Class property that contains a dictionary

Question

Could you please explain, why v2 doesn't contain {"A": 1, "B": 2, "C": 3} and v3 does?

class MyClass:

    def foo(self):

        v1 = self.d.get('A')
        print(v1)
        # 1

        v2 = self.d.update({"C": 3})
        print(v2)
        # None

        v3 = self.d
        v3.update({"C": 3})
        print(v3)
        # {'A': 1, 'B': 2, 'C': 3}

    @property
    def d(self):
        return {"A": 1, "B": 2}

mc = MyClass()
mc.foo()

`dict.update` is an *in-place* operation, it doesn't return anything (well, it returns `None`). If you just `print(self.d)` *before* creating `v3` you'll see `self.d["C"]` is already `3`. Canonical duplicate: https://stackoverflow.com/q/1452995/3001761. — jonrsharpe, Aug 06 '18 at 17:11
@jonrsharpe Thank you for the reply! I completely forgot about it. — Konstantin, Aug 06 '18 at 17:15

score 1 · Accepted Answer · answered Aug 06 '18 at 17:16

1

self.d.update({"C": 3}) is a void function. It returns a None value. In your case, v2 is being re-assigned to None.

answered Aug 06 '18 at 17:16

Alan Raso

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Class property that contains a dictionary

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