Running `mypy --strict`, the variable without a type hint doesn't get error, not strictly checking the code

[hyperkai]

*Memo:

• mypy --strict test.py
• mypy 1.18.2
• Python 3.14.0
• Windows 11

Running mypy --strict, the function without type hints gets the errors, strictly checking the code as shown below:

def func(x):
    return x
# error: Function is missing a type annotation

def func(x) -> str:
    return x
# error: Function is missing a type annotation for one or more arguments

def func(x: str):
    return x
# error: Function is missing a return type annotation

But, running mypy --strict, the variable without a type hint doesn't get error, not strictly checking the code as shown below:

v = 'Hello'
# No error

[tyralla]

That is because Mypy infers the type of v as str. Hence, it would detect that v + 1 crashes.

On the contrary, Mypy handles the parameter type (second example) or the return type (third example) of func as Any. Hence, it would not detect that func(1) + "x" (second example) and func("x") + 1 (third example) crash.

[A5rocks]

Inference is a core feature of mypy and it wouldn't make sense to add a flag to warn about it! You can probably quite easily make a lint rule to check this using the builtin ast module. (you really really shouldn't though)