5.7 深入条件控制 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.

in 和 not in 比较操作符审核值是否在一个区间之内。操作符 is is not 和比较两个对象是否相同；这只和诸如链表这样的可变对象有关。所有的比较操作符具有相同的优先级，低于所有的数值操作。

Comparisons can be chained. For example, a < b == c tests whether a is less than b and moreover b equals c.

比较操作可以传递。例如 a < b == c 审核是否 a 小于 b 并 b 等于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.

比较操作可以通过逻辑操作符 and 和or组合，比较的结果可以用 not 来取反义。这些操作符的优先级又低于比较操作符，在它们之中， not 具有最高的优先级， or 优先级最低，所以

A
and not B or C

等于

(A and (not B)) or
C

。当然，表达式可以用期望的方式表示。

The Boolean operators and and or are so-called short-circuit operators: their arguments are evaluated from left to right, and evaluation stops as soon as the outcome is determined. For example, 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 short-circuit operator, when used as a general value and not as a Boolean, is the last evaluated argument.

逻辑操作符 and 和 or 也称作短路操作符：它们的参数从左向右解析，一旦结果可以确定就停止。例如，如果 A 和 C 为真而 B 为假， A and B and C 不会解析 C。作用于一个普通的非逻辑值时，短路操作符的返回值通常是最后一个变量。

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

可以把比较或其它逻辑表达式的返回值赋给一个变量，例如：

>>> 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. C programmers may grumble about this, but it avoids a common class of problems encountered in C programs: typing = in an expression when == was intended.

需要注意的是Python与C不同，在表达式内部不能赋值。C 程序员经常对此抱怨，不过它避免了一类在C程序中司空见惯的错误：想要在解析式中使 == 时误用了 = 操作符。