Question

Found interesting thing in Python (2.7) that never mentioned before.

This:

a = []
a += "a"

does work and result is:

>>> a
>>> ["a"]

But

a = []
a = a + "a"

gives

>>> TypeError: can only concatenate list (not "str") to list

Can someone explain why? Thanks for your answers.

Answer 1

Python distinguishes between the + and += operators and provides separate hooks for these; __add__ and __iadd__. The list() type simply provides a different implementation for the latter.

It is more efficient for lists to implement these separately; __add__ has to return a completely new list, while __iadd__ can just extend self then return self.

In the C code, __iadd__ is implemented by list_inplace_concat(), which simply calls listextend(), or, in python code, [].extend(). The latter takes any sequence, by design.

The __add__ method on the other hand, represented in C by list_concat, only takes a list as input, probably for efficiency’s sake; it can loop directly over the internal C array and copy items over to the new list.

In conclusion, the reason __iadd__ accepts any sequence is because when PEP 203 (the Augmented Add proposal) was implemented, for lists it was simplest just to reuse the .extend() method.