What does a colon and comma stand in a python list?

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Generally speaking:

foo[somestuff]

calls either __getitem__, or __setitem__. (there’s also __getslice__ and __setslice__, but those are now deprecated, so let’s not talk about that). Now, if somestuff has a comma in it, python will pass a tuple to the underlying function:

foo[1,2]  # passes a tuple

If there is a :, python will pass a slice:

foo[:]  # passes `slice(None, None, None)`
foo[1:2]  # passes `slice(1, 2, None)`
foo[1:2:3]  # passes `slice(1, 2, 3)
foo[1::3]  # passes `slice(1, None, 3)

Hopefully you get the idea. Now if there is a comma and a colon, python will pass a tuple which contains a slice. in your example:

foo[:, 1]  # passes the tuple `(slice(None, None, None), 1)`

What the object (foo) does with the input is entirely up to the object.

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