Detailed introduction to the actual usage of and and or in Python

Special properties of and and or

InPython, and and or perform Boolean logic operations, but they do not return a Boolean value; instead, Returns one of the values ​​they actually compare to. Let’s take a look at an example.

>>> 'a' and 'b'        
'b'
>>> '' and 'b'         
''
>>> 'a' and 'b' and 'c'
'c'

When using and, the value of the expression is evaluated from left to right in a Boolean context. 0, '', [], (), {}, None are false in a Boolean context; anything else is true. Well, almost everything. By default, class instances in a Boolean context evaluate to true, but you can define specific methods in the class that cause class instances to evaluate to false. You'll learn about classes and these special methods in Chapter 5. If all values ​​in the boolean context are true, then and returns the last value. In this example, the and calculus evaluates 'a' to true, then 'b' to true, and finally returns 'b'.
If a value in the Boolean context is false, then and returns the first false value. In this example, '' is the first false value.
All values ​​are true, so and returns the last true value, 'c'.

>>> 'a' or 'b'         
'a'
>>> '' or 'b'          
'b'
>>> '' or [] or {}     
{}
>>> def sidefx():
...     print "in sidefx()"
...     return 1
>>> 'a' or sidefx()    
'a'

When using or, values ​​are evaluated from left to right in a Boolean context, just like and. If a value is true, or returns that value immediately. In this case, 'a' is the first truth value.
or evaluates '' to false, then evaluates 'b' to true, and returns 'b' .
If all values ​​are false, or returns the last false value. or evaluates '' to false, then evaluates [] to false, then evaluates {} to false, and finally returns {}.
Note that or in a Boolean context will continue to evaluate the expression until the first true value is found, and then ignore the remaining comparison values. This property is important if some values ​​have side effects. Here, Function sidefx will never be called, because the or operation evaluates 'a' to be true, so it returns 'a' immediately.

If you are a C language hacker, you must be familiar with the bool ? a : b expression. If bool is true, the expression evaluates to a, otherwise it is b. Based on the way and and or work in Python, you can accomplish the same thing.

Example . Introduction to and-or skills

>>> a = "first"
>>> b = "second"
>>> 1 and a or b
'first'
>>> 0 and a or b
'second'

This syntax looks similar to the bool ? a : b expression in C language. The entire expression is evaluated from left to right, so the and expression is evaluated first. 1 and 'first' evaluate to 'first', then 'first' or 'second' evaluate to 'first'.
0 and 'first' evaluate to False, then 0 or 'second' evaluate to 'second'.

However, since this Python expression only performs Boolean logical operations and is not a specific component of the language, this is the and-or technique and the bool ? a : b syntax in C language is very important s difference. If a is false, the expression does not work as you expect. (Do you know that I have been troubled by this problem? More than once?)

Example. The situation where the and-or technique is invalid

>>> a = ""
>>>b = "second"
>>> 1 and a or b        
'second'

Because a is an empty character String , the empty string is considered false in Python's boolean context, 1 and '' evaluate to '', and finally '' or 'second' evaluates to 'second'. oh! This value is not what you want.

and-or trick, the bool and a or b expression, when a evaluates to false in a boolean context, does not work like the C language expression bool ? a : b .

The real trick behind the and-or trick is to ensure that the value of a is never false. The most common way is to make a [a] and b [b], and then use the first element of the return value list, which should be one of a or b.

Example: SafetyUse the and-or technique

>>> a = ""
>>> b = "second"
>>> (1 and [a] or [b])[0]
''

Since [a] is a non-empty list, it will never be false. Even if a is 0 or '' or some other false value, list [a] is true because it has one element.

By now, this tip may seem more problematic than it's worth. After all, the same thing can be accomplished using an if statement, so why go through all the trouble? Well, in many cases you'll be choosing between two constant values, and since you know a will always evaluate to true, you can use this simpler syntax and not worry. There are still good reasons for using more sophisticated forms of security. For example, if statements are not allowed in certain situations in the Python language, such as in lambda functions.

Note: The statements here are interpreted from left to right. We can understand it this way.
and statement:
0and＊ There is no need to consider whether * is 0 or 1, the result is 0
1and＊ You need to consider whether * is 0 or 1 to determine the result.

1or＊ There is no need to consider the following *, the result is 1
0or＊ You need to consider the following * to determine the result

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