Type hint for instance of subclass

I want to allow type hinting using Python 3 to accept instances which are children of a given class. I’m using the enforce module to check the function typing. E.g.:

import abc
class A(metaclass=abc.ABCMeta)
      pass
class B(A)
      def __init__(self,a)
      self.a = a
      pass
x = B(3)

@enforce.runtime_validation
def function(x:A)
     print(x.a)

but it seems like python 3 doesn’t allow for this syntax, returning:

Argument ‘x’ was not of type < class ‘A’ >. Actual type was B.

Any help?

Answer

By default enforce applies invariant type checking. The type has to match directly – or an error is thrown.

In order for an instance of a subclass to be accepted, the mode can be changed to covariant by adding :

enforce.config({'mode ': 'covariant'})

at a point in the code that is executed before any type checking is done (i.e. near the start).

Other modes are available as described in the documentation.

For further informations see : https://github.com/RussBaz/enforce