Playtime in the Dunders

These are materials to accompany my PyOhio talk on improving ones python skills through having fun with the double underscore (dunder) special python methods.

CarKeys

The misfit class:

import random

class CarKey:
    """
    Gets lost easily
    """
    def __hash__(self):
        return random.randint(1, 2)

Some ideas:

• Try making a dictionary keyed by CarKey objects.
  • See what happens when you assign to keys, retrieve values
• Look at sets that contain CarKey objects
  • Add a CarKey to the set multiple times.
• Try changing the CarKey hash to return either 1 or 9.
  • As of CPython 3.7.0, dictionaries start with eight hash buckets. Try adding the 1-9 CarKey to a dictionary. Then observe the change in behavior when more keys are added.
  • For the truly curious, you can see how buckets are selected from the hash from the CPython source code comments (as of this writing)

ChurchInt

The correctly adding version:

def square(x):
    return x*x

def yolo(s):
    return s + 'YOLO'


class ChurchInt(int):
    """
    A Church Number
    """
    def __call__(self, fun):

        def new_fun(arg):
            for idx in range(self):
                arg = fun(arg)
            return arg

        return new_fun

    def __add__(self, other):
        return ChurchInt(super().__add__(other))

Addition at work:

>>> three = ChurchInt(3)
>>> four = ChurchInt(4)
>>> seven = three + four
>>> y7 = seven(yolo)
>>> y7('spam')
'spamYOLOYOLOYOLOYOLOYOLOYOLOYOLO'

• Try adding other mathematical operations.
  • Once you implement multiplication you can even do things like square(three)(square)(2)!

NotDeadYet

“Resurrecting” version of the object:

limbo = []

class NotDeadYet:
    """
    Not quite dead
    """
    def __del__(self):
        print("I'm not dead yet!")
        limbo.append(self)

• Try making the object and deleting all references to it.

Add an init method with a self reference:

    def __init__(self):
        self.ref = self

Now the object will need to wait on the garbage collector for destruction due to the circular reference. You can do import gc; gc.collect() to make the garbage collector run.