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Using or With Common Objects

00:00 Let me go back and take a look at the last slide from your last lesson. I want to phrase this observation in a slightly different way. If I’m taking the or of <exp1> or <exp2> and exp1 is True, the result of the or operation is exp1.

00:21 True in both cases, exp1 is the result. If exp1 is False, then the result is exp2, whatever exp2’s value was.

00:38 Now, in this lesson, we’re going to take a look at how you can use the or operation with common objects in Python. Python allows you to use the word or with things other than Boolean expressions.

00:55 Some things that you need to know: All objects in Python have a truth value and most of them are True. A few of them are False. Items that have values of the keyword None or False are considered False, numbers of any type that have a value of 0 will be considered False, and empty collections—strings, sequences, tuples, et cetera—all of those evaluate to False.

01:23 And if you have a class that implements a method .__bool__() that returns False or has a method .__len__() which returns 0 for an object, then that object also will evaluate to False. But otherwise, an object is going to evaluate to True.

01:42 When you use the or operator in Python between non-Boolean expressions, the result is going to be one of the two expressions. It doesn’t convert everything and return a True or False, which some other languages do.

01:59 If you perform an or between two objects in Python, the result will be one of those two objects, and the rule is the same as the last observation that I made: If the first object evaluates to be True, the or operation returns the first object.

02:20 If the first object evaluates to be False, the or operation returns the second object.

02:29 And so with that in mind, we can actually perform or operations on things that aren’t Boolean expressions. So, for example, I can take the or of 3 or 7.

02:44 3 is True, and so we know the or operation is going to give us the first object, which is 3.

02:52 If I say empty string, "" or 19, here we have the first operand is False, and so we get back the second operand.

03:06 If I have an empty list or an empty tuple, because the first expression is False, I get the second expression back.

03:18 Let’s see, which case haven’t I done? I’ve not done where the first one is True, 18, or the second one is False. We get the first one back.

03:31 So again, an object can have a truth value associated with it. If we use objects in an or expression, then if the first object evaluates to be True, we get the first object.

03:44 If the first object evaluates to be False, we get the second object.

03:50 In your next lesson, we will take a look at what happens when you mix Boolean expressions with objects in an or statement.

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