initialize defaultdict in Python

Question

A dumb question after searching for Python Doc, for the constructor of defaultdict, there is only one parameter, which is type of value. Is there a way to specify type of key? I read a few samples and it seems no one specify type of key, and they just specify type of value. And whether it is needed or not -- interested to learn why defaultdict just needs to specify type of value.

thanks in advance, Lin

• AChampion: What would a default key be? How would you access a default key?
• Lin Ma: @AChampion, thanks for the reply. I do not get your points, what do you mean default key? Do you mean default value?
• Lin Ma: @AChampion, thanks for the clarification. If you could add a reply, I will mark it as answered to benefit more people. Thanks.
• Peter DeGlopper: Note that you're not specifying the "type" of the values. You're specifying a no-argument constructor, which must construct a default value. For instance, see this answer for a way to create an indefinitely nested defaultdict: stackoverflow.com/a/19189356/2337736 Or, for a trivial example, fives = defaultdict(lambda : 5) will product a defaultdict where the default value is 5.

Answer 1

Python is a dynamically typed language, your don't specify types (ignoring the newly added type hints). The argument to defaultdict is a function that returns a value when accessing a key that hasn't been added to the dictionary. Not a type

defaultdict(int)

Is equivalent to:

defaultdict(lambda:int()) # int() returns 0

• Lin Ma: Thanks AChampion for all the help, mark your reply as answer. :)