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:
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,
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. New in version 2.5.
x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.
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
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