Python's Logical Operator AND

问题

I\'m a little confused with the results I\'m getting with the logical operators in Python. I\'m a beginner and studying with the use of a few books, but they don\'t explain in as much detail as I\'d like.

here is my own code:

five = 5
two = 2

print five and two

>> 2

It seems to be just outputting the two variable.

five = 5
two = 2
zero = 0

print five and two and zero

So, I added another variable integer. Then I printed and got the following output:

>> 0

What is going on with Python in the background? Why isn\'t the output something like 7 or 5, 2.

回答1:

Python Boolean operators return the last value evaluated, not True/False. The docs have a good explanation of this:

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

回答2:

As a bit of a side note: (i don't have enough rep for a comment) The AND operator is not needed for printing multiple variables. You can simply separate variable names with commas such as print five, two instead of print five AND two. You can also use escapes to add variables to a print line such as print "the var five is equal to: %s" %five. More on that here: http://docs.python.org/2/library/re.html#simulating-scanf

Like others have said AND is a logical operator and used to string together multiple conditions, such as

if (five == 5) AND (two == 2):
    print five, two

回答3:

Boolean And operators will return the first value 5 if the expression evaluated is false, and the second value 2 if the expression evaluated is true. Because 5 and 2 are both real, non-false, and non-null values, the expression is evaluated to true.

If you wanted to print both variables you could concatenate them to a String and print that.

five = 5
two = 2
print five + " and " + two

Or to print their sum you could use

print five + two

This document explains how to use the logical Boolean operators.

回答4:

This AND in Python is an equivalent of the && in Java for instance. This doesn't mean the and in the English language. The AND is a logical operator. Assume five holds 5 and two holds 2. From Python documentation: The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned. Basically, it evaluates the last integer in your case which is true.

if (five and two):
...     print "True"
... else:
...     print "False"

The AND is a logical operator, to test the logic for a specific case, not an arithmetic operator. If you want to get results like 7 for five and two, you should rather use "+" which means adding two integers. See below:

>>> five = 5
>>> two = 2
>>> print five + two
7

回答5:

Try 0 and 9.

The result is 0 because the value of 0 is false. The operand on the left of the and operator is False so the whole expression is False and returns 0

回答6:

In Python any non-zero integer value is true; zero is false.

The OP’s values are both non-zero.

The AND operator tests from left to right,

with and, if all values are True, returns the last evaluated value. If any value is false, returns the first one.

Since both are non-zero, both are true, so the last value is returned

来源：https://stackoverflow.com/questions/18195322/pythons-logical-operator-and