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Comparisons: Normal and Chained

So far, we’ve been dealing with standard numeric operations (addition and multiplication), but numbers can also be compared. Normal comparisons work for numbers exactly as you’d expect—they compare the relative magnitudes of their operands and return a Boolean result (which we would normally test in a larger statement):

>>> 1 < 2                  # Less than
True
>>> 2.0 >= 1               # Greater than or equal: mixed-type 1 converted to 1.0
True
>>> 2.0 == 2.0             # Equal value
True
>>> 2.0 != 2.0             # Not equal value
False

Notice again how mixed types are allowed in numeric expressions (only); in the second test here, Python compares values in terms of the more complex type, float.

Interestingly, Python also allows us to chain multiple comparisons together to perform range tests. Chained comparisons are a sort of shorthand for larger Boolean expressions. In short, Python lets us string together magnitude comparison tests to code chained comparisons such as range tests. The expression (A < B < C), for instance, tests whether B is between A and C; it is equivalent to the Boolean test (A < B and B < C) but is easier on the eyes (and the keyboard). For example, assume the following assignments:

>>> X = 2
>>> Y = 4
>>> Z = 6

The following two expressions have identical effects, but the first is shorter to type, and it may run slightly faster since Python needs to evaluate Y only once:

>>> X < Y < Z              # Chained comparisons: range tests
True
>>> X < Y and Y < Z
True

The same equivalence holds for false results, and arbitrary chain lengths are allowed:

>>> X < Y > Z
False
>>> X < Y and Y > Z
False

>>> 1 < 2 < 3.0 < 4
True
>>> 1 > 2 > 3.0 > 4
False

You can use other comparisons in chained tests, but the resulting expressions can become nonintuitive unless you evaluate them the way Python does. The following, for instance, is false just because 1 is not equal to 2:

>>> 1 == 2 < 3        # Same as: 1 == 2 and 2 < 3
False                 # Not same as: False < 3 (which means 0 < 3, which is true)

Python does not compare the 1 == 2 False result to 3—this would technically mean the same as 0 < 3, which would be True (as we’ll see later in this chapter, True and False are just customized 1 and 0).