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Python Language Reference Manual
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 120 pages
ISBN 0954161785
RRP £12.95 ($19.95)

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5.10 Boolean operations

Boolean operations have the lowest priority of all Python operations:

expression or_test [if or_test else test]| lambda_form
or_test and_test| or_test "or" and_test
and_test not_test| and_test "and" not_test
not_test comparison | "not" not_test

In the context of Boolean operations, and also when expressions are used by control flow statements, the following values are interpreted as false: False, None, numeric zero of all types, and empty strings and containers (including strings, tuples, lists, dictionaries, sets and frozensets). All other values are interpreted as true.

The operator not yields True if its argument is false, False otherwise.

The expression x if C else y first evaluates C (not x); if C is true, x is evaluated and its value is returned; otherwise, y is evaluated and its value is returned. (Added in Python version 2.5)

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

The expression x or y first evaluates x; if x is true, its value is returned; otherwise, y is evaluated and the resulting value is returned.

(Note that neither and nor or restrict the value and type they return to False and True, but rather return the last evaluated argument. This is sometimes useful, e.g., if s is a string that should be replaced by a default value if it is empty, the expression s or 'foo' yields the desired value. Because not has to invent a value anyway, it does not bother to return a value of the same type as its argument, so e.g., not 'foo' yields False, not ".)

ISBN 0954161785Python Language Reference ManualSee the print edition