Python dictionary doesn't have all the keys assigned, or items

Question

I created the following dictionary

exDict = {True: 0, False: 1, 1: \'a\', 2: \'b\'}

and when I print exDict.keys(), well, it g

Answer 1

If you insert a key-value pair in a dict python checks if the key already exists and if it exists it will replace the current value.

This check does something like this:

def hash_and_value_equal(key1, key2):
    return hash(key1) == hash(key2) and key1 == key2

So not only must the values be equal but also their hash. Unfortunatly for you True and 1 but also False and 0 will be considered equal keys:

>>> hash_and_value_equal(0, False)
True

>>> hash_and_value_equal(1, True)
True

and therefore they replace the value (but not the key):

>>> a = {1: 0}
>>> a[True] = 2
>>> a
{1: 2}

>>> a = {False: 0}
>>> a[0] = 2
>>> a
{False: 2}

I've showed the case of adding a key manually but the steps taken are the same when using the dict literal:

>>> a = {False: 0, 0: 2}
>>> a
{False: 2}

or the dict-builtin:

>>> a = dict(((0, 0), (False, 2)))
>>> a
{0: 2}

---

This can be very important if you write own classes and want to use them as potential keys inside dictionaries. Depending on your implementation of __eq__ and __hash__ these will and won't replace the values of equal but not identical keys:

class IntContainer(object):
    def __init__(self, value):
        self.value = value

    def __eq__(self, other):
        return self.value == other

    def __hash__(self):
        # Just offsetting the hash is enough because it also checks equality
        return hash(1 + self.value)

>>> hash_equal(1, IntContainer(1))
False

>>> hash_equal(2, IntContainer(1))
False

So these won't replace existing integer keys:

>>> a = {1: 2, IntContainer(1): 3, 2: 4}
>>> a
{1: 2, <__main__.IntContainer at 0x1ee1258fe80>: 3, 2: 4}

or something that is considered as identical key:

class AnotherIntContainer(IntContainer):
    def __hash__(self):
        # Not offsetted hash (collides with integer)
        return hash(self.value)

>>> hash_and_value_equal(1, AnotherIntContainer(1))
True

These will now replace the integer keys:

>>> a = {1: 2, AnotherIntContainer(1): 5}
>>> a
{1: 5}

The only really important thing is to keep in mind that dictionary keys are consered equal if the objects and their hash is equal.