#!/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 _c3_merge(sequences): """Merges MROs in *sequences* to a single MRO using the C3 algorithm. Adapted from http://www.python.org/download/releases/2.3/mro/. """ result = [] while True: sequences = [s for s in sequences if s] # purge empty sequences if not sequences: return result for s1 in sequences: # find merge candidates among seq heads candidate = s1[0] for s2 in sequences: if candidate in s2[1:]: candidate = None break # reject the current head, it appears later else: break if not candidate: raise RuntimeError("Inconsistent hierarchy") result.append(candidate) # remove the chosen candidate for seq in sequences: if seq[0] == candidate: del seq[0] def _c3_mro(cls, abcs=None): """Computes the method resolution order using extended C3 linearization. If no *abcs* are given, the algorithm works exactly like the built-in C3 linearization used for method resolution. If given, *abcs* is a list of abstract base classes that should be inserted into the resulting MRO. Unrelated ABCs are ignored and don't end up in the result. The algorithm inserts ABCs where their functionality is introduced, i.e. issubclass(cls, abc) returns True for the class itself but returns False for all its direct base classes. Implicit ABCs for a given class (either registered or inferred from the presence of a special method like __len__) are inserted directly after the last ABC explicitly listed in the MRO of said class. If two implicit ABCs end up next to each other in the resulting MRO, their ordering depends on the order of types in *abcs*. """ for i, base in enumerate(reversed(cls.__bases__)): if hasattr(base, '__abstractmethods__'): boundary = len(cls.__bases__) - i break # Bases up to the last explicit ABC are considered first. else: boundary = 0 abcs = list(abcs) if abcs else [] explicit_bases = list(cls.__bases__[:boundary]) abstract_bases = [] other_bases = list(cls.__bases__[boundary:]) for base in abcs: if issubclass(cls, base) and not any( issubclass(b, base) for b in cls.__bases__ ): # If *cls* is the class that introduces behaviour described by # an ABC *base*, insert said ABC to its MRO. abstract_bases.append(base) for base in abstract_bases: abcs.remove(base) explicit_c3_mros = [_c3_mro(base, abcs=abcs) for base in explicit_bases] abstract_c3_mros = [_c3_mro(base, abcs=abcs) for base in abstract_bases] other_c3_mros = [_c3_mro(base, abcs=abcs) for base in other_bases] return _c3_merge( [[cls]] + explicit_c3_mros + abstract_c3_mros + other_c3_mros + [explicit_bases] + [abstract_bases] + [other_bases] ) def _compose_mro(cls, types): """Calculates the method resolution order for a given class *cls*. Includes relevant abstract base classes (with their respective bases) from the *types* iterable. Uses a modified C3 linearization algorithm. """ bases = set(cls.__mro__) # Remove entries which are already present in the __mro__ or unrelated. def is_related(typ): return (typ not in bases and hasattr(typ, '__mro__') and issubclass(cls, typ)) types = [n for n in types if is_related(n)] # Remove entries which are strict bases of other entries (they will end up # in the MRO anyway. def is_strict_base(typ): for other in types: if typ != other and typ in other.__mro__: return True return False types = [n for n in types if not is_strict_base(n)] # Subclasses of the ABCs in *types* which are also implemented by # *cls* can be used to stabilize ABC ordering. type_set = set(types) mro = [] for typ in types: found = [] for sub in typ.__subclasses__(): if sub not in bases and issubclass(cls, sub): found.append([s for s in sub.__mro__ if s in type_set]) if not found: mro.append(typ) continue # Favor subclasses with the biggest number of useful bases found.sort(key=len, reverse=True) for sub in found: for subcls in sub: if subcls not in mro: mro.append(subcls) return _c3_mro(cls, abcs=mro) def _find_impl(cls, registry): """Returns the best matching implementation from *registry* for type *cls*. 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 implicit ABC but there is another unrelated, # equally matching implicit ABC, refuse the temptation to guess. if (t in registry and t not in cls.__mro__ and match not in cls.__mro__ and not issubclass(match, t)): 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(cls): """generic_func.dispatch(cls) -> Runs the dispatch algorithm to return the best available implementation for the given *cls* 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[cls] except KeyError: try: impl = registry[cls] except KeyError: impl = _find_impl(cls, registry) dispatch_cache[cls] = impl return impl def register(cls, func=None): """generic_func.register(cls, func) -> func Registers a new implementation for the given *cls* on a *generic_func*. """ if func is None: return lambda f: register(cls, f) registry[cls] = func if ns.cache_token is None and hasattr(cls, '__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