use __contains__, index or split instead of str.find str.find return -1 when the substring is not found, -1 evaluate to True and is a valid index, which can lead to bugs. Using alternatives when possible makes the code clearer and less prone to bugs. (and __contains__ is faster in microbenchmarks)

# should print nothing
# should print ? b/x
? b/x
# should print A b/x
A b/x
# should forget b/x
forgetting b/x
# should print nothing