I want to pass in the key to the default_factory of defaultdict and I found
that defaultdict somehow can intercept my call to dict.__getitem__(self,
key), so my class's __getitem__ have to catch a TypeError instead instead
of KeyError. The following class is my code:
class DefaultDictWithEnhancedFactory(defaultdict):
"""Just like the standard python collections.dict,
but the default_factory takes the missing key as argument.
Args:
default_factory: A function that takes the missing key as the
argument
and return a value for the missing key.
*a: arguments passing to the defaultdict constructor
**kw: keyword arguments passing to the defaultdict constructor
"""
def __init__(self, default_factory, *a, **kw):
defaultdict.__init__(self, default_factory, *a, **kw)
def __getitem__(self, key):
try:
return dict.__getitem__(self, key)
except KeyError:
# Normally, you would expect this line to be
# called for missing keys...
return self.default_factory(key)
except TypeError as ex:
# However, this is actually getting called
# because for some reason, defaultdict still
# intercepts the __getitem__ call and raises:
# TypeError: <lambda>() takes exactly 1 argument (0 given)
# So we have to catch that instead...
if "lambda" in str(ex):
return self.default_factory(key)
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