If you want to measure import time, use

  python -m timeit -s "from sys import modules; modules_copy = modules.copy()" "import black; modules.clear(); modules.update(modules_copy)"

But I would be surprised to see significant difference in this case.

What Mark means, measure the time of creation of nested function.

  python -m timeit "def f(a: int, b: str) -> None: pass"

And maybe test with different number of arguments if there is a difference.