Any reason not to use ‘+’ to concatenate two strings?

There is nothing wrong in concatenating two strings with +. Indeed it’s easier to read than ''.join([a, b]).

You are right though that concatenating more than 2 strings with + is an O(n^2) operation (compared to O(n) for join) and thus becomes inefficient. However this has not to do with using a loop. Even a + b + c + ... is O(n^2), the reason being that each concatenation produces a new string.

CPython2.4 and above try to mitigate that, but it’s still advisable to use join when concatenating more than 2 strings.

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