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Is there a way to have a defaultdict(defaultdict(int)) in order to make the following code work?
for x in stuff: d[x.a][x.b] += x.c_int d needs to be built ad-hoc, depending on x.a and x.b elements.
I could use:
for x in stuff: d[x.a,x.b] += x.c_int but then I wouldn't be able to use:
d.keys() d[x.a].keys() 
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A defaultdict works exactly like a normal dict, but it is initialized with a function (“default factory”) that takes no arguments and provides the default value for a nonexistent key. A defaultdict will never raise a KeyError. Any key that does not exist gets the value returned by the default factory.
Defaultdict is a container like dictionaries present in the module collections. Defaultdict is a sub-class of the dictionary class that returns a dictionary-like object. The functionality of both dictionaries and defaultdict are almost same except for the fact that defaultdict never raises a KeyError.
The difference is that a defaultdict will "default" a value if that key has not been set yet. If you didn't use a defaultdict you'd have to check to see if that key exists, and if it doesn't, set it to what you want.
Yes like this:
defaultdict(lambda: defaultdict(int)) The argument of a defaultdict (in this case is lambda: defaultdict(int)) will be called when you try to access a key that doesn't exist. The return value of it will be set as the new value of this key, which means in our case the value of d[Key_doesnt_exist] will be defaultdict(int).
If you try to access a key from this last defaultdict i.e. d[Key_doesnt_exist][Key_doesnt_exist] it will return 0, which is the return value of the argument of the last defaultdict i.e. int().
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