Question

In Python, is there any difference (semantics, efficiency, etc.) between writing x = x+1 and x += 1?

Answer 1

Yes. Depending on how the class of x is coded, the short form has the option to modify x in-place, instead of creating a new object representing the sum and rebinding it back to the same name. This has an implication if you have multiple variables all referring to the same object – eg, with lists:

>>> a = b = []
>>> a += [5]
>>> a
[5]
>>> b
[5]
>>> a = a + [5]
>>> a
[5, 5]
>>> b
[5]

This happens because behind the scenes, the operators call different magic methods: + calls __add__ or __radd__ (which are expected not to modify either of their arguments) and += tries __iadd__ (which is allowed to modify self if it feels like it) before falling back to the + logic if __iadd__ isn’t there.