5.5 More on Conditions

The conditions used in while and if statements above can contain other operators besides comparisons.

The comparison operators in and not in check whether a value occurs (does not occur) in a sequence. The operators is and is not compare whether two objects are really the same object; this only matters for mutable objects like lists. All comparison operators have the same priority, which is lower than that of all numerical operators.

Comparisons can be chained: e.g., a < b == c tests whether a is less than b and moreover b equals c.

Comparisons may be combined by the Boolean operators and and or, and the outcome of a comparison (or of any other Boolean expression) may be negated with not. These all have lower priorities than comparison operators again; between them, not has the highest priority, and or the lowest, so that A and not B or C is equivalent to (A and (not B)) or C. Of course, parentheses can be used to express the desired composition.

The Boolean operators and and or are so-called shortcut operators: their arguments are evaluated from left to right, and evaluation stops as soon as the outcome is determined. E.g., if A and C are true but B is false, A and B and C does not evaluate the expression C. In general, the return value of a shortcut operator, when used as a general value and not as a Boolean, is the last evaluated argument.

It is possible to assign the result of a comparison or other Boolean expression to a variable. For example,

>>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
>>> non_null = string1 or string2 or string3
>>> non_null
'Trondheim'

Note that in Python, unlike C, assignment cannot occur inside expressions.