Question

I just make a foo class to explain what I mean:

class Foo:
    def __init__(self, fakevalue):
        self.fv = fakevalue

    @staticmethod
    def make_a_foo():
        return Foo(2)

    def try_change_foo_value(self):
        self = Foo.make_a_foo()
        print "in change function, self.fv:", self.fv

if(__name__ =='__main__'):
    foo_instance = Foo(1)
    print "foo_instance.fv:", foo_instance.fv
    foo_instance.try_change_foo_value()
    print "foo_instance.fv:", foo_instance.fv

I expect:

foo_instance.fv: 1
in change function, self.fv: 2
foo_instance.fv: 2

But the result is:

foo_instance.fv: 1
in change function, self.fv: 2
foo_instance.fv: 1

We can see the self value has already changed, but the instance value does not.
Why? And how to solve this problem?

Answer 1

In this case, self is a pointer to the caller instance. Though you change the pointer in try_change_foo_value, it is just like changing a parameter in a function: it has no effect outside the method.

To clarify this, you can consider

a.try_change_foo_value() as shorthand for Foo.try_change_foo_value(a). This should make it obvious that when you change self, you are changing a local variable, while a, the caller, remains unchanged.

To fix it, you should do

def try_change_foo_value(self):
    self.fv = 2

but as mgilson points out, it is more Pythonic to just say foo_instance.fv = 2.

If you want to actually change the value of foo_instance (rather than just fv), you should do that elsewhere, not in a method (which makes little conceptual sense).