Question

Its been a couple of days since I started learning python, at which point I stumbled across the == and is. Coming from a java background I assumed == does a comparison by object id and is by value, however doing

 >>> a = (1,2)
 >>> b = (1,2)
 >>> a is b
 False
 >>> a == b
 True

Seems like is is equivalent of java’s == and python’s == is equivalent to java’s equals(). Is this the right way to think about the difference between is and ==? Or is there a caveat?

Answer 1

is checks that both operands are the same object. == calls __eq__() on the left operand, passing the right. Normally this method implements equality comparison, but it is possible to write a class that uses it for other purposes (but it never should).

Note that is and == will give the same results for certain objects (string literals, integers between -1 and 256 inclusive) on some implementations, but that does not mean that the operators should be considered substitutable in those situations.