$ python3 -m unittest test_totp -v
test_rfc4226 (test_totp.Published.test_rfc4226) ... ok
test_rfc6238 (test_totp.Published.test_rfc6238) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.000s

OK

$ ./mutation_check.sh
13c13
<     num = struct.unpack(">I", mac[off:off + 4])[0] & 0x7FFFFFFF
---
>     num = struct.unpack(">I", mac[off:off + 4])[0] & 0x7FFFFFFE
----------------------------------------------------------------------
Ran 2 tests in 0.002s

FAILED (failures=9)
OK: broken copy was caught (tests red)

$ TOTP_SECRET=JBSWY3DPEHPK3PXP python3 totp.py   # made-up secret
483418

$ python3 totp.py   # no secret set
error: set TOTP_SECRET (base32 secret) in the environment
