is will return
True if two variables point to the same object (in memory),
== if the objects referred to by the variables are equal.
>>> a = [1, 2, 3] >>> b = a >>> b is a True >>> b == a True # Make a new copy of list `a` via the slice operator, # and assign it to variable `b` >>> b = a[:] >>> b is a False >>> b == a True
In your case, the second test only works because Python caches small integer objects, which is an implementation detail. For larger integers, this does not work:
>>> 1000 is 10**3 False >>> 1000 == 10**3 True
The same holds true for string literals:
>>> "a" is "a" True >>> "aa" is "a" * 2 True >>> x = "a" >>> "aa" is x * 2 False >>> "aa" is intern(x*2) True
Please see this question as well.