Setting instance members in function - what's going on here?

Question

Consider the following code, test_class.py:

class TestClass:
    def __init__(self,Na,Nb):
        self.a = list(range(Na))
        self.b = list(range(Nb))

    def swapA(self):
        a = self.a
        b = self.b
        a,b = b,a
        assert a is self.b
        assert b is self.a

    def swapB(self):
        a = self.a
        b = self.b
        a,b = b,a
        assert a is self.b
        assert b is self.a

        #why is this necessary?
        self.a = a
        self.b = b

if __name__ == '__main__':
    tc = TestClass(2,3)
    print('len(tc.a) = %d, len(tc.b) = %d' % (len(tc.a),len(tc.b)))
    tc.swapA()
    print('after swapA(): len(tc.a) = %d, len(tc.b) = %d' % (len(tc.a),len(tc.b)))
    tc.swapB()
    print('after swapB(): len(tc.a) = %d, len(tc.b) = %d' % (len(tc.a),len(tc.b)))

The output I get from running this (python test_class.py) is:

len(tc.a) = 2, len(tc.b) = 3
after swapA(): len(tc.a) = 2, len(tc.b) = 3
after swapB(): len(tc.a) = 3, len(tc.b) = 2

I can't understand why swapA() doesn't update self.a and self.b members as I would expect.. despite the assertion passing. I would expect that the behaviour of swapA() and swapB() would be identical, but it's clearly not.

Any explanation here would be greatly appreciated.

`a` and `b` are local names. You are not rebinding `self.a` and `self.b`, `a` and `b` are **independent** references. — Martijn Pieters, Feb 07 '19 at 15:51
`swapA` doesn't ever set `self.a` or `self.b`. `a` and `self.a` may be referencing the same object, but that doesn't mean that setting `a` also alters `self.a`. — Carcigenicate, Feb 07 '19 at 15:53
The above comments explains it all. More concretely, add the 2 statements `self.a = a` and `self.b = b` in `swapA()` function. — Supratim Haldar, Feb 07 '19 at 16:01
right, sorry... dumb question! thanks for pointing out my error — science404, Feb 07 '19 at 16:01
@science404 Not a dumb question. This is something many new programmers deal with. — Carcigenicate, Feb 07 '19 at 22:36

Setting instance members in function - what's going on here?

0 Answers0