python-比较字符串和布尔值 [英] python - compare string with boolean

问题描述

我遇到了python将字符串与True / False进行比较的奇怪行为。

I came across a strange behaviour of python comparing a string with True/False.

我认为python将在以下内容中打印：

I thought that python would print in the following:

if "Test" == True:
    print("Hello1")

但事实并非如此。
所以我写了一些测试用例，但我听不懂。

but it does not. So I wrote some Test cases and I do not understand some of them.

if "Test" == True:
    print("Hello1")

if "Test" == False:
    print("Hello2")

#This I understand
if bool("Test") == True:
    print("Hello3")

#This I understand too    
if bool("") == False:
    print("Hello4")

if "Test":
    print("Hello5")

输出

>> Hello3
>> Hello4
>> Hello5

所以我不明白：

• 如果没有打印Hello1，为什么还没有打印Hello2？


• 为什么打印Hello5，是否将强制转换为bool（ Test）的内容？

推荐答案

在前两个比较中，您正在检查字符串 测试 与对象 True 或 False 具有相同的值。这是一个值比较。

In the first two comparisons, you are checking whether the string "Test" has the same value as the object True or False. This is a value comparison.

如果它们的类型不同，则比较将返回 False 。在比较列表，数字等时，您也可以看到以下内容： [1] == 1 （false），（1，）== [1 ] （false）。

If they have a different type, the comparison will return False. You can see this also when comparing lists, numbers etc.: [1]==1 (false), (1,)==[1] (false).

在第三和第四次比较中，您仍在进行值比较，但是由于双方相同类型（布尔值），它将比较这些值。

In the third and fourth comparisons, you are still doing a value comparison, but since both sides are of the same type (boolean), it will compare the values.

Hello5 因为不是空字符串 。您可以通过尝试 Test！= None 看到此结果，它返回 True 。

Hello5 is printed because it is not the null string "". You can see this by trying "Test" != None, which returns True.

虽然与大多数课程的 None 相比（ None 是Python的null值），将Python的标准数据类型与其 null值进行比较，即：

While it is a comparison to None when it comes to most classes(None is Python's null value), Python's standard data types are compared to their "null" value, which are:

• 空字符串 表示字符串，


• [] 表示列表（对于元组，类似性（），对于字典， {} ），


• 0 用于整数和浮点数，

• The empty string "" for strings,
• [] for lists (similary () for tuples, {} for dictionaries),
• 0 for ints and floats,

就像布尔比较一样。因此，将 if expression 视为对 if bool（expression）的隐式转换是正确的。

just like a boolean comparison. Therefore it is not wrong to think of if expression as an implicit cast to if bool(expression).

内部进行的是对 __非零__ （python2.x）或 __ bool __ （python3.x）类的方法。

What is going on under the hood is the evaluation of the __non-zero__(python2.x) or __bool__(python3.x) method of the class.

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