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Mastering Python by Rick van Hattem

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dict comprehensions

dict comprehensions are very similar to list comprehensions, but the result is a dict instead. Other than this, the only real difference is that you need to return both a key and a value, whereas a list comprehension accepts any type of value. The following is a basic example:

>>> {x: x ** 2 for x in range(10)}
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}

>>> {x: x ** 2 for x in range(10) if x % 2}
{1: 1, 3: 9, 9: 81, 5: 25, 7: 49}

Note

Since the output is a dictionary, the key needs to be hashable for the dict comprehension to work.

The funny thing is that you can mix these two, of course, for even more unreadable magic:

>>> {x ** 2: [y for y in range(x)] for x in range(5)}
{0: [], 1: [0], 4: [0, 1], 16: ...

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