Here, we are using set as the missing value default, how can we use an undefined set for that purpose?
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What do you mean by "undefined"? – user2390182 Jul 07 '23 at 14:38
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1What is provided the defaultdict constructor is a *factory*, a no-argument callable that -- when called -- produces new independent default values. – user2390182 Jul 07 '23 at 14:41
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defaultdict doesn't take a value, it takes a callable thing that it can call (with no arguments) to construct the value when it needs to. Typically a constructor of a class, or a function, like lambda: 'Hello'.
Each new value will in this case be a set(), which is an empty set.
>>> d = collections.defaultdict(set)
>>> print(d['new_key'])
set()
Your datastructure is a dictionary of keys to sets, with an empty set as the default value.
Mikael Öhman
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2Well, doesn't have to be constructor. Any no-arg callable will do. – user2390182 Jul 07 '23 at 14:42
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Good point. Quite common to have a lambda in there so i updated the text. – Mikael Öhman Jul 07 '23 at 14:47
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Also, `set` *has* a constructor (`__new__`, though you probably meant `__init__`); it *is* a type. – chepner Jul 07 '23 at 16:07