Mypy allowing `List[str]` to have values of type `Tuple[str, List[str]]`

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The following minified example was originally found while making changes to my work's code base. I have the line x: List[str] = [] so I expect that mypy will enforce that, but in this example it seems it does not. If I want mypy to catch this, what should I do differently?

from typing import List

x: List[str] = []
y = ["world"]
z = ["hello"]

def go(boop):
    return f"{boop}!", y

x = list(map(go, z))
print(x)

# output:
# [('hello!', ['world'])]

$ mypy mypy_lists.py   
Success: no issues found in 1 source file

$ python3 mypy_lists.py
[('hello!', ['world'])]

EDIT:

For context, the , y part of the definition of go was a copy-paste error, and I expected to be protected against that by just the x: List[str] annotation, but maybe I have to update my internal model of mypy type checking, and just trust it less.

EDIT 2:

Given the accepted answer, the amount I can trust mypy with type hints seems to be directly related to the way mypy is configured. Not terribly surprising, but a good thing to remember.

mypy python
2 Answers

It seems that adding type annotations to the go function causes mypy to correctly pick up on the problem:

$ mypy -c 'from typing import List, Tuple

x: List[str] = []
y = ["world"]
z = ["hello"]

def go(boop: str) -> Tuple[str, List[str]]:
    return f"{boop}!", y

x = list(map(go, z))
print(x)'
<string>:10: error: Argument 1 to "map" has incompatible type "Callable[[Any], Tuple[str, List[str]]]"; expected "Callable[[str], str]"
Found 1 error in 1 file (checked 1 source file)
user_32824360

By default mypy ignores untyped function definitions.

This helps gradually introduce type annotations into an existing codebase without having to add them all at once.

Personally for new projects I run mypy with the --strict option to avoid accidentally leaving functions unannotated and bypassing type checks.

user_124796390
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