In-depth analysis of python's and or return value

In Python, and and or perform Boolean logic, as you would expect, but they do not return a Boolean value; instead, they return one of the values ​​they actually compare against.

1. and:

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

evaluates the expression from left to right in a Boolean context. If all values ​​in the Boolean context are true, then and returns the last value.

If a value in the Boolean context is false, then and returns the first false value

2. or:

>>> 'a' or 'b'
'a'
>>> '' or 'b'
'b'
>>> '' or [] or {}
{}
>>> 0 or 'a' or 'c'
'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

If all values ​​are false, or returns the last false value

Note that or in a Boolean context will continue to perform expression calculations until the first true value is found. Then the remaining comparison values ​​will be ignored

3. and-or:

and-or combines the previous two syntaxes and can be reasoned.

>>> a='first'
>>> b='second'
>>> 1 and a or b
'first'
>>> (1 and a) or b
'first'
>>> 0 and a or b
'second'
>>> (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'.

and-or is mainly used to imitate the ternary operator bool?a:b, that is, when the expression bool is true, take a, otherwise take b.

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 .

4. Use and-or safely

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

A responsible programmer should encapsulate the and-or technique into a function:

def choose(bool,a,b):
    return (bool and [a] or [b])[0]
print  choose(1,'','second')    #''