this post was submitted on 16 Mar 2026
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PEP: Is initializing typed variable with None bad practice? (lemmy.world)
submitted 1 day ago by AstroLightz@lemmy.world to c/python@programming.dev
10 comments fedilink hide all child comments
 

For example:

class FooBar:
    def __init__(self):
        self.a: int = None
        self.foo: str = None

Is this bad practice/go against PEP guidelines or is it fine?

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[–] Vulwsztyn@programming.dev 1 points 6 hours ago* (last edited 6 hours ago)

yes, bad practice; yes, against PEP

[–] joyjoy@lemmy.zip 16 points 23 hours ago (1 children)

Implicit optional is deprecated. Explicitly use self.foo: int | None = None

[–] AstroLightz@lemmy.world 2 points 23 hours ago

Thanks! Good to know.

[–] jtrek@startrek.website 6 points 1 day ago (1 children)

That seems like that's going to give you an error in most type checkers. You said it's always an int and then immediate made that a lie and made it None instead.

Why are you trying to do this?

[–] AstroLightz@lemmy.world 2 points 23 hours ago (1 children)

I'm initializing variables that would be used later in the class in different functions. I wasn't sure if I needed to do a var: <type> | None = None or if just setting it to None was fine.

[–] solrize@lemmy.ml 3 points 16 hours ago* (last edited 16 hours ago) (1 children)

I'm going to say initializing them to None and updating them later is a code smell. If you can do so, make them non-optional and always initialize them to actual meaningful values.

[–] jtrek@startrek.website 2 points 13 hours ago

Yeah, if you initialize them to None then for the entire rest of the class you're going to have to account for the possibility that they're None. If it's unavoidable that they might be None, you should type it as such.

If you type them as like str | None then later when you do like return foo[0] it will warn you that you can't do that with None.

[–] solrize@lemmy.ml 1 points 16 hours ago

If it can be None then the type is Optional[int] or Optional[str] as the case may be. Or I guess now int | None etc. I'm used to older versions of mypy so idk if they support that alternate syntax.

Optional[int] would seem to be preferable but it turns out it's not really an option type. Like Optional[None] being None is ambiguous.

[–] sbv@sh.itjust.works 3 points 23 hours ago (1 children)

Why not require values in the constructor?

[–] AstroLightz@lemmy.world 1 points 23 hours ago

This is an example. For my actual use case, they would be private vars that would be set by class functions instead of passed to the constructor.