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Python Reference Manual

Previous: 5.9 Comparisons Up: 5. Expressions Next: 5.11 Lambdas

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5.10 Boolean operations

Boolean operations have the lowest priority of all Python operations:

expression	::=	or_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
lambda_form	::=	"lambda" [parameter_list]: expression

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

The operator not yields 1 if its argument is false, 0 otherwise.  

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 0 and 1, 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 0, not ''.)

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Python Reference Manual

Previous: 5.9 Comparisons Up: 5. Expressions Next: 5.11 Lambdas

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Release 2.2.3, documentation updated on 30 May 2003.