Specify type of instance variables using python 2 type comments

Question

I am trying to specify the type for an instance variable using PEP 484's python 2 syntax. However, I haven't found any way to add types without initializing the variable in python 2, equivalent to the following python 3:

value: int

My normal work around for this is to declare the type of the variable in __init__ when instantiating the variable. However, this doesn't work for Protocols where the type of the instance variable should be part of the Protocol (types in __init__ seem to not count). Here's an example in Python 3 where I use a default implementation:

from typing_extensions import Protocol
class A(Protocol):
    value: int

    def get_value(self) -> int:
        return self.value

This would then highlight errors if value isn't initialized properly:

class B(A):
    pass
B()  # error: Cannot instantiate abstract class 'B' with abstract attribute 'value'

However, converting this to python 2 type comments fails to pass mypy. It gives the same error with or without the __init__ declaration.

class A(Protocol):
    def __init__(self):
        # type: () -> None
        self.value = 0  # type: int
    def get_value(self):
        # type: () -> int
        return self.value  # error: "A" has no attribute "value"

Is there some special syntax for declaring variable types without initializing them in python 2?

I guess that's why Protocol is in typing_extensions in 2.7 rather than typing. — Quantum7
– Quantum7, Commented Sep 11, 2019 at 10:04
@BoarGules This is an extremely unhelpful comment because it implies this is an explicit decision made in mypy. It seems much more like a bug. — Markus Unterwaditzer
– Markus Unterwaditzer, Commented Sep 11, 2019 at 17:39

Markus Unterwaditzer · Accepted Answer · 2019-09-17 20:52:47Z

1

Mypy's protocols use class variables to define attributes. Otherwise mypy does not make a particularly fine distinction between class vs instance variables. Taking these two things together, you can write code like the following:

from typing_extensions import Protocol

class A(Protocol):
    value = None  # type: int

    def get_value(self):
        # type: () -> int
        return self.value

# below here it's just to validate that the protocol works

class B(object):
    def __init__(self, value):
        # type: (int) -> None
        self.value = value

    def get_value(self):
        # type: () -> int
        return self.value


a = B(42)  # type: A

edited Sep 17, 2019 at 20:52

answered Sep 11, 2019 at 17:37

Markus Unterwaditzer

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3 Comments

Quantum7 Over a year ago

This works, to my surprise! A.value is a class variable not an instance variable; however, mypy seems to also accept objects with instance variables as implementing A. Changing B to have def __init__(self): self.value = 42 (instance variable) typechecks fine, which was exactly what I was looking for.

Quantum7 Over a year ago

If you update your answer to use an instance variable in B I'll accept this.

Markus Unterwaditzer Over a year ago

Done! I think in the process of updating this answer I started to understand the question better.

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