Python and, or, and and-or syntax summaries _python

I. and:

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

Copy Code code as follows:

>>> ' A ' and ' B ' B ' >>> ' and ' B ' ' >>> ' a ' and ' B ' and ' C '

The value of an expression from left to right in a Boolean context, and returns the last value if all the values in the Boolean context are true.

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

Two, or:

Copy Code code as follows:

>>> ' A ' or ' B ' ' a ' >>> ' or ' B ' ' B ' >>> ' or [] or {}
{} >>> 0 or ' a ' or ' C ' ' a '
[Code]
When using or, the value is evaluated from left to right in the Boolean context, like and. If a value is true, or immediately returns the value

If all of the values are false, or the last false value is returned

Note that the or in a Boolean context will always perform an expression calculus until the first true value is found, and then the remaining comparison values are ignored

Third, And-or:

And-or combines the two preceding syntaxes, reasoning.

[Code]
>>> a= ' i ' >>> b= ' second ' >>> 1 and A or B ' I ' >>> (1 and a) or B ' a ' >& Gt;> 0 and A or B ' second ' >>> (0 and a) or B ' second ' >>>

This syntax looks similar to bool in C language. An A:B expression. The entire expression is performed from left to right, so the calculation of the and expression is performed first. The 1 and ' a ' is the value of ' a ', and then ' second ' is evaluated as ' a '.

The 0 and ' a ' calculation value is False, then the 0 or ' second ' calculus value is ' second '.

And-or is mainly used to imitate the three-mesh operator bool?a:b, that is, when the expression bool is true, take a or B.

And-or technique, bool and a or B expressions, when a value in the Boolean context is false, it is not like C-language expression bool? A:B work like that.

Iv. Safe use of and-or

Copy Code code as follows:

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

Because [a] is a non-empty list, it is never false. Even if a is 0 or "or any other false value, the list [a] is true because it has an element.

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

Copy Code code as follows:

def choose (bool,a,b): Return (bool and [a] or [b]) [0] Print choose (1, ', ' second ') # "