Importing classes just for typing in python?

user_7195897 0
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I am trying to use typing in Python 3.8 and I am a bit stuck. Example: I am mainly developing in main.py. I also have a class util.py that contains some helper functions an classes. But these classes also need to import classes from main.py for Typing. Now, when I want to use functions from util.py in main.py I also need to import it - but then I'll get an error because of circular importing (and rightly so).

Is there a way around this?

Thanks in advance!

python-typing python-import python
3 Answers

Circular imports are not an immediate error in Python; they're only an error if you use them in a particular way. I believe you're looking for forward references

Example main.py:

import util

class SomeClass:
    pass

Example util.py

import main

# We can't use main.SomeClass in the type signature because of the cycle,
# but we can forward reference it, which the type system understands.
def make_some_class() -> "main.SomeClass":
    return main.SomeClass()
user_22886590

Sylvio's answer is correct, but I want to address just for typing in your title.

Importing classes just for typing in python?

If the only reason you are importing a class is to address some typing consideration, you can use the typing.TYPE_CHECKING constant to make that import conditional.

Quoting that documentation:

Runtime or type checking?

Sometimes there's code that must be seen by a type checker (or other static analysis tools) but should not be executed. For such situations the typing module defines a constant, TYPE_CHECKING, that is considered True during type checking (or other static analysis) but False at runtime. Example:

import typing

if typing.TYPE_CHECKING:
    import expensive_mod

def a_func(arg: 'expensive_mod.SomeClass') -> None:
    a_var = arg  # type: expensive_mod.SomeClass
    ...

(Note that the type annotation must be enclosed in quotes, making it a "forward reference", to hide the expensive_mod reference from the interpreter runtime. In the # type comment no quotes are needed.)

Bottom line is that you may very well not have to import it all, except for when you are type-checking.

user_13943530

I think the best way to prevent that is to move every class in their own file, and import them where you need them. Declaring multiple class in a single file is not good for code maintenance and can make big application harder to debug.

And I don't think you can avoid circular importing the way you describe it, if every class depends on each other

user_154420050
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