WEBVTT 1 00:00:00.550 --> 00:00:01.960 Hi guys and welcome back. 2 00:00:01.960 --> 00:00:03.800 In this video, we learn about 3 00:00:03.800 --> 00:00:06.643 default parameter values in Python. 4 00:00:07.480 --> 00:00:08.320 Let's get started. 5 00:00:08.320 --> 00:00:10.050 We've got a function here called add 6 00:00:10.050 --> 00:00:11.430 and it takes two arguments. 7 00:00:11.430 --> 00:00:14.220 So it has two parameters, x and y. 8 00:00:14.220 --> 00:00:15.053 And then it's gonna print 9 00:00:15.053 --> 00:00:16.330 the result of adding them together. 10 00:00:16.330 --> 00:00:18.780 So normally, you would call this add function 11 00:00:18.780 --> 00:00:20.200 with two arguments. 12 00:00:20.200 --> 00:00:22.020 So five comma eight. 13 00:00:22.020 --> 00:00:24.380 Gives them five and eight to x and y. 14 00:00:24.380 --> 00:00:27.010 However, you can have default values 15 00:00:27.010 --> 00:00:29.370 for one or two of these parameters 16 00:00:29.370 --> 00:00:31.420 by just saying in the function definition, 17 00:00:31.420 --> 00:00:32.763 y equals eight. 18 00:00:33.910 --> 00:00:36.780 Again, we usually don't put spaces around this equal sign 19 00:00:36.780 --> 00:00:39.130 when we use it for a default parameter value 20 00:00:39.130 --> 00:00:41.930 and what this is that x is a required parameter 21 00:00:41.930 --> 00:00:44.410 and y is now an optional parameter. 22 00:00:44.410 --> 00:00:47.520 If you don't give y a value when you call the function, 23 00:00:47.520 --> 00:00:49.080 it will use the value eight, 24 00:00:49.080 --> 00:00:50.800 which is defined here. 25 00:00:50.800 --> 00:00:53.420 So, instead of doing add five comma eight, 26 00:00:53.420 --> 00:00:55.570 you can just do add five. 27 00:00:55.570 --> 00:00:57.620 And five will become the value for x, 28 00:00:57.620 --> 00:01:00.470 since that is the first parameter and the first argument, 29 00:01:00.470 --> 00:01:02.640 and because there is no second argument, 30 00:01:02.640 --> 00:01:05.760 y will use its default value eight. 31 00:01:05.760 --> 00:01:07.520 So if we run this code here, 32 00:01:07.520 --> 00:01:10.260 you'll see that 13 comes out. 33 00:01:10.260 --> 00:01:12.870 Similarly, you can do, of course, 34 00:01:12.870 --> 00:01:15.030 x equal five, if you like, 35 00:01:15.030 --> 00:01:17.250 and still y doesn't have a value, 36 00:01:17.250 --> 00:01:19.900 so it will use the default value of eight. 37 00:01:19.900 --> 00:01:22.030 So this means exactly the same thing. 38 00:01:22.030 --> 00:01:26.190 What can't do though is do this, y equal five, 39 00:01:26.190 --> 00:01:28.700 because x is a required parameter, 40 00:01:28.700 --> 00:01:30.230 you need to give it a value, 41 00:01:30.230 --> 00:01:31.860 and here, we're not given it, 42 00:01:31.860 --> 00:01:33.550 so if we were to run this code, 43 00:01:33.550 --> 00:01:35.540 you would get the add is missing 44 00:01:35.540 --> 00:01:37.840 one required positional argument, x. 45 00:01:37.840 --> 00:01:39.633 So you need to give x a value. 46 00:01:40.640 --> 00:01:44.490 As usual, you can still do x equal five, y equal five, 47 00:01:44.490 --> 00:01:45.510 that's totally fine. 48 00:01:45.510 --> 00:01:48.660 And of course, you cannot do x equals five comma five, 49 00:01:48.660 --> 00:01:50.050 because as we learned earlier on, 50 00:01:50.050 --> 00:01:53.170 you can't put a positional argument after a keyword 51 00:01:53.170 --> 00:01:55.290 or named argument. 52 00:01:55.290 --> 00:01:57.490 Similarly, that also applies for 53 00:01:57.490 --> 00:01:59.160 the add function definition. 54 00:01:59.160 --> 00:02:02.290 You cannot have a default parameter 55 00:02:02.290 --> 00:02:05.080 be followed by a non default parameter. 56 00:02:05.080 --> 00:02:06.490 So here, it'll tell you 57 00:02:06.490 --> 00:02:09.310 that a default value must be specified here, 58 00:02:09.310 --> 00:02:12.310 in y, because you've specified one earlier on. 59 00:02:12.310 --> 00:02:15.690 So default parameter values must go at the end, 60 00:02:15.690 --> 00:02:19.943 after all the required non default parameters. 61 00:02:22.500 --> 00:02:24.310 Something to take into consideration 62 00:02:24.310 --> 00:02:25.980 when you are defining functions 63 00:02:25.980 --> 00:02:28.790 is to usually not do this. 64 00:02:28.790 --> 00:02:31.570 So here we've got a variable called default_y, 65 00:02:31.570 --> 00:02:32.970 and it's equal to three. 66 00:02:32.970 --> 00:02:34.610 And then we're defining an add function 67 00:02:34.610 --> 00:02:39.610 and making the y parameter equal to our default_y variable. 68 00:02:39.740 --> 00:02:42.220 So, y will equal three, 69 00:02:42.220 --> 00:02:45.280 and that will be the default value for the y parameter. 70 00:02:45.280 --> 00:02:48.320 Now we can call add with the value two, let's say, 71 00:02:48.320 --> 00:02:51.380 and then we do for x and three for y, 72 00:02:51.380 --> 00:02:53.520 giving us a total of five. 73 00:02:53.520 --> 00:02:58.110 However, if we then go and change default y to four, 74 00:02:58.110 --> 00:03:02.170 and then we add again, let's say add two, 75 00:03:02.170 --> 00:03:04.960 now you'll see that actually, 76 00:03:04.960 --> 00:03:07.640 we get the same value back from the function. 77 00:03:07.640 --> 00:03:10.610 So changing a variable that has been used 78 00:03:10.610 --> 00:03:12.770 as a default parameter value, 79 00:03:12.770 --> 00:03:14.630 does not modify the function. 80 00:03:14.630 --> 00:03:17.770 So this value here, this three, 81 00:03:17.770 --> 00:03:20.550 is defined when the function gets created 82 00:03:20.550 --> 00:03:22.190 and it doesn't change after that, 83 00:03:22.190 --> 00:03:24.020 if you reassign the variable. 84 00:03:24.020 --> 00:03:26.470 So this is why I don't usually recommend doing this, 85 00:03:26.470 --> 00:03:29.200 because it can get really confusing really quickly. 86 00:03:29.200 --> 00:03:32.650 So stick to putting the actual value on the function, 87 00:03:32.650 --> 00:03:34.990 unless you really have to do something different, 88 00:03:34.990 --> 00:03:36.850 and then make it clear that that value 89 00:03:36.850 --> 00:03:41.180 is not going to change after defining the function. 90 00:03:41.180 --> 00:03:42.490 So that is how you define 91 00:03:42.490 --> 00:03:44.940 default parameter values in Python. 92 00:03:44.940 --> 00:03:47.410 Just put them in the function definition, 93 00:03:47.410 --> 00:03:48.260 just make sure that they are 94 00:03:48.260 --> 00:03:50.700 at the end of the parameter lists. 95 00:03:50.700 --> 00:03:52.500 Thank you for joining me in this video 96 00:03:52.500 --> 00:03:54.150 and I'll see you in the next one.