Unexpected dictionary behaviour when building it rom enumerated list

Question

This is weird, first time I'd seen this behaviour when creating a dictionary from enumerated list. Let's say you have a list like this:

>>> a = [1, 8, 6, 2, 5, 4, 8, 3, 7]

>>> for x, y in enumerate(a):
    print(x, y)

Out: 
0 1
1 8
2 6
3 2
4 5
5 4
6 8
7 3
8 7

This makes sense. And so does this:

>>> foo = {x: y for x, y in enumerate(a)}
>>> foo

Out: {0: 1, 1: 8, 2: 6, 3: 2, 4: 5, 5: 4, 6: 8, 7: 3, 8: 7}

But if I switch index and value around then I get all sorts of weirdness happening:

>>> foo = {y: x for x, y in enumerate(a)}
>>> foo
Out:
{1: 0, 8: 6, 6: 2, 2: 3, 5: 4, 4: 5, 3: 7, 7: 8}

Why did index 1 disappear? Why does value 8 has index 6 instead of 1?

I don't understand why this is happening and how to correctly write a dictionary comprehension in this case?

Answer 1

Because dictionaries can not contain duplicate keys, and they preserve the last element with the same key.