conventions/requirements for 'is' vs '==', 'not vs '!=', etc

[Terry Reedy]

"Duncan Booth" <duncan.booth at invalid.invalid> wrote in message 
news:Xns9AA470833CCA8duncanbooth at 127.0.0.1...
| In short, two equal strings may or may not be identical and any code 
which
| makes assumptions based on observed behaviour is broken. If you want to 
be
| able to test identity on strings safely then use the 'intern()' builtin 
to
| get repeatable behaviour.

Intern() is gone in 3.0