4. Conditionals — How To Think Like A Computer Scientist

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4.2. Boolean values and expressions¶

The Python type for storing true and false values is called bool, named after the British mathematician, George Boole. George Boole created Boolean algebra, which is the basis of all modern computer arithmetic.

There are only two boolean values: True and False. Capitalization is important, since true and false are not boolean values.

>>> type(True) <type 'bool'> >>> type(true) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'true' is not defined

A boolean expression is an expression that evaluates to a boolean value. The operator == compares two values and produces a boolean value:

>>> 5 == 5 True >>> 5 == 6 False

In the first statement, the two operands are equal, so the expression evaluates to True; in the second statement, 5 is not equal to 6, so we get False.

The == operator is one of the comparison operators; the others are:

x != y # x is not equal to y x > y # x is greater than y x < y # x is less than y x >= y # x is greater than or equal to y x <= y # x is less than or equal to y

Although these operations are probably familiar to you, the Python symbols are different from the mathematical symbols. A common error is to use a single equal sign (=) instead of a double equal sign (==). Remember that = is an assignment operator and == is a comparison operator. Also, there is no such thing as =< or =>.

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