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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.