5. 表达式 Expressions

This chapter explains the meaning of the elements of expressions in Python.

本章描述了Python中表达式的组成元素的含义。

Syntax Notes: In this and the following chapters, extended BNFnotation will be used to describe syntax, not lexical analysis. When (one alternative of) a syntax rule has the form

句法注意：在本章和之后的章节中，描述句法时使用与词法分析时不同的扩展BNF记法。当某个句法规则（可能是可选的）具有如下形式

name

::=

othername

and no semantics are given, the semantics of this form of name are the same as for othername.

并且未给出特定语义时，name的这种形式规则的意义就是其与othername含义相同。

5.1 数值型间的转换 Arithmetic conversions

When a description of an arithmetic operator below uses the phrase ``the numeric arguments are converted to a common type,'' the arguments are coerced using the coercion rules listed at the end of chapter 3. If both arguments are standard numeric types, the following coercions are applied:

当用以下短语"数值型参数转换为通用类型"描述数值型操作数时, 参数使用第三章中结尾处的强制规则进行强制转换.如果两个参数都属于标准数值型的, 就使用以下的强制规则:

• If either argument is a complex number, the other is converted to complex; 如果其中一个参数是复数, 另一个也要转换成复数;
• otherwise, if either argument is a floating point number, the other is converted to floating point; 否则, 如果其中一个参数是浮点数, 另一个也要转换成浮点数;
• otherwise, if either argument is a long integer, the other is converted to long integer; 否则, 如果其中一个参数是长整数, 另一个也要转换成长整数;
• otherwise, both must be plain integers and no conversion is necessary. 否则, 两个都是普通整数, 不需要转换.

Some additional rules apply for certain operators (e.g., a string left argument to the `%' operator). Extensions can define their own coercions.

对于某些运算符有特殊的规则(例如, "