variable changing despite of copy

Question

I've written code for a sudoku-solver and chase the bugs right now. There is a function "insert" that should copy the dictionary "mglwerte" and change it. But the function changes not only the copy but the original dictionary, despite using copy at the beginning of the function. Here is the call of the function:

    print(ebene, ": 748: ", mglwerte["748"], " / 779: ", mglwerte["779"])
    sudokuneu, mglwerteneu, geklappt = insert(int(var[0]), int(var[1]), wert, sudoku, mglwerte)
    print(ebene, ": 748: ", mglwerte["748"], " / 779: ", mglwerte["779"])

And the function itself:

def insert(z,s,v,sudokualt,mglwertealt):
    sudoku = sudokualt.copy()
    mglwerte = mglwertealt.copy()
    geschafft = True
    q = 3*int((z-1)/3) + int((s-1)/3) + 1
    keyfull = str(z) + str(s) + str(q)

    if v not in mglwerte[keyfull]:
        print(v, "not in ", keyfull)
        return None, None, False

    sudoku[keyfull] = v
    mglwerte[keyfull] = {}    

    for key,value in mglwerte.items():
        if len(value) > 0:
            if str(key[0]) == str(z) or str(key[1]) == str(s) or str(key[2]) == str(q):
                value.discard(v)
                mglwerte[key] = value
                if len(value) < 1:
                    geschafft = False
    return sudoku, mglwerte, geschafft

The print-statements in the first piece of code show me, that "mglwerte" is changed after the call of the function. How can I change this?

you may need `copy.deepcopy()` if your values are mutable. else you just copy the references. — Jean-François Fabre, Mar 22 '17 at 08:43
thank you for the clarification. So copy makes a new dictionary but since the values are sets the new dictionary still points to the same sets, right? — MrData, Mar 22 '17 at 08:54
exactly. you got that right. problem with `deepcopy` is when you don't want to copy everything... so in that case a custom copy is required. — Jean-François Fabre, Mar 22 '17 at 08:55

variable changing despite of copy

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