Using and in Non-Boolean Contexts

00:00 In this lesson, you’ll see how to use an and operation separate from any if statement or while loop condition. Consider this small code segment.

00:11 The purpose of this code is to assign a Boolean value to the variable flag to tell a part of the program further along whether or not some condition was true.

00:22 It’s anticipated that this variable will be used in some other if or while statement, but right now you’re only concerned with how it gets assigned.

00:31 There are two conditions which need to be true for this variable to be assigned True.

00:37 The list in question must have at least one element in it. In other words, it can’t be empty. And the first element must contain the string "expected value".

00:47 If those two conditions are true, then you want to set the flag variable to True. Otherwise, it will retain its previously assigned value of False.

00:58 However, you don’t need to use an if statement to make this happen. You simply want flag to be the result, True or False of the and operation.

01:09 So you should write your code in just that way.

01:13 flag will be set to the truth value of length of the list is greater than 0 and element 0 has the string "expected value". If both conditions are true, then flag will have the value True. If either of those conditions are false, then flag will be False. Exactly what you wanted.

01:34 Observe also how this statement is taking advantage of short-circuit evaluation. The second expression will actually cause an error if the list is empty.

01:45 However, you test for that in the first expression. If the list is empty, then that part of the and expression is false, and Python knows the whole expression evaluates to False without needing to check the second expression.

02:01 So Python will only check the second expression if the first expression is true, meaning the list is not empty, and a check for the first element in the list won’t cause a program error.

02:14 This is an excellent use of short-circuit evaluation, so you should be sure to recall this in your own programs as needed.

02:23 Next, you’ll see more examples of using the and operator.