python function parameters should not contain mutable objects #28

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1. myagley commented on Sep 16, 2013

   

   myagley

   on Sep 16, 2013

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   We can certainly update this to reflect your example. However, we don't mutate those parameters inside the function. When do you see an issue in practice? Can you provide an example?

   Thanks!
2. myagley commented on Sep 16, 2013

   

   myagley

   on Sep 16, 2013

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   Is this blowing up in PEP8?
3. alecklandgraf commented on Sep 16, 2013

   

   alecklandgraf

   on Sep 16, 2013

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   We had some issues internally that were traced back to this where we were looping over a function and getting strange results. It's just one of python's gotchas.
   Something like this:

   def increment_list(l=[]):
       l.append(1)
       return l
   
   x = increment_list() # x = [1]
   y = increment_list() # y = x = [1, 1]
   x.append(2) # x = y = [1, 1, 2]
   z = increment_list([1]) # z = [1, 1]
   
   
   def increment_list_better(l=None):
       if l is None:
           l=[]
       l.append(1)
       return l
   
   x = increment_list_better() # x = [1]
   y = increment_list_better() # y = [1], x = [1]
   
4. myagley commented on Sep 16, 2013

   

   myagley

   on Sep 16, 2013

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   Yeah. I'm trying to figure out how urgent this is for our client. I don't think our client exhibits this problem because we don't mutate the parameters. However it should be cleaned up. We'll get it updated for the next release. Thanks for pointing it out! Appreciate it.
5. alecklandgraf commented on Sep 16, 2013

   

   alecklandgraf

   on Sep 16, 2013

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   Glad to help. Hope it saves a headache down the road.