Question

These two class definitions are structured identically, except in the weird() class definition the attribute ‘d’ is a list, and in the normal() class definition its an int. I dont understand why the weird() class results in self.d = d, while this is not the case with the class normal(). Why does Python treat int and list differently in this situation?

class weird:
    def __init__(self):
        d = [1,2,3]
        self.d = d
        for x in range(3):
            d[x] = 10+x 
        print "d =", d
        print "self.d =", self.d

class normal:
    def __init__(self):
        d = 1
        self.d = d
        d = 11
        print "d =", d
        print "self.d =", self.d

And when I run the code, I get

>>> a=weird()
d = [10, 11, 12]
self.d = [10, 11, 12]
>>> b=normal()
d = 11
self.d = 1

Answer 1

You are confusing assignment with mutation of an object.

In one method, you assigned new values to d, changing what it points to:

d = 11

but in the other method you changed values contained in d:

d[x] = 10 + x

Note the [x] there; you are, in essence, telling python to do this:

d.__setitem__(x, 10 + x)

The value of the variable d, which points to a list, is never changed. The values in the list referred to by d, do change.

Take a look at this previous answer of mine, which goes into more detail as to what the difference is between these two statements: Python list doesn't reflect variable change.