Unresolved reference to a class in a class constructor - Type Annotation

Question

I am just learning Python and I would like to write a module to process some data files. I am using Type Annotations and I would like to write a class (e.g. MyData) that when instantiating takes an optional argument of the type MyData. This is to compare the structure of new data to the previously loaded. However, when I run the code, I got the NameError (see below). PyCharm is highlighting "MyData" in '__init__' function and gives a message "Unresolved reference 'MyData'.

There is definitely something that I am getting wrong. Could you please enlighten me what's going on? I don't even know how to properly formulate a question to ask uncle G.

Thanks in advance for your help.

from typing import Optional

class MyData():
    def __init__(self, file_path: str, reference_data: Optional[MyData] = None):
        self.file_path = file_path
        self.reference_data = reference_data
        self.validate_data()

    def validate_data(self):
        # do some stuff with the data and check whether it has
        # similar structure to the reference_data
        pass

Traceback (most recent call last):
  File "some/path/somecode.py", line 4, in <module>
    class MyData():
  File "some/path/somecode.py", line 5, in MyData
    def __init__(self, file_path: str, reference_data: Optional[MyData] = None):
NameError: name 'MyData' is not defined