Delegation-based prototypes: implementing JavaScript semantics in Python

delegation-based-prototypes.py

###
# Explicit delegation-based prototypes implementation.
#
# Implementing JavaScript semantics.
#
# Here we show that "delegation" is just a mechanism (a pattern) for
# implementation of the "inheritance" concept. In this specific case we
# have direct inheritance from objects, skipping the "class" step.
#
# See "OO Relationships" article for some details:
# https://medium.com/@DmitrySoshnikov/oo-relationships-5020163ab162
#
# Note: Python itself is already delegation-based ;)
#
# by Dmitry Soshnikov <[email protected]>
# MIT Style License
##

# Because JS :P
undefined = None

# To bind a method at resolution
from functools import partial

class JSObject(object):
    def __init__(self, props = None):
        self.__props = props or {}

    # Implement dynamic dispatch
    def __getattr__(self, prop):

        resolved = self.__props.get(prop)

        # Resolved own prop.
        if not resolved is None:
            # In case if we resolved a function, bind
            # it to needed context, making it a method.
            if hasattr(resolved, '__call__'):
                resolved = partial(resolved, self)

            return resolved

        # Else, lookup in the prototype chain via delegation.
        __proto__ = self.__props.get('__proto__')

        # Reached base inheritance link
        if __proto__ is None:
            return undefined

        # Continue lookup.
        return __proto__.__getattr__(prop)

foo = JSObject({'x': 10, 'getX': lambda self: self.x})

bar = JSObject({'y': 20, '__proto__': foo})

print(bar.x) # 10, inherited
print(bar.y) # 20, own

print(bar.getX()) # whoo-hoo! methods work too!, 10

##
## Homework exercise: implement `set` and `delete` operations.
##

def __init__(self, props = None):
    self.__props = props or {}

I'd just use kwargs:

def __init__(self, **props):
    self.__props = props

@kevinastone, yep, kwargs is a good idea 👍