Comment by th | Hacker News Reader

Comment by th | Hacker News Reader

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This one is an odd one because initially most people I show this one to assume the oddity is that immutable types cannot be mutated in-place.

But tuples can be mutated in-place: >>> b = (1,) >>> b += (2,) >>> b (1, 2)

The odd thing is that lists can be in-place added (+='d) to any iterable, while tuples can only be in-place added to tuples.

I think it works this way so that += on lists works consistently with the extend method on lists, which also accepts any iterable.

> But tuples can be mutated in-place: >>> b = (1,) >>> b += (2,) >>> b (1, 2)

That's not an in-place mutation any more than

    >>> a = 1
    >>> a += 2

is.

The oddity is that most people assume `a += b` is the same as `a = a + b`, but Python allows overloading both separately and list.__iadd__ is an alias for list.extend, hence being able to += any iterable onto a list whereas `list + non_list` generates a type error.

The original intent is an optimisation (to avoid copying the subject list) but it's a broken one as it behaves very inconsistently with regular concatenation/addition: it mutates the subject in-place and allows a different (wider) set of parameters.

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