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Date: Fri, 27 May 2011 10:42:16 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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To: python-list@python.org
Subject: Re: Python's super() considered super!
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Duncan Booth wrote:
> Steven D'Aprano <steve+comp.lang.python@pearwood.info> wrote:
> 
>> I was thrilled to learn a new trick, popping keyword arguments before 
>> calling super, and wondered why I hadn't thought of that myself. How on 
>> earth did I fail to realise that a kwarg dict was mutable and therefore 
>> you can remove keyword args, or inject new ones in?
>>
> Probably because most of the time it is better to avoid mutating kwargs. 
> Instead of popping an argument you simply declare it as a named argument in 
> the method signature. Instead of injecting new ones you can pass them as 
> named arguments.
> 
> 
> def foo(x=None, **kwargs):
> 	bar(y=2, **kwargs)
> 
> 	
> def bar(**kwargs):
> 	print(kwargs)
> 
>>>> foo(x=1, z=3)
> {'y': 2, 'z': 3}
>>>> foo(x=1, y=2, z=3)
> Traceback (most recent call last):
>   File "<pyshell#8>", line 1, in <module>
>     foo(x=1, y=2, z=3)
>   File "<pyshell#4>", line 2, in foo
>     bar(y=2, **kwargs)
> TypeError: bar() got multiple values for keyword argument 'y'

And the above error is exactly why you don't want to use named arguments 
in MI -- because you don't know in what order the methods will be 
called, you cannot know which named arguments to supply to the method 
that super() will call next.

~Ethan~