Question

Why do I get this result at the REPL?

>>> a = [1, 2, 3]
>>> b = a
>>> b is a
True

My understanding is that is only evaluates to True if the two variables point to the same object, when in this case they’re different objects with the same value. == would evaluate to True, but is shouldn’t.

Going further:

>>> b.reverse()
>>> (a, b)
([3, 2, 1], [3, 2, 1])

It seems that as far as the interpreter is concerned, they are the same object, and operations on b will automatically be performed on a. Again, why?

Answer 1

When you do a = [1, 2, 3] you’re binding the name a to a list object. When you do b = a, you’re binding the name b to whatever a is – in this case the list object. Ergo, they’re the same… An object can have multiple names. It’s worth reading up on the Python Data Model.

If you wanted to make a copy of your listobj, then you can look at b = a[:] to use slice to create a shallow copy, or copy.copy for a shallow copy (should work on arbitary objects), or copy.deepcopy for strangely – a deep copy.

You’ll also notice something surprising in CPython which caches short strings/small integers…

>>> a = 4534534
>>> b = a
>>> a is b
True
>>> b = 4534534
>>> a is b
False
>>> a = 1
>>> b = a
>>> a is b
True
>>> b = 1
>>> a is b
True