Defaultdict，其值默认为负无穷大

Question

I want to create a default dictionary in which the default values are negative infinity. I tried doing defaultdict(float("-inf")) but it is not working. How do I do this?

Answer 1

As the traceback specifically tells you :

>>> from collections import defaultdict
>>> dct = defaultdict(float('-inf'))

Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    dct = defaultdict(float('-inf'))
TypeError: first argument must be callable

and per the documentation (emphasis mine):

If default_factory [the first argument to defaultdict ] is not None , it is called without arguments to provide a default value for the given key, this value is inserted in the dictionary for the key, and returned.

float('-inf') is not callable. Instead, you could do eg:

dct = defaultdict(lambda: float('-inf'))

providing a callable "lambda expression" that returns the default value. It's for the same reason that you see code with eg defaultdict(int) rather than defaultdict(0) :

>>> int()  # callable
0  # returns the desired default value

You would also get similar issues when eg trying to nest defaultdict s within each other (see Python: defaultdict of defaultdict? ).