148 – Set operations with operators

Many set operations, like containment checks, can be written with Python’s operators instead of method calls. This allows for succinct but expressive code:

import string

allowed_chars = set(string.ascii_letters)

s = "I like trains."

if not (used := set(s)) <= allowed_chars:
    print("Illegal characters found:")
    print(used - allowed_chars)

# Illegal characters found:
# {' ', '.'}

The comparison used <= allowed_chars is the same as used.issubset(allowed_chars) and checks whether used is contained within allowed_chars.

The operation used - allowed_chars is the same as used.difference(allowed_chars) and computes all elements of used that are not in allowed_chars.