A function which receives a string and returns a value of ourselves different symbols, without register.

Question

f=lambda s='abcA'.lower(): len(set(s))

#It returned 4, but it is necessary to 3.

Answer 1

Solved.
Answer is:
f=lambda s='abcA'.lower(): len(set(map(str.casefold, s)))

Answer 2

Maybe it can be made in other way?