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