import UserDict
class Chainmap(UserDict.DictMixin):
"""Combine multiple mappings for sequential lookup.
For example, to emulate Python's normal lookup sequence:
import __builtin__
pylookup = Chainmap(locals(), globals(), vars(__builtin__))
"""
def __init__(self, *maps):
self._maps = maps
def __getitem__(self, key):
for mapping in self._maps:
try:
return mapping[key]
except KeyError:
pass
raise KeyError(key)
if __name__ == "__main__":
d1 = {'a':1, 'b':2}
d2 = {'a':3, 'd':4}
cm = Chainmap(d1, d2)
assert cm['a'] == 1
assert cm['b'] == 2
assert cm['d'] == 4
try:
print cm['f']
except KeyError:
pass
else:
raise Exception('Did not raise KeyError for missing key')
assert 'a' in cm and 'b' in cm and 'd' in cm
assert cm.get('a', 10) == 1
assert cm.get('b', 20) == 2
assert cm.get('d', 30) == 4
assert cm.get('f', 40) == 40
