Mainly, because tuples are immutable. Assume the following works:
>>> l = [1, 2, 3]
>>> t = (1, 2, 3)
>>> x = {l: 'a list', t: 'a tuple'}
Now, what happens when you do l.append(4)
? You've modified the key in your dictionary! From afar! If you're familiar with how hashing algorithms work, this should frighten you. Tuples, on the other hand, are absolutely immutable. t += (1,)
might look like it's modifying the tuple, but really it's not: it simply creating a new tuple, leaving your dictionary key unchanged.
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