Typing a delayed initialised variable

[alexisdrakopoulos]

I have lots of variables in __init__ that currently are not typing compliant as they do something like:

self.var: T = None

where T can be any number of types like str, int or complex types.

I could wrap it with Optional[] but it's not Optional, it's just not initialised yet.

I've looked into using Sentinel objects or other features but none seem to really work in an elegant manner. I could also just leave them purely as annotations like self.var: T, but this causes ugly AttributeErrors and introduces more issues.

I've seen discussions such as:

• https://nedbatchelder.com/blog/202302/late_initialization_reconsidered.html
• What to do about Undefined #81
• Declaring the existence of a variable while using --strict-optional mypy#2179

But none have any relevant solutions for modern Python versions (3.10 onwards) with mypy (or other) support.

What is the "recommended" way to type hint these types of variables?

[Akuli]

You can simply delete the = None. The attribute will not exist until it is set.

For example:

class Foo:
    var: str

    def __init__(self) -> None:
        pass

    def set_up_stuff(self) -> None:
        self.var = "hello"

f = Foo()
f.set_up_stuff()
reveal_type(f.var)  # Revealed type is "builtins.str"

Note that you lose some type safety this way (but based on your question, it looks like you want this). This will pass the type check, but fail when you run the code, because the attribute does not exist yet:

f = Foo()
print(f.var)  # AttributeError: 'Foo' object has no attribute 'var'