1
00:00:00,000 --> 00:00:03,800
All right. Let's talk about a more advanced subject here called

2
00:00:03,800 --> 00:00:04,800
an interface.

3
00:00:04,800 --> 00:00:08,400
Now we're getting into the real nitty gritty of Python as

4
00:00:08,400 --> 00:00:09,400
a coding language.

5
00:00:09,420 --> 00:00:10,830
Interfaces are a cool thing.

6
00:00:10,840 --> 00:00:13,020
A lot of people actually don't know them. But when you're

7
00:00:13,030 --> 00:00:16,090
working with a larger framework, you're going to be using

8
00:00:16,100 --> 00:00:17,800
interfaces without even knowing about it.

9
00:00:17,840 --> 00:00:19,190
So what is an interface?

10
00:00:19,200 --> 00:00:22,500
An interface is literally just a template for your classes.

11
00:00:22,500 --> 00:00:25,900
It's actually just, it's a blueprint for designing your classes.

12
00:00:26,150 --> 00:00:28,760
So they usually have some sort of base implementation, but

13
00:00:28,770 --> 00:00:31,550
they also have a set of rules for you to implement changes

14
00:00:31,550 --> 00:00:33,299
on a new subclass.

15
00:00:34,100 --> 00:00:37,100
So let's go ahead and sort of recreate the example from our

16
00:00:37,100 --> 00:00:42,200
last video where we have 'class Animal', and we have 'def speak():

17
00:00:42,900 --> 00:00:50,200
print("Says a thing")', and then we have 'class Cat', which inherits

18
00:00:50,200 --> 00:00:54,400
from 'Animal', and doesn't need to do anything yet.

19
00:00:55,400 --> 00:00:59,300
And if we do, 'henry = Cat()',

20
00:00:59,700 --> 00:01:03,900
'henry.speak()', "Says a thing".

21
00:01:03,900 --> 00:01:06,400
But what if this is our blueprint?

22
00:01:06,459 --> 00:01:11,020
And on every 'Animal' that we subclass to, this is called a

23
00:01:11,030 --> 00:01:12,550
subclass, and this is the base class.

24
00:01:13,100 --> 00:01:16,700
What if we said on every subclass we needed to change this?

25
00:01:16,700 --> 00:01:22,400
Let's go ahead and force this with a rule by raising an exception,

26
00:01:22,400 --> 00:01:24,700
which is the thing we'll talk about in a few lessons from

27
00:01:24,700 --> 00:01:28,400
now I believe, we're going to raise an exception called

28
00:01:28,400 --> 00:01:31,200
'NotImplementedError',

29
00:01:31,200 --> 00:01:35,700
And we can give this an error that says, "Not implemented

30
00:01:35,700 --> 00:01:39,300
please add an animal sound."

31
00:01:40,600 --> 00:01:41,800
Okay, let's run that.

32
00:01:41,800 --> 00:01:46,700
Let's rerun this cell, rerun this cell, and we get the, 'NotImplementedError',

33
00:01:47,100 --> 00:01:51,100
and that's because 'speak' is going to raise an error for us

34
00:01:51,100 --> 00:01:52,800
when we run it from 'Cat'.

35
00:01:52,800 --> 00:01:56,200
Now we're likely never going to run it from 'Animal' since that's a base class,

36
00:01:56,200 --> 00:01:59,500
but in a subclass, we need this to be overwritten.

37
00:01:59,500 --> 00:02:02,000
And what this basically says is, 'That's cool.

38
00:02:02,000 --> 00:02:05,100
Yeah, there's a 'speak' class, but Hey, if you try to use it,

39
00:02:05,160 --> 00:02:06,510
it's not going to work.

40
00:02:06,510 --> 00:02:10,199
You need to overwrite this on every single subclass".

41
00:02:10,280 --> 00:02:12,259
So that's what we're going to do.

42
00:02:12,900 --> 00:02:17,200
This is now our template saying, "Hey, this needs to be implemented".

43
00:02:17,200 --> 00:02:18,900
So let's implement it.

44
00:02:18,900 --> 00:02:20,900
[no audio]

45
00:02:20,900 --> 00:02:26,500
And this is just going to say "Meow". We'll rerun these cells,

46
00:02:26,900 --> 00:02:28,900
and 'henry' now says, "Meow".

47
00:02:29,500 --> 00:02:33,200
But let's take a look at an example with a 'Dog' class.

48
00:02:33,200 --> 00:02:35,300
We've got a 'Dog', it inherits from 'Animal',

49
00:02:35,300 --> 00:02:37,300
and it's not going to do anything either.

50
00:02:38,100 --> 00:02:43,300
We have a new 'doggo', 'doggo.speak()'.

51
00:02:43,300 --> 00:02:46,400
We're going to see the same implementation raised error

52
00:02:46,400 --> 00:02:48,700
[no audio]

53
00:02:48,700 --> 00:02:51,800
where it says 'Not implemented', and 'Not implemented

54
00:02:51,800 --> 00:02:53,200
please add an animal sound".

55
00:02:54,300 --> 00:02:57,900
And that's again, because 'Dog' is a subclass of 'Animal', and

56
00:02:57,900 --> 00:03:01,400
every subclass, we're saying needs to overwrite this.

57
00:03:01,400 --> 00:03:07,400
So 'def speak()', always take 'self' because it is a method on a class.

58
00:03:08,300 --> 00:03:14,900
'print("Woof woof"). "Woof woof". Rerun those cells and it works for us.

59
00:03:14,900 --> 00:03:17,450
Now let's talk about this in here.

60
00:03:17,460 --> 00:03:18,950
This is a little bit different.

61
00:03:20,200 --> 00:03:23,600
We haven't seen the keyword 'raise' yet, but we will be seeing

62
00:03:23,600 --> 00:03:24,600
that eventually.

63
00:03:24,600 --> 00:03:28,900
And all that does is bubbles up an exception, or an error,

64
00:03:28,900 --> 00:03:30,300
or a specific type of error.

65
00:03:30,300 --> 00:03:33,900
This one comes built-in with Python, called the 'NotImplementedError',

66
00:03:33,900 --> 00:03:38,600
and it looks a lot like a function, although it's not lowercase.

67
00:03:39,400 --> 00:03:43,700
It's camel case, a lot like what our classes have been looking like lately.

68
00:03:43,780 --> 00:03:48,340
So this is actually raising a class, and its parameter, its

69
00:03:48,340 --> 00:03:50,400
first parameter, is an error message.

70
00:03:50,400 --> 00:03:53,800
And in a future lesson, we are actually going to be able

71
00:03:53,800 --> 00:03:55,900
to raise an exception like this.

72
00:03:55,900 --> 00:03:58,400
This is called an exception. So we can raise an exception

73
00:03:59,400 --> 00:04:02,900
and we can catch it so that our program doesn't just totally

74
00:04:03,000 --> 00:04:09,890
die. So, for instance, if I comment that out, save, save,

75
00:04:09,900 --> 00:04:14,280
and we get this 'NotImplementedError' on our 'Dog'. We can

76
00:04:14,290 --> 00:04:18,600
actually catch this error, and work with it instead of having it

77
00:04:18,600 --> 00:04:19,899
error out on us like this,

78
00:04:19,920 --> 00:04:23,279
because moving forward, if we have a program that errors

79
00:04:23,279 --> 00:04:25,700
out, chances are it's going to just totally die.

80
00:04:25,720 --> 00:04:28,420
It's not going to know how to handle the error, and it's

81
00:04:28,430 --> 00:04:31,720
just going to totally quit your program, which is terrible.

82
00:04:31,730 --> 00:04:33,100
You never want that.

83
00:04:33,110 --> 00:04:36,670
And so by raising an exception, we can then catch that exception

84
00:04:36,670 --> 00:04:38,500
and work with it gracefully.

85
00:04:38,560 --> 00:04:42,190
And we'll talk about raising exceptions and catching them

86
00:04:42,200 --> 00:04:43,600
a little bit later in depth.

87
00:04:43,680 --> 00:04:47,940
But what I want to talk about here was, this in particular

88
00:04:49,200 --> 00:04:52,300
is going to force us to always implement the 'speak' method

89
00:04:52,300 --> 00:04:54,100
on any subclass that we use.

90
00:04:54,140 --> 00:04:58,310
So the idea here is to not actually use the 'Animal' class,

91
00:04:58,320 --> 00:05:03,570
except to inherit from other classes, or by other classes

92
00:05:03,580 --> 00:05:06,180
rather. So we would never really use this on its own.

93
00:05:06,180 --> 00:05:08,200
It would really only show up in here.

94
00:05:08,900 --> 00:05:11,500
And this is the rule saying that, "Hey, if anytime you're going to

95
00:05:11,520 --> 00:05:14,840
make that animal speak, you actually have to implement

96
00:05:14,840 --> 00:05:17,500
something on your own. That's our blueprint. That's our guide.

97
00:05:17,500 --> 00:05:21,600
That's a perfect template for creating other classes,

98
00:05:21,600 --> 00:05:22,900
subclasses we call them.

99
00:05:22,900 --> 00:05:28,700
And that, in a nutshell, is a class interface in Python.