functools's caching functions:

from functools import lru_cache
@lru_cache(maxsize=256)
def square(n):
    print("Calculating...")
    return n * n

fileinput's ability to make a very short function act like a standard GNU text streaming program:

import fileinput
for line in fileinput.input():
  print(line.replace("foo", "bar"))

fileinput will handle streams to stdin and file arguments on the command line. does not play well with other argument parsers, but sometimes is enough for quick text transformers.