__new__() does not return anything, on singletong pattern

[Mario Figueiredo]

I'm fairly new to Python, so I don't know if the following is me
abusing the programming language idioms, or simply a mistake of my IDE
code inspection routine.

I have a singleton Map class which is defined like so:

class Map:
	_instance = None

    def __new__(cls):
        if Map._instance is None:
            Map._instance = super(Map, cls).__new__(cls)

        return Map._instance

	def __init__(self, filename):
		# Instantiates from the contents of a binary file

I am now trying to add another way of constructing an instance of this
class. (I need to be able to create a dirty empty instance that is
going to be used by the separate map editor script).

I added the following method to the class definition, above:

    @classmethod
    def generate(cls, width, height, fill=terrain[6]):
        if Map._instance is None:
            Map._instance = super(Map, cls).__new__(cls)
        else:
            raise Exception('Cannot generate an instance of Map.')

        Map._instance.author = None
        Map._instance.name = None
        Map._instance.description = None
		# etc...

		self.cells = [Cell(fill)] * width * height

        return Map._instance


The following code runs just fine. But PyCharm flags the assignment
with a warning telling me that generate() does not return anything and
the I lose code completion on the mmap variable.

if __name__ == '__main__':
    mmap = Map.generate(12, 24)
    print(mmap.width, mmap.height, mmap.author)

I need to understand if this is just a glitch of the IDE or I am doing
indeed something that is frowned upon and ended up caught in a
misleading static analysis warning.