Dictionary creation with fromkeys and mutable objects. A surprise [duplicate]

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Your Python 2.6 example is equivalent to the following, which may help to clarify:

>>> a = []
>>> xs = dict.fromkeys(range(2), a)

Each entry in the resulting dictionary will have a reference to the same object. The effects of mutating that object will be visible through every dict entry, as you’ve seen, because it’s one object.

>>> xs[0] is a and xs[1] is a
True

Use a dict comprehension, or if you’re stuck on Python 2.6 or older and you don’t have dictionary comprehensions, you can get the dict comprehension behavior by using dict() with a generator expression:

xs = dict((i, []) for i in range(2))

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