difference in binding between strings and tuples?

[Terry Reedy]

"Iwan van der Kleyn" <null at null.com> wrote in message
news:3ec151a7$0$137$e4fe514c at dreader4.news.xs4all.nl...
>  >>> a = 1
>  >>> b = 1
>  >>> a is b
1

You are generalizing too quickly.  Consider
>>> a = 1000
>>> b = 1000
>>> a is b
0

>  >>> x = 'test'
>  >>> y = 'test'
>  >>> x is y
> 1

This is a version dependent internal implementation optimization.

>  >>> x += 's'
>  >>> x == y
> 1

No, 'tests' != 'test'!  You left something out.

>  >>> q = (0, 1)
>  >>> r = (0, 1)
>  >>> q is r
> 0

Again, implentation choice.

> Ok? Testing for equality gives no suprises, but testing for identity
> does, especially considering the differences between strings and
tuples
> (both of which are immutable)

Whether the implementation merges duplicate immutables or not is an
internal optimization matter.  The main use for 'is' is 'x is None'
instead of 'x == None' which is dependable because None is explicitly
a singleton -- like the empty set in set theory.  The next most common
use is for newbies to confuse themselves with insufficient data ;-)
(You are not the first to only test a=1; b=1 and stop there with
ints.)

As for your other questions, 'immutable += something' is (in the
absence of side effects) an abbreviation for 'immutable = immutable +
something'.  The name is rebound to a new object.  However list += seq
abbreviates list.extend(seq) to modify in place.

Terry J. Reedy