Unexpected behaviour from the 'in' operator

[Terry Reedy]

> [testing for identity instead of equality in seq]

for x in seq:
  if x is y:
    match = 1
    break
else:
   match = 0

# or

len(filter(lambda x: x is y, seq))
# does not short-circuit, counts multiple occurences

> If this is true I guess the equality operator could be optimized
> to first check for identity, and only test non-identical objects
> for equality.

For an intermediate check, compute and store a hash value with every
instance.  Test hash equlity first and only then detail equality.  For
instance, current stringobject.c/string_richcompare() uses the
equivalent of this:

def str_eq(a,b):
  return len(a)==len(b) and a[0]==b[0] and a==b

It uses length and first char as quick hashes that eliminate most
non-matches.

Terry J. Reedy