Comparisons,Equality in Python

All python objects also respond to comparison.

For instance, a comparison of list objects compares all their components automatically:

>>> L1 = [1, ('a', 3)]         # Same value, unique objects
>>> L2 = [1, ('a', 3)]
>>> L1 == L2, L1 is L2         # Equivalent? Same object?
(1, 0)

Here, L1 and L2 are assigned lists that are equivalent, but distinct objects. Because of the nature of Python references (studied in Chapter 4), there are two ways to test for equality:

• The == operator tests value equivalence. Python performs an equivalence test, comparing all nested objects recursively

• The is operator tests object identity. Python tests whether the two are really the same object (i.e., live at the same address in memory).

In the example, L1 and L2 pass the == test (they have equivalent values because all their components are equivalent), but fail the is check (they are two different objects, and hence two different pieces of memory). Notice what happens for short strings:

>>> S1 = 'spam'
>>> S2 = 'spam'
>>> S1 == S2, S1 is S2
(1, 1)

Here, we should have two distinct objects that happen to have the same value: == should be true, and is should be false. Because Python internally caches and reuses short strings as an optimization, there really is just a single string, 'spam', in memory, shared by S1 and S2; hence, the is identity test reports a true result. To trigger the normal behavior, we need to use longer strings that fall outside the cache mechanism:

>>> S1 = 'a longer string'
>>> S2 = 'a longer string'
>>> S1 == S2, S1 is S2
(1, 0)