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220 lines
8.1 KiB
Python
220 lines
8.1 KiB
Python
#!/usr/bin/env python
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# -*- coding: utf-8 -*-
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from __future__ import absolute_import
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from __future__ import division
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from __future__ import print_function
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from __future__ import unicode_literals
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__all__ = ['singledispatch']
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from functools import update_wrapper
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from weakref import WeakKeyDictionary
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from singledispatch_helpers import MappingProxyType, get_cache_token
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################################################################################
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### singledispatch() - single-dispatch generic function decorator
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################################################################################
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def _c3_merge(sequences):
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"""Merges MROs in *sequences* to a single MRO using the C3 algorithm.
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Adapted from http://www.python.org/download/releases/2.3/mro/.
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"""
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result = []
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while True:
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sequences = [s for s in sequences if s] # purge empty sequences
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if not sequences:
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return result
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for s1 in sequences: # find merge candidates among seq heads
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candidate = s1[0]
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for s2 in sequences:
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if candidate in s2[1:]:
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candidate = None
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break # reject the current head, it appears later
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else:
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break
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if not candidate:
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raise RuntimeError("Inconsistent hierarchy")
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result.append(candidate)
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# remove the chosen candidate
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for seq in sequences:
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if seq[0] == candidate:
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del seq[0]
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def _c3_mro(cls, abcs=None):
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"""Computes the method resolution order using extended C3 linearization.
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If no *abcs* are given, the algorithm works exactly like the built-in C3
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linearization used for method resolution.
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If given, *abcs* is a list of abstract base classes that should be inserted
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into the resulting MRO. Unrelated ABCs are ignored and don't end up in the
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result. The algorithm inserts ABCs where their functionality is introduced,
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i.e. issubclass(cls, abc) returns True for the class itself but returns
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False for all its direct base classes. Implicit ABCs for a given class
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(either registered or inferred from the presence of a special method like
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__len__) are inserted directly after the last ABC explicitly listed in the
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MRO of said class. If two implicit ABCs end up next to each other in the
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resulting MRO, their ordering depends on the order of types in *abcs*.
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"""
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for i, base in enumerate(reversed(cls.__bases__)):
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if hasattr(base, '__abstractmethods__'):
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boundary = len(cls.__bases__) - i
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break # Bases up to the last explicit ABC are considered first.
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else:
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boundary = 0
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abcs = list(abcs) if abcs else []
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explicit_bases = list(cls.__bases__[:boundary])
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abstract_bases = []
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other_bases = list(cls.__bases__[boundary:])
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for base in abcs:
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if issubclass(cls, base) and not any(
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issubclass(b, base) for b in cls.__bases__
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):
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# If *cls* is the class that introduces behaviour described by
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# an ABC *base*, insert said ABC to its MRO.
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abstract_bases.append(base)
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for base in abstract_bases:
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abcs.remove(base)
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explicit_c3_mros = [_c3_mro(base, abcs=abcs) for base in explicit_bases]
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abstract_c3_mros = [_c3_mro(base, abcs=abcs) for base in abstract_bases]
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other_c3_mros = [_c3_mro(base, abcs=abcs) for base in other_bases]
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return _c3_merge(
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[[cls]] +
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explicit_c3_mros + abstract_c3_mros + other_c3_mros +
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[explicit_bases] + [abstract_bases] + [other_bases]
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)
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def _compose_mro(cls, types):
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"""Calculates the method resolution order for a given class *cls*.
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Includes relevant abstract base classes (with their respective bases) from
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the *types* iterable. Uses a modified C3 linearization algorithm.
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"""
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bases = set(cls.__mro__)
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# Remove entries which are already present in the __mro__ or unrelated.
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def is_related(typ):
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return (typ not in bases and hasattr(typ, '__mro__')
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and issubclass(cls, typ))
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types = [n for n in types if is_related(n)]
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# Remove entries which are strict bases of other entries (they will end up
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# in the MRO anyway.
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def is_strict_base(typ):
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for other in types:
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if typ != other and typ in other.__mro__:
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return True
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return False
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types = [n for n in types if not is_strict_base(n)]
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# Subclasses of the ABCs in *types* which are also implemented by
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# *cls* can be used to stabilize ABC ordering.
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type_set = set(types)
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mro = []
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for typ in types:
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found = []
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for sub in typ.__subclasses__():
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if sub not in bases and issubclass(cls, sub):
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found.append([s for s in sub.__mro__ if s in type_set])
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if not found:
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mro.append(typ)
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continue
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# Favor subclasses with the biggest number of useful bases
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found.sort(key=len, reverse=True)
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for sub in found:
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for subcls in sub:
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if subcls not in mro:
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mro.append(subcls)
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return _c3_mro(cls, abcs=mro)
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def _find_impl(cls, registry):
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"""Returns the best matching implementation from *registry* for type *cls*.
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Where there is no registered implementation for a specific type, its method
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resolution order is used to find a more generic implementation.
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Note: if *registry* does not contain an implementation for the base
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*object* type, this function may return None.
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"""
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mro = _compose_mro(cls, registry.keys())
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match = None
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for t in mro:
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if match is not None:
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# If *match* is an implicit ABC but there is another unrelated,
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# equally matching implicit ABC, refuse the temptation to guess.
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if (t in registry and t not in cls.__mro__
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and match not in cls.__mro__
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and not issubclass(match, t)):
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raise RuntimeError("Ambiguous dispatch: {0} or {1}".format(
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match, t))
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break
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if t in registry:
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match = t
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return registry.get(match)
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def singledispatch(func):
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"""Single-dispatch generic function decorator.
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Transforms a function into a generic function, which can have different
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behaviours depending upon the type of its first argument. The decorated
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function acts as the default implementation, and additional
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implementations can be registered using the register() attribute of the
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generic function.
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"""
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registry = {}
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dispatch_cache = WeakKeyDictionary()
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def ns(): pass
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ns.cache_token = None
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def dispatch(cls):
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"""generic_func.dispatch(cls) -> <function implementation>
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Runs the dispatch algorithm to return the best available implementation
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for the given *cls* registered on *generic_func*.
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"""
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if ns.cache_token is not None:
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current_token = get_cache_token()
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if ns.cache_token != current_token:
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dispatch_cache.clear()
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ns.cache_token = current_token
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try:
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impl = dispatch_cache[cls]
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except KeyError:
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try:
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impl = registry[cls]
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except KeyError:
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impl = _find_impl(cls, registry)
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dispatch_cache[cls] = impl
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return impl
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def register(cls, func=None):
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"""generic_func.register(cls, func) -> func
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Registers a new implementation for the given *cls* on a *generic_func*.
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"""
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if func is None:
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return lambda f: register(cls, f)
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registry[cls] = func
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if ns.cache_token is None and hasattr(cls, '__abstractmethods__'):
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ns.cache_token = get_cache_token()
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dispatch_cache.clear()
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return func
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def wrapper(*args, **kw):
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return dispatch(args[0].__class__)(*args, **kw)
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registry[object] = func
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wrapper.register = register
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wrapper.dispatch = dispatch
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wrapper.registry = MappingProxyType(registry)
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wrapper._clear_cache = dispatch_cache.clear
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update_wrapper(wrapper, func)
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return wrapper
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