Understanding is in Python

Every Python object has three essential properties: identity, type, and value.

The identity uniquely identifies an object. The is operator compares two objects by that identity.

None is None
# True

a = 1; b = 1
a is b
# True

a = "myX"; b = "myX";
a is b
# True

a = 1.8; b = 1.8
a is b
# False

a = 3L; b = 3L
a is b
# False

a = 1+2j; b = 1+2j
a is b
# False

a = (1, 2); b = (1, 2)
a is b
# False

a = [1, 2]; b = [1, 2]
a is b
# False

a = {"male":1}; b = {"male": 1}
a is b
# False

Full example on GitHub Gist.

Everything in Python is an object. Strings and integers are immutable objects; strings may be stored in a string intern pool, while integers may be stored in a small-integer pool. In the example above, both a and b can point to the same memory used by "myX", so their identities match and a is b is True.

Large integers are not necessarily shared. On my 64-bit Ubuntu machine, the largest integer for which this identity comparison held was 256; with 257, it returned False.

Originally published on SegmentFault.