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]:          


        

Answer 1

It is now documented in Python 3.4+ and Python 2.7:

4.6.3. Mutable Sequence Types

The operations in the following table are defined on mutable sequence types. The collections.abc.MutableSequence ABC is provided to make it easier to correctly implement these operations on custom sequence types.

[Below] s is an instance of a mutable sequence type, t is any iterable object and x is an arbitrary object that meets any type and value restrictions imposed by s (for example, bytearray only accepts integers that meet the value restriction 0 <= x <= 255).

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s.extend(t) or s += t

extends s with the contents of t (for the most part the same as s[len(s):len(s)] = t)

So it is now documented that for any mutable sequence type s, s += t is synonymous with s.extend(t).