What does the asterisk in the output of `reveal_type` mean?

Question

reveal_type(1) # Revealed type is 'builtins.int'
bla = [1,2,3]
reveal_type(bla[0]) # Revealed type is 'builtins.int*'
reveal_type(bla[0] * 2) # Revealed type is 'builtins.int'

What is the difference between int and int*?

Answer 1

It means that particular type was inferred by mypy as a part of performing type variable substitution.

For example, blah[0] is actually doing blah.__getitem__(0): it turns out that the __getitem__ method is defined to return some value of type _T, where _T is whatever type is contained within the list*.

Mypy understands that blah contains ints, and so inferred that the _T return type is of type int.

In contrast, there's no type variable inference going on with just 1 and blah[0] * 2. The former is a literal; the latter is invoking the int.__mul__(...) method, which is typed to return specifically an int.

_{*Well, that's not actually the exact signature, but close enough.}

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For the most part, you can ignore this and just treat it as an implementation detail of mypy. It exists mostly because being able to tell whether or not a type was inferred is occasionally useful to have when you're tinkering with or debugging mypy internals.