Why does the output for following code differs in python,
>>> A = [1,2,3]
>>> B = A
>>> B += [4]
>>> print A,B
Output: A = [1,2,3,4] , B = [1,2,3,4]
But if we replace B += [4] to B = B + [4] the output changes to:
>>> A = [1,2,3] , B = [1,2,3,4]
Please explain.
+= is augmented addition; for mutable types it alters the object in place.
You would get the same effect if you did:
B.extend([4])
Assigning B to A does not create a copy, so B and A refer to the same object, and any changes to that object are visible through both references.
When, however you use B = B + [4] you create a new list object, and rebind B to that new object. Because a new list object is created, the original list object (previously referenced by B and still referenced by A) is unaffected.
In effect, by using straight concatenation, you created a shallow copy of the original list object.