Python 2.7 type hinting for subclasses with stubs

Question

You can use type hinting with stub files in Python 2.7 per https://www.python.org/dev/peps/pep-0484/#stub-files

However I cannot make it work for method signatures in subclasses.

In stub file a.pyi:

class A(object):
    def foo(self, timestamp: float): ...

In Python 2.7 file b.py

class B(A):
    def foo(self, timestamp):
        print(timestamp)  # Inferred type of timestamp is not float!

PyCharm 2017.3.3 does not infer timestamp as float. I did not check the behavior of mypy.

There are two workarounds which bring code redundancies (not preferred)

Workaround 1

In stub file b.pyi

class B(A):
    def foo(self, timestamp: float): ...

Workaround 2

In Python 2.7 file b.py

class B(A):
def foo(self, timestamp):  # type: (float) -> None
    print(timestamp)

Michael0x2a Michael0x2a · Accepted Answer · 2018-05-09T22:58:22

If you do not add type annotations to a function, you are indicating that you do NOT want PEP-484 compliant type checkers to check that function.

What this means is that you need to do approach 2: explicitly add type hints so that Pycharm (and mypy) knows you want that function to be typechecked.

Note that your workaround 1 doesn't really work: if you add a *.pyi file, you're telling the typechecker to completely ignore the corresponding *.py file. This is probably not what you want to happen here.

More broadly, it would actually be incorrect for Pycharm (or mypy) to infer that timestamp is always of type float: it's legal for the subtype to actually widen the parameter type. For example, it could be the case that B's timestamp method accepts floats OR strs:

class B(A):
    def timestamp(self, timestamp):
        # type: (Union[float, str]) -> None
        print(timestamp)

Or maybe it could be widened to accept any type:

class B(A):
    def timestamp(self, timestamp):
        # type: (object) -> None
        print(timestamp)

Both of these definitions are valid subtypes of A: they both match A.timestamp's signature without violating the Liskov substitution principle.

Consequently, since we can't easily automatically infer what the subtype's signatures should be, Pycharm (and mypy) don't try.

Python 2.7 type hinting for subclasses with stubs

Workaround 1

Workaround 2

1 Answers