On , I learnt ...

Mypy skips empty functions

Mypy doesn’t check types of empty functions, such as those whose implementation is pass, ... or only a docstring.

For example, running mypy on this class:

class C:
    def pass_only(self) -> int:

    def elipsis_only(self) -> int:

    def docstring_only(self) -> int:
        Just a docstring, which implicitly returns `None`.

    def docstring_and_none_return_type(self) -> int:
        Now explicitly retiurn `None.
        return None

    def incorrect_return_type(self) -> int:
        return "x"

only picks up errors on the latter two methods where there is an explicit return value:

$ mypy --strict types_empty_methods.py
example.py:17: error: Incompatible return value type (got "None", expected "int")  [return-value]
example.py:20: error: Incompatible return value type (got "str", expected "int")  [return-value]
Found 2 errors in 1 file (checked 1 source file)

Such empty functions are normally used to indicate that a subclass should provide a concrete implementation. However, real errors can occur if this is omitted:

>>> C().pass_only() + 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

This is true as of Mypy v0.982 and below, but looks like it will be fixed in a upcoming release.

Thanks to Sam Searles-Bryant, who showed this to me.