Python typing: How to type-hint a variable as a bound method?

Question

So I have a function (let's call it fun1) that accepts a function as a parameter. But inside fun1, I need to access the parameter's __self__, which exists only if the function is a bound method.

The function itself must accept two str args and return a bool.

In other words, like this:

MyFuncType = Callable[[str, str], bool] # fun1 is an unbound function def fun1(func: MyFuncType): ... o = func.__self__ ... # Some logic on the "o" object, such as logging the object's class, # doing some inspection, etc. ...

If I use MyFuncType like the above, PyCharm will complain that __self__ is not an attribute of func.

So, what type hint should I annotate func with, so that PyCharm (and possibly mypy) won't protest on that line?

(I'm using Python 3.6 by the way)

This may help you: stackoverflow.com/questions/58085648/…. — Mario Ishac
– Mario Ishac, Commented Oct 7, 2020 at 16:59
Yup sadly, but you might be able to get around it. What prevents you from taking Callable[[<class>, str, str], bool]? You would need to pass Class.method instead of self.method, does your scenario allow that? Then you would declare o to be the first argument of that Callable. — Mario Ishac
– Mario Ishac, Commented Oct 7, 2020 at 19:17
The reason is to detect what object func was bound to and log that, plus doing some inspection on the object. So, passing a reference to an object to the func definitely won't help. Notice in the example that fun1 was a standalone function, it has no self to pass to func. — pepoluan
– pepoluan, Commented Oct 8, 2020 at 3:44
I just noticed I made a typo in the sample code that might be the reason of the confusion. Fixed. Sorry about that 😅 — pepoluan
– pepoluan, Commented Oct 8, 2020 at 4:02

pepoluan · Accepted Answer · 2020-10-08 03:59:49Z

Okay after some experimentation, I settle with this:

class BoundFunc: __self__: object MyFuncType = Callable[[str, str], bool] MyBoundFuncType = Union[MyFuncType, BoundFunc] def fun1(func: MyBoundFuncType): ... o = func.__self__ ...

This does NOT warn me if I pass an unbound function to fun1, but at least it suppresses PyCharm's warning when I try to access the __self__ property.

I figure a proper docstring on fun1 explicitly saying that func MUST be a bound method should be enough for adults...

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