python中字符串的真值 [英] Truth value of a string in python

问题描述

if <boolean> :
   # do this

布尔值必须为 True 或 False.

boolean has to be either True or False.

那为什么

if "poi":
   print "yes"

输出:是的

我不明白为什么是打印，因为poi"是真或假.

i didn't get why yes is printing , since "poi" is nether True or False.

推荐答案

当表达式需要布尔值时，Python 将尽最大努力评估该表达式的真实性".

Python will do its best to evaluate the "truthiness" of an expression when a boolean value is needed from that expression.

字符串的规则是空字符串被认为是False，非空字符串被认为是True.同样的规则也适用于其他容器，因此空字典或列表被视为 False，包含一个或多个条目的字典或列表被视为 True.

The rule for strings is that an empty string is considered False, a non-empty string is considered True. The same rule is imposed on other containers, so an empty dictionary or list is considered False, a dictionary or list with one or more entries is considered True.

None 对象也被认为是错误的.

The None object is also considered false.

0 的数值被认为是假的(尽管 '0' 的字符串值被认为是真).

A numerical value of 0 is considered false (although a string value of '0' is considered true).

所有其他表达式都被视为 True.

All other expressions are considered True.

详细信息(包括用户定义的类型如何指定真实性)可以在这里找到:http://docs.python.org/release/2.5.2/lib/truth.html.

Details (including how user-defined types can specify truthiness) can be found here: http://docs.python.org/release/2.5.2/lib/truth.html.

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