Question

… the is keyword that can be used for equality in strings.

>>> s = 'str'
>>> s is 'str'
True
>>> s is 'st'
False

I tried both __is__() and __eq__() but they didn’t work.

>>> class MyString:
...   def __init__(self):
...     self.s = 'string'
...   def __is__(self, s):
...     return self.s == s
...
>>>
>>>
>>> m = MyString()
>>> m is 'ss'
False
>>> m is 'string' # <--- Expected to work
False
>>>
>>> class MyString:
...   def __init__(self):
...     self.s = 'string'
...   def __eq__(self, s):
...     return self.s == s
...
>>>
>>> m = MyString()
>>> m is 'ss'
False
>>> m is 'string' # <--- Expected to work, but again failed
False
>>>

Answer 1

Testing strings with is only works when the strings are interned. Unless you really know what you’re doing and explicitly interned the strings you should never use is on strings.

is tests for identity, not equality. That means Python simply compares the memory address a object resides in. is basically answers the question “Do I have two names for the same object?” – overloading that would make no sense.

For example, ("a" * 100) is ("a" * 100) is False. Usually Python writes each string into a different memory location, interning mostly happens for string literals.