149 – NotImplementedError

The built-in exception NotImplementedError is supposed to be used in base classes to indicate that a given method must be implemented by the derived class:

class Parser:
    def parse(self, data):
        raise NotImplementedError

You can also use the exception NotImplementedError to create a stub for a method but also signal that its real implementation still needs to be added:

class MyParser(Parser):
    def parse(self, data):
        self._validate_data(data)
        # Parse the data...
        
    def _validate_data(self, data):
        raise NotImplementedError

The documentation also notes that you should not use NotImplementedError to indicate that a method isn’t supposed to be supported at all by a (sub)class.

NotImplementedError should not be confused with the built-in singleton NotImplemented.