[Python-Dev] chained assignment weirdity

Chris Withers chris at simplistix.co.uk
Tue Nov 6 07:18:07 CET 2012

Hi All,

I bumped into this using Michael Foord's Mock library.
It feels like a bug to me, but thought I'd ask here before logging one 
in the tracker in case people know that we won't be able to fix it:

On 05/11/2012 13:43, Michael Foord wrote:
>>>>>> class Foo(object):
>>> ...  def __setattr__(s, k, v):
>>> ...   print k
>>> ...
>>>>>> f = Foo()
>>>>>> f.g = f.x = 3
>>> g
>>> x
>> Hmm, that's definitely counter-intuitive. Worth raising on python-dev?
> Well, I guess "it's always been that way", so changing it would be backwards incompatible. Feel free to raise it though. :-)

Here's the actual problem I had:

>>>>>>> mock = Mock()
>>>>>>> inst = mock.Popen.return_value = mock.Popen_instance = Mock(spec=Popen)
>>> Here the *return_value* is set first. So when mock.Popen_instance is assigned it already has a parent! (Setting a mock as the return value of another mock also establishes the child / parent relationship.)
>>> This is why calls to the instance are then not recorded in "method_calls".



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