch generic function decorator.
Transforms a function into a generic function, which can have different
behaviours depending upon the type of its first argument. The decorated
function acts as the default implementation, and additional
implementations can be registered using the 'register()' attribute of
the generic function.
"""
registry = {}
dispatch_cache = WeakKeyDictionary()
def ns(): pass
ns.cache_token = None
def dispatch(typ):
"""generic_func.dispatch(type) -> <function implementation>
Runs the dispatch algorithm to return the best available implementation
for the given `type` registered on `generic_func`.
"""
if ns.cache_token is not None:
current_token = get_cache_token()
if ns.cache_token != current_token:
dispatch_cache.clear()
ns.cache_token = current_token
try:
impl = dispatch_cache[typ]
except KeyError:
try:
impl = registry[typ]
except KeyError:
impl = _find_impl(typ, registry)
dispatch_cache[typ] = impl
return impl
def register(typ, func=None):
"""generic_func.register(type, func) -> func
Registers a new implementation for the given `type` on a `generic_func`.
"""
if func is None:
return lambda f: register(typ, f)
registry[typ] = func
if ns.cache_token is None and hasattr(typ, '__abstractmethods__'):
ns.cache_token = get_cache_token()
dispatch_cache.clear()
return func
def wrapper(*args, **kw):
return dispatch(args[0].__class__)(*args, **kw)
registry[object] = func
wrapper.register = register
wrapper.dispatch = dispatch
wrapper.registry = MappingProxyType(registry)
wrapper._clear_cache = dispatch_cache.clear
update_wrapper(wrapper, func)
return wrapper
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