Case-insensitive set intersection

Question

What would be the best way to do the following case-insensitive intersection:

a1 = ['Disney', 'Fox']
a2 = ['paramount', 'fox']
a1.intersection(a2)
> ['fox']

Normally I'd do a list comprehension to convert both to all lowercased:

>>> set([_.lower() for _ in a1]).intersection(set([_.lower() for _ in a2]))
set(['fox'])

but it's a bit ugly. Is there a better way to do this?

Answer 1

Using the set comprehension syntax is slightly less ugly:

>>> {str.casefold(x) for x in a1} & {str.casefold(x) for x in a2}
{'fox'}

The algorithm is the same, and there is not any more efficient way available because the hash values of strings are case sensitive.

Using str.casefold instead of str.lower will behave more correctly for international data, and is available since Python 3.3+.