int() and str.is*** functions accepts other than ASCII.
But we want to accept only ASCII in some cases.  (e.g. ipaddress module)

We can use try-except to check ASCII, but it's inefficient.

try:
    s.encode('ascii')
except UnicodeEncodeError:
    ascii = False
else:
    ascii = True

CPython can check string is ASCII efficiently.

(Voting on python-ideas ML now)