Why does Python extend output [[...]]


This question already has an answer here:


The ... indicates that the list contains a recursive loop, i.e., at some level something contains itself. This is because you extended x with x, so you essentially put x inside itself.

There is no ... in the second example because y is a distinct object. Although it happens to be equal to x[0] in that both are empty lists, they are not the same empty list.

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