Using mypy with with lazy initialization of instance attributes

Question

UPDATE: trying to check/fill values in another function

I'm trying to use mypy in my projects, but many of the instance attributes I use are only initialized after __init__, and not inside it. However, I do want to keep the good practice of declaring all instance attributes at __init__, so I need some complicated solutions to make this work.

An example to how I want this to behave (currently mypy is complaining):

from typing import Optional

class Foo:
    def __init__(self, x: int):
        self.x = x
        self.y: int = None  # will initialize later, but I know it will be an int

    def fill_values(self):
        self.y = x**2

    def do(self) -> int:
        return self.x + self.y

Currently mypy complains about the assignment of self.y, and wants it to be Optional or None.

If I agree with it and change the line to self.y: Optional[int] = None, then mypy complains on the return value of do, because self.y might be None.

The only way I found around it is to add as assert before using self.y, like: assert self.y is not None, which mypy picks up and understands. However, starting each method with many asserts is quite hard. I have many such values, and usually one method that initializes all of them, and all other methods runs after it.

I understand that mypy is rightfully complaining (the method do can be called before fill_values), but even when I try to prevent it I can't get mypy to accept this. I can extend this example by adding more functionality but mypy can't infer this:

from typing import Optional

class Foo:
    def __init__(self, x: int):
        self.x = x
        self.y: int = None  # will initialize later, but I know it will be an int

    def fill_values(self):
        self.y = x**2

    def check_values(self):
        assert self.y is not None

    def do(self) -> int:
        if self.y is None:
            self.fill_values()
        self.check_values()
        return self.x + self.y

Any idea of a more elegant solution that multiple assert statements and Optional types which obscure the code?

If there's no reasonable default for `self.y`, what would you do if `do` was called before `fill_values`? You probably would want to pass this decision back up to where it was being called, by perhaps raising an exception? — dspencer, Mar 30 '20 at 06:31
I've updated the question to reflect that option, but mypy is still complaining — Beka, Mar 30 '20 at 07:15
You can ignore the wrong assignment via a trailing `# type: ignore[assignment]` comment, or you can turn this behaviour off completely by passing [`--no-implicit-optional`](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-optional), but you'll loose the none-checks in the situations like @dspencer described. As for validation of optionals: `mypy` only understands direct assertions, not when you put them in a separate method as the inference becomes too cumbersome. — hoefling, Mar 30 '20 at 08:37

Rory Sullivan · Accepted Answer · 2023-01-26T23:46:51.773

14

I have found this to work for me:

class Foo:
def __init__(self, x: int):
    self.x = x
    self.y: int # Give self.y a type but no value

def fill_values(self):
    self.y = self.x**2

def do(self) -> int:
    return self.x + self.y

Essentially all you are doing is telling mypy that self.y will be an integer when (and if) it is initialised. Trying to call self.y before it is initialised will raise an error and you can check if it has been initialised using hasattr(self, "y").

edited Jan 26 '23 at 23:46

answered Aug 05 '20 at 09:39

Rory Sullivan

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2
5

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That's brilliant! it also solves the issue of "communicating" that `self.y` is eventually gonna be part of the class without actually initializing it. – Beka Aug 06 '20 at 14:03
Any reason this is not working with `mypy==0.982` and `python 3.9`? MyPy still complains – zerohedge Oct 10 '22 at 15:56
@zerohedge this is most likely due to a change to mypy. Try an older version of mypy. See if you can figure out the exact version where this behaviour changed and check the release notes for that version. – Rory Sullivan Nov 01 '22 at 16:20
it should be `self.y = self.x**2` – Anton Jan 26 '23 at 11:33

Using mypy with with lazy initialization of instance attributes

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