In Python, assert statement produces no code if command line optimization options -O or -OO are passed. Does it happen for typing.assert_never()? Is it safe to declare runtime assertions that will not be optimized out?
Consider the case
from typing import assert_never
def func(item: int | str):
match item:
case int():
...
case str():
...
case _:
assert_never(item)
Is it guaranteed that the default branch will work even in optimized mode?
3
No. assert_never() is just a normal function consisting of a raise statement, at least in CPython. It is not removed at runtime, as with other typing features.
It should be emphasized that assert_never() has nothing special: It is just a function that takes in an argument of type Never and never returns anything (i.e., it raises):
def assert_never(arg: Never) -> Never:
raise AssertionError("Expected code to be unreachable")
Its inclusion in the stdlib was simply due to the expectation that it would be commonly used. To quote the typing guides:
Because the
assert_never()helper function is frequently useful, it is provided by the standard library astyping.assert_neverstarting in Python 3.11, and is also present intyping_extensionsstarting at version 4.1.— §
assert_never()and Exhaustiveness Checking | Python typing guides
The same section also says:
However, it is also possible to define a similar function in your own code, for example if you want to customize the runtime error message.
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