Class name isn't defined when method parameter is of the same class

Question

class Folder:

    def copy_files_with(self, extension: str, to_location: Folder):
        pass

    def __eq__(self, other):
        if isinstance(other, Folder):
            return (self._parent, self._subdirectory) == (other._parent, other._subdirectory)
        return NotImplemented

    def __hash__(self):
        return hash((self._parent, self._subdirectory))

I'm using Visual Studio code with PyLint and it returns an error with the copy_files_with method.

line 20, in Folder
def copy_files_with(self, extension: str, to_location: Folder)

I removed all the unnecessary code, but line 20 is where the method copy_files_with is located.

I don't understand why, the __eq__ method can see the Folder class in the isinstance call. I want the to_location to be a Folder, and I want to specify that in the type hint, how do I do this?

Answer 1

When a class and its methods are processed, in the moment a module is imported, the lines containing the "class" and "def" statements are run. The method bodies - the code inside the "def" blocks, are only run when the class is later instantiated and have its methods called.

So, it is natural that a class is not defined inside its own body - the class body have to be processed before Python will create the class itself - and only then it will be associated with its name.

The way to make this work - references to the own class as well as refrences to class that are forward in the same file, is to use their name as strings in the annotations - in this case, you should use

 def copy_files_with(self, extension: str, to_location: 'Folder'):

and it should work with most tools that make use of the annotations.