assignment order

Variable assignment in a chained "=" goes left-to-right.

>>> a = [None]

>>> a[0][0] = a  # fails, as expected

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

TypeError: 'NoneType' object has no attribute '__getitem__'

>>> a[0] = a[0][0] = a  # succeeds

The second form is equivalent to:

>>> a[0] = a

>>> a[0][0] = a

With a bit of code we can explicitly see that the assignments are happening in left-to-right order:

>>> class Squawker(object):

...    def __init__(self, name): self.name = name

...    def __setitem__(self, key, val): print self.name

...

>>> Squawker("a")[0] = Squawker("b")[0] = 1

a

b