Question

Say I have a string, "ab".

I want to replace "a" with "b" and "b" with "a" in one swoop.

So in the end, the string should be "ba" and not "aa" or "bb" and not use more than one line. Is this doable?

Answer 1

When you need to swap variables, say x and y, a common pattern is to introduce a temporary variable t to help with the swap: t = x; x = y; y = t.

The same pattern can also be used with strings:

>>> # swap a with b
>>> 'obama'.replace('a', '%temp%').replace('b', 'a').replace('%temp%', 'b')
'oabmb'

This technique isn’t new. It is described in PEP 378 as a way to convert between American and European style decimal separators and thousands separators (for example from 1,234,567.89 to 1.234.567,89. Guido has endorsed this as a reasonable technique.