[Python] What do square brackets mean in Python function definitions? [duplicate]

While watching a video about Python features, I came across the following code snippet:

def compose[_First, _Second, _Thirt](
first: Callable[[_First], _Second],
second: Callable[[_Second], _Third],
) -> Callable[[_First], _Third]:
return lambda x: second(first(x))


I’m curious about the syntax with square brackets right after the function name, specifically [_First, _Second, _Third]. What is this syntax called? When and how is it used in Python?


P.S. Summary for future readers:

1. This is called Type Parameter Syntax
2. It was introduced in Python 3.12 and is detailed in PEP 695
3. A partial answer to the question is provided in this article on SO
4. This specific case is described in the documentation on generic functions

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