How can I use @property setters and make mypy happy?

Question

I have the following example.py file:

class Location(object):
    def __init__(self, latitude, longitude):
        self.latitude = latitude
        self.longitude = longitude

    @property
    def latitude(self):
        return self._latitude

    @property
    def longitude(self):
        return self._longitude

    @latitude.setter
    def latitude(self, latitude):
        """Setter for latiutde."""
        if not (-90 <= latitude <= 90):
            raise ValueError('latitude was {}, but has to be in [-90, 90]'
                             .format(latitude))
        self._latitude = latitude

    @longitude.setter
    def longitude(self, longitude):
        """Setter for longitude."""
        if not (-180 <= longitude <= 180):
            raise ValueError('longitude was {}, but has to be in [-180, 180]'
                             .format(longitude))
        self._longitude = longitude

    def __repr__(self):
        return 'Location({}, {})'.format(self.latitude, self.longitude)

    __str__ = __repr__


munich = Location(48.137222222222, 11.57555)
print(munich)
try:
    munich.latitude = 200
    print("This should not work")
except ValueError:
    pass

When I run mypy example.py (mypy version 0.73) I get a couple of errors:

$ mypy example.py 
example.py:14: error: Name 'latitude' already defined on line 6
example.py:14: error: "Callable[[Any], Any]" has no attribute "setter"
example.py:22: error: Name 'longitude' already defined on line 10
example.py:22: error: "Callable[[Any], Any]" has no attribute "setter"
example.py:39: error: Property "latitude" defined in "Location" is read-only
Found 5 errors in 1 file (checked 1 source file)

Why do I get those and how can I fix it?

Mypy may not like that you are passing an argument with the exact same name as the method: def longitude(self, longitude):. — Error - Syntactical Remorse
– Error - Syntactical Remorse, Commented Sep 26, 2019 at 16:25
Yeah. I don't know there logic nor why they won't fix it but I guess its a feature :). — Error - Syntactical Remorse
– Error - Syntactical Remorse, Commented Sep 26, 2019 at 16:31
Do you want to post it as an answer or should I make a community wiki answer? :-) — Martin Thoma
– Martin Thoma, Commented Sep 26, 2019 at 16:34

Martin Thoma · Accepted Answer · 2019-09-26 17:36:47Z

23

The issue is probably related to Property setter not accepted if not next to getter.

The following code incorrectly raises an error:

class Config(object):

    @property
    def my_proprty(self):
        return None

    def _other(self):
        pass

    @my_proprty.setter
    def my_proprty(self, val):
        pass Error:

mypytest.py: note: In class "Config": mypytest.py:12: error:
Callable[[Any], Any] has no attribute "setter"

MyPy closed the issue and marked it as a false-positive. It appears they have no intentions of fixing it as of now.

We don't have immediate plans for fix this issue, but we are happy to receive a PR. - JukkaL

Moving your getters and setters next to each other (with the getter first) should fix the issue.

Consider it a feature :)

edited Sep 26, 2019 at 17:36

Martin Thoma

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answered Sep 26, 2019 at 16:40

Error - Syntactical Remorse

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