Python note: swapping objects

The best way to swap two objects is:

a, b = b, a

It swaps the names of two object by the use of a tuple, without altering the objects.

The way to swap objects in many languages involves copying. However, this method can be tricky with Python. This is particularly because object assignment in Python does not copy objects. It simply links a variable name to the existing object. Suppose

class X(object):
__init__(self, x)
self.x = x

a = X([1, 2, 3])
b = X([])

Then we want this:

b = a
a.x = []

The above will not work, because b.x will become [] too. To avoid copying, use

a, b = b, a
a.x = []

In another occasion, if a will be kept while b becomes a copy of a's value, define a copying function or use the copy module.

With the new-style class, everything is an object. So the rule for object assignment applies to lists, dicts etc. To make a copy of the original list, use l2 = l1[:].