operators - Is the behaviour of Python's list += iterable documented anywhere?

Question

It would appear that in Python, list += x works for any iterable x:

In [6]: l = []

In [7]: l += [1]

In [8]: l += (2, 3)

In [9]: l += xrange(5)

In [10]: l
Out[10]: [1, 2, 3, 0, 1, 2, 3, 4]

Is this behaviour documented anywhere?

To contrast this with list + x, the latter only works if x is also a list. This is spelled out in the documentation.

Answer 1

From Guido van Rossum:

It works the same way as .extend() except that it also returns self. I can't find docs explaining this. :-(

Here is the relevant source code taken from listobject.c:

list_inplace_concat(PyListObject *self, PyObject *other)
{
     PyObject *result;

     result = listextend(self, other);
     if (result == NULL)
         return result;
     Py_DECREF(result);
     Py_INCREF(self);
     return (PyObject *)self;
}

I've raised a bug report to have the documentation fixed: http://bugs.python.org/issue16701