Python FTW: Under the Hood/

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Disorder Within Order

Disorder Within Order

We'll cover the following...

How can Python dictionaries be chaotic? They were supposed to keep the order. Let’s see what’s going on.

Python 3.5
Saved
from collections import OrderedDict
dictionary = dict()
dictionary[1] = 'a'; dictionary[2] = 'b';
ordered_dict = OrderedDict()
ordered_dict[1] = 'a'; ordered_dict[2] = 'b';
another_ordered_dict = OrderedDict() # same key-values, different order
another_ordered_dict[2] = 'b'; another_ordered_dict[1] = 'a';
class DictWithHash(dict):
    """
    A dict that also implements __hash__ magic.
    """
    __hash__ = lambda self: 0
class OrderedDictWithHash(OrderedDict):
    """
    An OrderedDict that also implements __hash__ magic.
    """
    __hash__ = lambda self: 0
print(dictionary == ordered_dict) # If a == b
print(dictionary == another_ordered_dict) # and b == c
print(ordered_dict == another_ordered_dict) # then why isn't c == a ??

Recall that a set consists of only unique elements. Let’s try making a set of these dictionaries and see what happens.

C++
Saved
len({dictionary, ordered_dict, another_ordered_dict})

It makes sense, since dict and OrderedDict don’t have __hash__ implemented let’s use our wrapper classes DictWithHash and OrderedDictWithHash. ...