1 00:00:00,000 --> 00:00:02,990 As you are writing code in Python, there are a lot of opportunities for you to 2 00:00:03,000 --> 00:00:04,990 use various expressions. 3 00:00:05,000 --> 00:00:08,990 And some of these expressions might have a number of operators and a number of factors. 4 00:00:09,000 --> 00:00:12,990 For example, let's say that you have an expression that looks something like 5 00:00:13,000 --> 00:00:18,990 this: 5 * 25 + 14 / 2. 6 00:00:19,000 --> 00:00:21,990 And you might look at an expression like that, maybe you didn't code it 7 00:00:22,000 --> 00:00:23,990 yourself, maybe somebody else did, 8 00:00:24,000 --> 00:00:28,990 and you are looking at some code and you see that and you say, hmm, is that 5 9 00:00:29,000 --> 00:00:35,990 times the sum of 25 + 14, with the result of that divided by 2, or is it 5 times 10 00:00:36,000 --> 00:00:42,990 25 plus the result of 14 divided by 2, or some other combination? 11 00:00:43,000 --> 00:00:47,990 The answer to this question lies in the subject of operator precedence. 12 00:00:48,000 --> 00:00:51,990 When there are a number of different operators in one expression, which 13 00:00:52,000 --> 00:00:54,990 operations get evaluated first? 14 00:00:55,000 --> 00:01:00,990 And the results of that operation would then be used in operating on the other operators. 15 00:01:01,000 --> 00:01:06,990 So in this case, I happened to know, because I looked it up, that multiplication 16 00:01:07,000 --> 00:01:11,990 and division have a higher precedence than addition and subtraction. 17 00:01:12,000 --> 00:01:16,990 And so I know that the first thing that's going to happen is 5 * 25 will be 18 00:01:17,000 --> 00:01:23,990 evaluated, and then 14 / 2 will be evaluated, and those two results will be added 19 00:01:24,000 --> 00:01:30,990 together, giving us the answer of 132, because 5 * 25 is 125, and 14 / 2 is 7. 20 00:01:31,000 --> 00:01:35,990 125 + 7 is 132. 21 00:01:36,000 --> 00:01:39,990 If we want a different result, obviously we could use parenthesis. 22 00:01:40,000 --> 00:01:47,990 We could say 5 * and put in parenthesis here 25 + 14 / 2, and we can get 23 00:01:48,000 --> 00:01:49,990 that different result. 24 00:01:50,000 --> 00:01:54,990 I strongly suggest when you are writing your own code that you use the 25 00:01:55,000 --> 00:01:58,990 parentheses to explicitly say what it is that you mean to say, 26 00:01:59,000 --> 00:02:03,990 that you don't write bare expressions with a lot of different operators like 27 00:02:04,000 --> 00:02:07,990 the first one here in this example, that you just don't do that. 28 00:02:08,000 --> 00:02:10,990 If you are reading other people's code and you see that and you want to know 29 00:02:11,000 --> 00:02:14,990 what it is that they meant or at least what the result is that they got, then 30 00:02:15,000 --> 00:02:18,990 you can refer to the Operator Precedence chart, and I've provided one handy in 31 00:02:19,000 --> 00:02:20,990 your exercise files. 32 00:02:21,000 --> 00:02:23,990 Let's take a look at what operator precedence looks like in Python. 33 00:02:24,000 --> 00:02:26,990 There is no need for you to memorize this, but it's good for you to become 34 00:02:27,000 --> 00:02:31,990 at least familiar with its existence and know where to find it when you need to look it up. 35 00:02:32,000 --> 00:02:37,990 In this chart, you'll see there's three columns: Associativity, Operators, and Description. 36 00:02:38,000 --> 00:02:42,990 Associativity means which way is it evaluated, if you have several of these 37 00:02:43,000 --> 00:02:44,990 things from this column? 38 00:02:45,000 --> 00:02:48,990 So for example, if you have several Boolean or operators, they'll be 39 00:02:49,000 --> 00:02:50,990 evaluated left to right. 40 00:02:51,000 --> 00:02:55,990 If you have several Comparison operators, they'll be evaluated right to left. 41 00:02:56,000 --> 00:02:58,990 More often than not, they are left to right in Python. 42 00:02:59,000 --> 00:03:02,990 The designers of Python have done a really good job of trying to make this stuff 43 00:03:03,000 --> 00:03:03,990 consistent as they possibly can. 44 00:03:04,000 --> 00:03:07,990 And where it's not consistent, it's because it makes better sense that way. 45 00:03:08,000 --> 00:03:12,990 Starting at the top of the list, we have lambda, and then we have or, and, not. 46 00:03:13,000 --> 00:03:14,990 These are the Boolean, or, and, and not. 47 00:03:15,000 --> 00:03:19,990 Then we have all of the Comparison operators lumped together in one priority, and 48 00:03:20,000 --> 00:03:21,990 those are evaluated right to left. 49 00:03:22,000 --> 00:03:28,990 And then continuing, there is the Bitwise operators or, exclusive or, and and. 50 00:03:29,000 --> 00:03:32,990 Then there is the Bitwise shift operators, and then we have addition and 51 00:03:33,000 --> 00:03:37,990 subtraction, multiplication, division, and remainder, the Unary operators, 52 00:03:38,000 --> 00:03:39,990 the Exponent operator, 53 00:03:40,000 --> 00:03:44,990 and then bundled together and right to left again slices, function calls, and 54 00:03:45,000 --> 00:03:49,990 attribute references, and then finally, left to right the Binding operator, 55 00:03:50,000 --> 00:03:54,990 which is the parentheses when it's not meant to be a tuple and then tuple, 56 00:03:55,000 --> 00:03:56,990 lists and dictionaries. 57 00:03:57,000 --> 00:03:58,990 So this is operator precedence in Python. 58 00:03:59,000 --> 00:04:04,990 Again, it's much better for you to not rely on this, to use parentheses to 59 00:04:05,000 --> 00:04:08,990 say exactly what it is that you mean to say when you are writing complex expressions. 60 00:04:09,000 --> 00:04:12,990 When you are reading other people's code that have not been that kind to you, 61 00:04:13,000 --> 00:04:16,990 it's good to know that this table exists and where to find it, so that you can 62 00:04:17,000 --> 00:04:27,000 read that code and know exactly how it gets evaluated.