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112 lines
3.5 KiB
Python
112 lines
3.5 KiB
Python
# from https://hg.python.org/cpython/file/default/Lib/collections/__init__.py
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from collections import MutableMapping
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class ChainMap(MutableMapping):
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""" A ChainMap groups multiple dicts (or other mappings) together
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to create a single, updateable view.
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The underlying mappings are stored in a list. That list is public and can
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be accessed or updated using the *maps* attribute. There is no other
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state.
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Lookups search the underlying mappings successively until a key is found.
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In contrast, writes, updates, and deletions only operate on the first
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mapping.
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"""
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def __init__(self, *maps):
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"""Initialize a ChainMap by setting *maps* to the given mappings.
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If no mappings are provided, a single empty dictionary is used.
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"""
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self.maps = list(maps) or [{}] # always at least one map
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def __missing__(self, key):
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raise KeyError(key)
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def __getitem__(self, key):
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for mapping in self.maps:
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try:
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# can't use 'key in mapping' with defaultdict
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return mapping[key]
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except KeyError:
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pass
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# support subclasses that define __missing__
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return self.__missing__(key)
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def get(self, key, default=None):
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return self[key] if key in self else default
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def __len__(self):
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# reuses stored hash values if possible
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return len(set().union(*self.maps))
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def __iter__(self):
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return iter(set().union(*self.maps))
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def __contains__(self, key):
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return any(key in m for m in self.maps)
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def __bool__(self):
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return any(self.maps)
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def __repr__(self):
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return '{0.__class__.__name__}({1})'.format(
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self, ', '.join(map(repr, self.maps)))
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@classmethod
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def fromkeys(cls, iterable, *args):
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"""Create a ChainMap with a single dict created from the iterable."""
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return cls(dict.fromkeys(iterable, *args))
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def copy(self):
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"""New ChainMap or subclass with a new copy of maps[0] and refs to maps[1:]"""
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return self.__class__(self.maps[0].copy(), *self.maps[1:])
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__copy__ = copy
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def new_child(self, m=None): # like Django's Context.push()
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"""New ChainMap with a new map followed by all previous maps.
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If no map is provided, an empty dict is used.
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"""
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if m is None:
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m = {}
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return self.__class__(m, *self.maps)
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@property
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def parents(self): # like Django's Context.pop()
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"""New ChainMap from maps[1:]."""
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return self.__class__(*self.maps[1:])
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def __setitem__(self, key, value):
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self.maps[0][key] = value
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def __delitem__(self, key):
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try:
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del self.maps[0][key]
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except KeyError:
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raise KeyError(
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'Key not found in the first mapping: {!r}'.format(key))
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def popitem(self):
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"""Remove and return an item pair from maps[0]. Raise KeyError is maps[0] is empty."""
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try:
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return self.maps[0].popitem()
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except KeyError:
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raise KeyError('No keys found in the first mapping.')
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def pop(self, key, *args):
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"""Remove *key* from maps[0] and return its value. Raise KeyError if *key* not in maps[0]."""
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try:
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return self.maps[0].pop(key, *args)
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except KeyError:
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raise KeyError(
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'Key not found in the first mapping: {!r}'.format(key))
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def clear(self):
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"""Clear maps[0], leaving maps[1:] intact."""
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self.maps[0].clear()
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