python dictionary passed as an input to a function acts like a global in that function rather than a local

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Python’s “parameter evaluation strategy” acts a bit different than the languages you’re probably used to. Instead of having explicit call by value and call by reference semantics, python has call by sharing. You are essentially always passing the object itself, and the object’s mutability determines whether or not it can be modified. Lists and Dicts are mutable objects. Numbers, Strings, and Tuples are not.

You are passing the dictionary to the function, not a copy. Thus when you modify it, you are also modifying the original copy.

To avoid this, you should first copy the dictionary before calling the function, or from within the function (passing the dictionary to the dict function should do it, i.e. testfun4(dict(d)) and defining the function as def testfun4(d):).

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