python-metaclasses

A collection of useful metaclasses for python

Single Dispatch

Inheriting the metaclass SingleDispatchMetaClass makes it possible to use function annotations to overload a method

from elymetaclasses import SingleDispatchMetaClass

class MyClass(metaclass=SingleDispatchMetaClass):
    def myfunc(first, second)
        """ Default method """
        return first
        
    def myfunc(first: int, second: int)
        """ Invoked if both arguments are of type int """
        return first * second
                
    def myfunc(first, second: int)
        """ invoked if second is of type int but first is not """
        return second * 2

An overloaded method will only be called if all annotated arguments passes: isinstance(arg, annotation). Arguments with no annotation are "wild cards", i.e they will match any input. Arguments are matched from left to right. If no methods matches the function signatures the first method declared in the class body will be called.

Inheritance

If a subclass has the metaclass SingleDispatchMetaClass overloading will extend to it.

class MyClass(metaclass=SingleDispatchMetaClass):
    def myfunc(first: int, second: int)
        """ Invoked if both arguments are of type int """
        return first * second
    
    def myfunc(first, second: int)
         """ never invoked in MyClass2 because it is shadowed """
         return first * second
    
    def myfunc(first, second)
         """ Effectively the default class, also for MyClass2! """
         return first * second

class MyClass3:
    def myfunc(first: str, second):
        """ never invoked in MyClass2 because it does not have the SingleDispatchMetaClass"""
            pass
            
class MyClass2(MyClass):
    def myfunc(first, second: int)
        """ invoked if second is of type int but first is not """
        return second * 2