> As for `operator.concat`, any reason why the check is made beforehand to see if the first argument has a `__getitem__` method?

Concatenation is an operation of sequence (+ could be used both for concatenation and addition), so you have to check __getitem__ to make sure it's a sequence.

> Couldn't that just be removed allowing the exception from `concat(1, '')` to just propagate to the caller?

If removed, `concat(1, '')` could propagate exceptions but not `concat(1, 1)`.