In python, a string is a string until you add special characters.

In browsing twitter this morning I came accross this tweet, that showed that you can use is accross two strings if they do not contain special characters.

I popped open ipython to play with this. I could confirm on 3.9.7, short strings that I typed in worked as expected.

waylonwalker ↪main v3.9.7 ipython
❯ a = "asdf"

waylonwalker ↪main v3.9.7 ipython
❯ b = "asdf"

waylonwalker ↪main v3.9.7 ipython
❯ a is b
True

Using the upper() method on these strings does break down.

waylonwalker ↪main v3.9.7 ipython
❯ a.upper() is b.upper()
False

waylonwalker ↪main v3.9.7 ipython
❯ a = "ASDF"

waylonwalker ↪main v3.9.7 ipython
❯ b = "ASDF"

waylonwalker ↪main v3.9.7 ipython
❯ a is b
True

If You can also see this in the id of the objects as well, which is the memmory address in CPython.

waylonwalker ↪main v3.9.7 ipython
❯ id(a)
140717359289568

waylonwalker ↪main v3.9.7 ipython
❯ id(b)
140717359289568

waylonwalker ↪main v3.9.7 ipython
❯ id(a.upper())
140717359581824

waylonwalker ↪main v3.9.7 ipython
❯ id(b.upper())
140717360337824

Finally just as the post shows if you add a special character in there it also breaks.

waylonwalker ↪main v3.9.7 ipython
❯ a = "ASDF!"

waylonwalker ↪main v3.9.7 ipython
❯ b = "ASDF!"

waylonwalker ↪main v3.9.7 ipython
❯ a is b
False

What should you do

First and foremost, these are the exact pitfalls that flake8 guards you against. So the very first things you should take away here is that there is a lot of wisdom and value in flake8.

Second, the is comparison should be used for things that you want to compare to exact memmory addresses. These include booleans and None. Don't use is accross two assigned variables.