I am a bit confused on why you need a lambda function for nesting defaultdict
Why can't you do it like this?
test = defaultdict(defaultdict(list))
instead of
test = defaultdict(lambda:defaultdict(float))
test = defaultdict(defaultdict(list))
Because defaultdict
requires that you give it something that can be called to create keys for missing values. list
is such a callable, but defaultdict(list)
is not. It's a defaultdict
instance, and you can't call a defaultdict
.
The lambda
is a function that, when called, returns a value that can be used in the dictionary, so it works.
Essentially, defaultdict(list)
is going to be evaluated before your defaultdict
is instantiated, and you want to defer that until a missing key is encountered. This is why a callable object (a type or a function) is used here.
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