SyntaxError: can't assign to a function call

[Fuzzyman]

What gives ?

>>> a = []
>>> def f():
	return a

>>> f()
[]
>>> a.append(3)
>>> f()
[3]
>>> a += [3]
>>> a
[3, 3]
>>> f()
[3, 3]
>>> f() += [4]
SyntaxError: can't assign to function call
>>> 

Fuzzyman
http://www.voidspace.org.uk/python/index.shtml