Question

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Python '==' vs 'is' comparing strings, 'is' fails sometimes, why?

I am going to skip the part where i tell you how i tested my code and jump straight to the problem.

Python seems to be having some problem matching split of a unicode string to another inline unicode string in an if statement.

>>>zone = u'domain.com.'
>>>zone[-1:]
u'.'

>>>u'.' is u'.' #works fine
True
>>> z[-1:] == u'.' #works fine
True
>>> zone[-1:] is u'.' # FAILS !
False

here is my actual code snippet

>>>if zone[-1:] is not u'.':
>>>    #this line will always run !

if i change ‘is not’ to != the code works fine !

Does anyone know why “is” caused the comparison to fail ?

Answer 1

It’s because strings are objects in Python — when you slice a string, you create a new one.

It’s slightly more complicated than that, but that’s the gist of it.

Solution: use == and != instead of is and is not.