The reason behind this is because the python lists (a in your case) implement the __iadd__ method, which in turns calls the __iter__ method on the passed parameter.
The following code snippet illustrates this better:
class MyDict(dict):
def __iter__(self):
print "__iter__ was called"
return super(MyDict, self).__iter__()
class MyList(list):
def __iadd__(self, other):
print "__iadd__ was called"
return super(MyList, self).__iadd__(other)
a = MyList(['a', 'b', 'c'])
b = MyDict((('d1', 1), ('d2', 2), ('d3', 3)))
a += b
print a
The result is:
__iadd__ was called
__iter__ was called
['a', 'b', 'c', 'd2', 'd3', 'd1']
The python interpreter checks if an object implements the __iadd__ operation (+=) and only if it doesn't it will emulate it by doing a add operation followed by an assignment.
