Relational Expressions

The test expressions used in Python are relational expressions, and more specifically Boolean expressions. Relational expressions test the relation between quantities and produce a Boolean value, either True or False*. Python provides the relational operators

<        <=        ==        !=        >=        >

The two on either end should be easy to interpret. They are, respectively, less than (<), less than or equal to (<=), greater than or equal to (>=) and greater than (>). The two in the center are not as easily recognized. The first, ==, tests for equality. Why not just use a single equals sign? Because the equals sign, =, is already taken: remember that = is the assignment operator. The second expression, !=, is used to test for inequality (or "not equals"). Here are some examples of simple relational expressions.

x < 27.8
age >= 65
temp != max

Try them out in the Python shell.

Python also provides logical operators that can be used to build compound relational expressions, usually referred to as Boolean expressions. The logical operators are,

and
or
not

An and relation is only True if both its arguments are True. An or relation is True as long as at least one of its arguments is True. The not operator negates its argument. Consider the following example:

age >= 20 and age < 30

This expression will only be True if the value of the variable age is greater than or equal to 20 and it is also less than 30. Thus the entire expression is True when, for example, age is 24, but False when it is 12 (because this makes the first relation False), and also when it is 62 (because this makes the second relation False).

age >= 20 or income < 18.45

This expression is True when either the value of age is greater than or equal to 20, or the value of income is less than 18.45, or both.

not tested

This expression is True when the value stored in tested is False, and False when the value stored in tested is True.

* The values True and False are Boolean constants in Python in the same way that 1 and -5 are numerical constants. Note however that while there are an infinite number of numerical constants there are exactly two Boolean constants: True and False.