python / PyCharm: access to a protected member of another object of the same class

Question

In a method of my class MyHeader i access the private property _label of another MyHeader object new_header:

class MyHeader:
    def __init__(self, label, n_elem):
        self._label = label
        self._n_elem = n_elem

    def check_header_update(self, new_header):
        # check that label is preserved
        if new_header._label != self._label:
            raise Exception("new header must have the same label")

In PyCharm, this results in the syntax highlighting error "Access to a protected member _label of a class".

I tried specifying the type of the new_header parameter:

    def check_header_update(self, new_header: MyHeader):

but this is not recognized, and at run-time this leads to the error "NameError: name 'MyHeader' is not defined".

Any idea how to access the protected member in an accepted way?

Answer 1

The correct way to type your function would be to use forward references, and type your check_header_update like so. Note that I'm also adding the return type, for completeness:

def check_header_update(self, new_header: 'MyHeader') -> None:

The reason why the type needs to be a string is because when you're defining check_header_update, MyHeader hasn't been fully defined yet, so isn't something you can refer to.

However, I don't remember if this will end up fixing the problem or not. If it doesn't, then I would either:

1. Make _label non-private by removing that underscore
2. Make some kind of getter method or use properties to let other people access that data