Why are variables shared across scopes in python when using django views

Question

Please Note this is a very Django specific question, so if you know the Django Rest Framework you will understand it

So I have a view in Django rest framework with an update function over ridden and a _has_permission helper function

def update(self, request, *args, **kwargs):
        permission, message = self._has_permission(data=request.data)
        if not permission:
            return Response({'error': message}, status=status.HTTP_403_FORBIDDEN)

        return super().update(request, *args, **kwargs)



    def _has_permission(self, data):
        """
       Resolves if the logged in user has permission to update the data given the type of data
       :param data:
       :return:
       """


        data['user_type'] = data['user_type'] if ('user_type' in data and data['user_type']) else None

        ....
        some checks
        ....
        return True, 'Has Permission'

In the case where the function _has_permission() sees that 'user_type' is not in data, it sets data['user_type'] = None in the function, but when it comes out request.data['user_type'] now exists and becomes None as well

How is this dictionary being shared across two different scopes. I thought functions have their own scopes

I don't understand your question. You're explicitly passing the dictionary from `update` to `_has_permission`. — Daniel Roseman, May 03 '16 at 15:38
Maybe i have been coding for too many hours straight, for the past 5 years i was of the belief that if you pass a variable to a function it will now be in a different scope — Dr Manhattan, May 03 '16 at 15:42
dictionary is mutable, so no, not a copy. a shallow copy would be enough here so pass `data.copy()`. — fips, May 03 '16 at 15:42
But this has nothing to do with scope. Why do you think it does? — Daniel Roseman, May 03 '16 at 15:43
Perhaps you're confusing python with PHP? https://stackoverflow.com/questions/2030906/are-arrays-in-php-passed-by-value-or-by-reference — Anonymous, May 03 '16 at 15:46

score 1 · Accepted Answer · answered May 03 '16 at 15:43

1

Contrary to your assertion, this is not at all specific to DRF, but a very general Python principle.

Passing any argument to a function always passes the object itself. If you then mutate that object in the function, all other references to that object will see that modification, because they remain references to the same object.

answered May 03 '16 at 15:43

Daniel Roseman

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i meant specific to DRF because i did not paste the whole class and only specific DRF functions – Dr Manhattan May 03 '16 at 15:52

trinchet · Answer 2 · 2016-05-03T19:34:15.923

0

By default, in Python, all args are passed by reference (or a kind of), which mean, you are passing a reference to the object, not a copy, so if you alter the object inside a function, you are affecting the object itself, not a copy of it. You can see more details at http://robertheaton.com/2014/02/09/pythons-pass-by-object-reference-as-explained-by-philip-k-dick/

edited May 03 '16 at 19:34

answered May 03 '16 at 19:19

trinchet

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Why are variables shared across scopes in python when using django views

2 Answers2