Compare two Python strings with hyphen/dash

Question

I am trying to compare to strings in Python and noticed that when a dash/hyphen is present in the string it will not equate identical strings. For example:

>>>teststring = 'newstring'
>>>teststring is 'newstring'
True

Then, if I add a dash

>>>teststring = 'new-string'
>>>teststring is 'new-string'
False

Why is that the case, and what would be the best way to compare strings with dashes?

Answer 1

you should never use is to compare equality anyway. is tests for identity. Use == .

Frankly I don't know why 'newstring' is 'newstring' . I'm sure it varies based on your Python implementation as it seems like a memory-saving cache to re-use short strings.

However:

teststring = 'newstring'
teststring == 'newstring' # True

nextstring = 'new-string'
nextstring == 'new-string' # True

basically all is does is test id s to make sure they're identical.

id('new-string') # 48441808
id('new-string') # 48435352
# These change
id('newstring') # 48441728
id('newstring') # 48441728
# These don't, and I don't know why.

Answer 2

You should not use is for string comparison. Is checks if both objects are same. You should use equality operator == here. That compares the values of objects, rather than ids of objects.

In this case, looks like Python is doing some object optimizations for string objects and hence the behavior.

>>> teststring = 'newstring'
>>> id(teststring)
4329009776
>>> id('newstring')
4329009776
>>> teststring = 'new-string'
>>> id(teststring)
4329009840
>>> id('new-string')
4329009776
>>> teststring == 'new-string'
True
>>> teststring is 'new-string'
False