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Waylon Walker
Waylon Walker

Posted on • Originally published at waylonwalker.com

Python string of letters is a string of letters, but not with special

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
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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
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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
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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
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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.

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