问题
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))
回答1:
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.
来源：https://stackoverflow.com/questions/30609117/why-do-you-need-lambda-to-nest-defaultdict