WEBVTT

1
00:00:01.939 --> 00:00:05.950
so there's several different programming
paradigms and up until now in this

2
00:00:05.950 --> 00:00:09.440
course we've been mainly using the
imperative style of programming

3
00:00:09.440 --> 00:00:14.119
imperative programming involves
providing a series of instructions for

4
00:00:14.119 --> 00:00:18.510
the computer to follow in a defined
order now object oriented programming

5
00:00:18.510 --> 00:00:24.430
aims to combine data and the processes that
act on that data into objects which is

6
00:00:24.430 --> 00:00:26.460
called encapsulation

7
00:00:26.460 --> 00:00:30.390
before I continue it's important to
realize that the different paradigms are

8
00:00:30.390 --> 00:00:34.550
not necessarily exclusive and
object-oriented programming makes use of

9
00:00:34.550 --> 00:00:38.429
imperative programming within the
methods that objects used to manipulate

10
00:00:38.429 --> 00:00:44.440
the data also as we will see shortly our
imperative programming in Python has

11
00:00:44.440 --> 00:00:49.530
actually made extensive use of objects
we just didn't know that so the approach that

12
00:00:49.530 --> 00:00:54.569
we've been using so far in the course is
very similar to a recipe for cooking a

13
00:00:54.569 --> 00:00:58.710
meal so the recipe starts with the
ingredients and utensils it will be need

14
00:00:58.710 --> 00:01:03.479
which corresponds to a programs data
and then continues with the description of

15
00:01:03.479 --> 00:01:07.890
the steps that must be performed on that
data to produce in this case for the

16
00:01:07.890 --> 00:01:08.630
recipe

17
00:01:08.630 --> 00:01:13.490
finished meal an object oriented
approach relies on the objects such as

18
00:01:13.490 --> 00:01:18.630
the eggs milk spoon etc knowing
how to perform certain operations so

19
00:01:18.630 --> 00:01:21.010
the program just tells them to do so

20
00:01:21.010 --> 00:01:26.000
concept of an egg that knows how to fry
itself is a little bizarre but that

21
00:01:26.000 --> 00:01:29.560
really shows that the imperatives
approach to cooking a meal is more

22
00:01:29.560 --> 00:01:32.560
appropriate than an object-oriented
approach

23
00:01:33.280 --> 00:01:37.270
now a good example of the encapsulation of
data and function might be self lighting

24
00:01:37.270 --> 00:01:40.680
cigarettes well actually its a
bad example and I certainly wouldn't

25
00:01:40.680 --> 00:01:45.210
want to suggest that smoking is good but
it does demonstrate the concept quite well

26
00:01:45.210 --> 00:01:50.090
so without object oriented programming
the steps to smoking a cigarette might

27
00:01:50.090 --> 00:01:54.910
be place cigarette in mouth that's number
one number two light match number three

28
00:01:54.910 --> 00:02:00.020
hold match to cigarette and number four
inhale now encapsulating the functions

29
00:02:00.020 --> 00:02:04.830
that can be performed on the cigarette
into the cigarette reduces the code to

30
00:02:04.830 --> 00:02:10.600
place cigarette in mouth and inhale so
the cigarette knows how to light itself so

31
00:02:10.600 --> 00:02:13.560
that's not something that the smoker
needs to be concerned with

32
00:02:13.560 --> 00:02:18.330
the details of how it lights itself are
built into it when it's manufactured or

33
00:02:18.330 --> 00:02:21.980
in the case of programming it's
programmed into it when it's defined

34
00:02:21.980 --> 00:02:26.620
and the actual implementation is of
no interest to anything that uses the

35
00:02:26.620 --> 00:02:30.849
object now I've exercise a little license
they're self lighting cigarettes were

36
00:02:30.849 --> 00:02:35.360
produced for short time the 1960's
coincidentally around the same time that

37
00:02:35.360 --> 00:02:39.650
object oriented programming was
conceived and had a igniter

38
00:02:39.650 --> 00:02:44.680
built into them that had to be struck like a match
the advanced type that I described only

39
00:02:44.680 --> 00:02:49.750
existed in science fiction stories now
modern electronic cigarettes take this

40
00:02:49.750 --> 00:02:53.160
concept a little bit further but it's
only the details of the implementation

41
00:02:53.160 --> 00:02:57.930
that have changed so the method use
remains pretty much the same as the

42
00:02:57.930 --> 00:03:03.060
science-fiction variety now that's important
concept of object oriented

43
00:03:03.060 --> 00:03:07.400
programming as we will see replacing one
object with another one that performs

44
00:03:07.400 --> 00:03:11.700
the same task without having to concern
ourselves with the details of how it

45
00:03:11.700 --> 00:03:17.220
performs the task is central to object
oriented programming so another and

46
00:03:17.220 --> 00:03:21.549
hopefully much more healthier example
might be an electric kettle now the

47
00:03:21.549 --> 00:03:26.350
imperative approach to boiling water
for hot drink might be step 1 fill kettle with

48
00:03:26.350 --> 00:03:26.940
water

49
00:03:26.940 --> 00:03:33.739
step 2 light stove step 3 place kettle on stove step 4 for monitor kettle until

50
00:03:33.739 --> 00:03:38.340
water boils step 5 make the drink now it would then have to be a function that

51
00:03:38.340 --> 00:03:41.989
deals with lighting the stove which might
involve further functions to collect

52
00:03:41.989 --> 00:03:47.150
water or coal or was switched on the gas a
function to light the match and so on but

53
00:03:47.150 --> 00:03:51.829
in object orientated terms a kettle would have a
boil method that is triggered by

54
00:03:51.829 --> 00:03:56.269
operating a switch so the object
oriented program comes down to step

55
00:03:56.269 --> 00:03:57.690
one fill kettle with water

56
00:03:57.690 --> 00:04:04.000
step 2 switch kettle on step 3 when kettle switches off make drink so the

57
00:04:04.000 --> 00:04:07.400
imperative approach will also have to
identify the type of stove being used as

58
00:04:07.400 --> 00:04:11.660
its futile and probably a bit dangerous to hold a match to electric cooker

59
00:04:11.660 --> 00:04:16.410
now these examples may make it look like
we've got something for nothing because

60
00:04:16.410 --> 00:04:20.280
performing the tasks using an object
oriented approach involves fewer steps

61
00:04:20.280 --> 00:04:25.340
but there's no magic involved here now the
steps are still had to be programmed

62
00:04:25.340 --> 00:04:29.650
but their built into the objects
so that anything that using the object doesn't have to be

63
00:04:29.650 --> 00:04:34.340
concerned about them so the object orientated kettle has been built to contain an

64
00:04:34.340 --> 00:04:38.320
element or whatever method it employs could
be a ceramic disc or even a small

65
00:04:38.320 --> 00:04:42.970
nuclear reactor and the operation of
the heating mechanism has been wired up to

66
00:04:42.970 --> 00:04:47.550
the switch where the kettle was made so a lot
of work has gone into the production on

67
00:04:47.550 --> 00:04:51.740
the kettle in other words far more so that
the work that goes into making a kettle

68
00:04:51.740 --> 00:04:55.740
for use on the stove but subsequent use
of the kettle is there much easier

69
00:04:55.740 --> 00:05:00.389
because it knows how to perform its
function itself so one thing that may

70
00:05:00.389 --> 00:05:06.030
not be obvious and I haven't yet mentioned is that
everything in Python is an object so Python

71
00:05:06.030 --> 00:05:11.750
is unusual in this respect because even though Python uses objects extensively you

72
00:05:11.750 --> 00:05:15.850
don't actually have to be aware of that
to use so Python can be used for

73
00:05:15.850 --> 00:05:20.389
object-oriented programming but can also
be used to write purely imperative

74
00:05:20.389 --> 00:05:25.720
programming code so in fact the
Wikipedia entry for Python which I'm going to just load

75
00:05:25.720 --> 00:05:33.690
up the screen now you can see the link
in the top left hand corner you can see

76
00:05:33.690 --> 00:05:38.960
just down here it says that Python
supports multiple programming paradigms

77
00:05:38.960 --> 00:05:43.020
including object oriented imperative and
functional programming or procedural

78
00:05:43.020 --> 00:05:48.700
styles so in Java which is considered a
class-based object-oriented language

79
00:05:48.700 --> 00:05:51.139
there are primitive types such as an

80
00:05:51.139 --> 00:05:55.440
int and a double so if you've come to Python
from Java the following code which I'm

81
00:05:55.440 --> 00:06:01.419
about to show might seem very odd at
first so lets just go back to IntelliJ and I've created a

82
00:06:01.419 --> 00:06:06.720
new project here and a new Python file so if
you come back come from Java type in

83
00:06:06.720 --> 00:06:17.090
code like this....

84
00:06:17.090 --> 00:06:26.520
....so if we just run this see that it does

85
00:06:26.520 --> 00:06:34.440
work and we get the same result each time so line 3...and

86
00:06:34.440 --> 00:06:37.560
that's pretty normal for a programming
language like Java

87
00:06:37.560 --> 00:06:44.010
but the code on line 4 would probably be a bit bizarre and that's because the a even though

88
00:06:44.010 --> 00:06:48.430
it's been defined as you can see on line
12 as a number you could see that a has

89
00:06:48.430 --> 00:06:52.550
got an add method even though its an integer so if I command click add

90
00:06:53.580 --> 00:06:58.910
and that is control click on a PC or Linux you can
see that there's a method this is the

91
00:06:58.910 --> 00:07:03.970
built-ins definition of the add method and
interestingly enough if I go back to oop

92
00:07:03.970 --> 00:07:10.700
our file and click on the plus note that it has gone to the same method in the built ins

93
00:07:10.700 --> 00:07:15.570
so what you can see by that is that whether
you use this __add__

94
00:07:15.570 --> 00:07:20.139
or use a plus it's calling
the same code that is built-in

95
00:07:20.139 --> 00:07:25.760
builtins.py file and in this case add
two numbers together and smart enough to

96
00:07:25.760 --> 00:07:29.810
figure out the parameters you can see
there that the second argument in the

97
00:07:29.810 --> 00:07:35.110
case of line 3 is then added and line 4
the parameters b is added to the value

98
00:07:35.110 --> 00:07:39.940
of a now if this is your first
programming language and either you see

99
00:07:39.940 --> 00:07:44.020
nothing at all strange about this
or it will make very little sense but don't worry

100
00:07:44.020 --> 00:07:48.450
I'm gonna explain what it all means but
I wanted to warn you that you know if

101
00:07:48.450 --> 00:07:53.060
you've come from other programming
languages such as Java or C++ you

102
00:07:53.060 --> 00:07:56.820
probably gonna see some aspects of Python that seem surprising to you at

103
00:07:56.820 --> 00:08:00.580
least initially now when I said that
everything in Python is an object I meant

104
00:08:00.580 --> 00:08:05.710
everything even types are
implemented as classes now I will come

105
00:08:05.710 --> 00:08:09.470
back to that but for now I'm gonna focus
on making sense of what I've just done

106
00:08:09.470 --> 00:08:15.620
their so object-oriented programming uses
classes and methods to provide object

107
00:08:15.620 --> 00:08:20.680
that encapsulates both data and the
functions that operate on that Data now the

108
00:08:20.680 --> 00:08:24.400
method is just another word for function
and you have seen me use those terms

109
00:08:24.400 --> 00:08:29.310
interchangeably in this course but being big
correct here when a function is part of

110
00:08:29.310 --> 00:08:33.729
a class in Python we call it a method
now in Python there is a slight

111
00:08:33.729 --> 00:08:37.690
difference between a function and a method but writing a method is the same

112
00:08:37.690 --> 00:08:42.039
as writing functions now what we gonna
do is start with a very simple example

113
00:08:42.039 --> 00:08:47.430
to introduce the concept of a class and
show how classes work in Python now this

114
00:08:47.430 --> 00:08:50.290
example be a simple implementation of
the electric

115
00:08:50.290 --> 00:08:54.639
kettle so let's go ahead and do that's
so close the run window and I'm just gonna

116
00:08:54.639 --> 00:09:01.750
delete this code and we start by creating
a kettle class and its gonna model an

117
00:09:01.750 --> 00:09:02.880
electric kettle

118
00:09:02.880 --> 00:09:13.329
so.....and notice how its has been indented for us

119
00:09:13.329 --> 00:09:16.329
automatically and we're going to create a
method so....

120
00:09:17.509 --> 00:09:25.819
....

121
00:09:25.819 --> 00:09:28.690
....will talk about this shortly...

122
00:09:28.690 --> 00:09:35.860
we just type in the code for now so...

123
00:09:35.860 --> 00:09:38.860
....

124
00:09:39.769 --> 00:09:46.440
...so looking at this it can be helpful to
think of a class as a template from

125
00:09:46.440 --> 00:09:51.529
which objects can be created so when we
create objects of this kettle class they

126
00:09:51.529 --> 00:09:56.660
all have a name and a price now they won't
have the same name nor the same price

127
00:09:56.660 --> 00:10:02.769
each instance of the class will have its
own values for name and price so classes

128
00:10:02.769 --> 00:10:07.029
as I said is a template from which objects
can be created and all the objects

129
00:10:07.029 --> 00:10:12.040
created from the same class will share
the same characteristics now an instance

130
00:10:12.040 --> 00:10:18.139
is just another name for an object created from a
class definition so if I now create a

131
00:10:18.139 --> 00:10:23.850
kettle called Kenwood then Kenwood will be an
instance of the kettle class you can also

132
00:10:23.850 --> 00:10:28.810
say that kenwood is an object of
type kettle so I'm gonna create a couple of

133
00:10:28.810 --> 00:10:33.420
kettle objects so we can see this in
practice so let's continue typing and

134
00:10:33.420 --> 00:10:41.560
we will go to line 8 here and type...

135
00:10:41.560 --> 00:11:08.320
....

136
00:11:08.320 --> 00:11:14.690
.....

137
00:11:14.690 --> 00:11:19.140
.....and lets make another kettle...

138
00:11:19.140 --> 00:11:31.660
...and I should really have two lines there

139
00:11:31.660 --> 00:11:36.070
separating the class definition to be
consistent with our code so the code on

140
00:11:36.070 --> 00:11:41.660
line 9 creates an instance of the kettle
class and we've given it the name

141
00:11:41.660 --> 00:11:46.120
kenwood and other instances is created on line 16 and this time it's called the

142
00:11:46.120 --> 00:11:51.600
Hamilton so each instance has its own
values for naming and price and their access

143
00:11:51.600 --> 00:11:56.570
by using . annotations so we type in Kenwood.price or Hamilton.make for

144
00:11:56.570 --> 00:12:02.610
argument sake to access that information
so a good analogy here is for plans for

145
00:12:02.610 --> 00:12:07.250
building a house so the plan itself just
describes what the house will look like

146
00:12:07.250 --> 00:12:11.710
its size how many rooms it will have etc
now this is similar

147
00:12:11.710 --> 00:12:16.430
the plan to the class which is just a
template for creating objects once you

148
00:12:16.430 --> 00:12:20.310
have a plan you can then create it as many
houses as you want based on that

149
00:12:20.310 --> 00:12:25.540
plan and the same is true of classes
once a class has been defined like how

150
00:12:25.540 --> 00:12:30.000
kettle class that we define on line one
we can then create as many instances of that

151
00:12:30.000 --> 00:12:34.710
classes as we want now just as you can't do
anything with the house plan other than

152
00:12:34.710 --> 00:12:38.700
build houses based on it you can't
really do anything with the class other

153
00:12:38.700 --> 00:12:43.730
than create instances of it once the
instances have been created you can then call

154
00:12:43.730 --> 00:12:49.580
their methods and access their variables so
on line 11 Kenwood.price retrieves

155
00:12:49.580 --> 00:12:54.630
the price from the instance called
Kenwood and Hamilton.price will get the

156
00:12:54.630 --> 00:12:59.860
price of the Hamilton instance we can
also give the price a new value as we see on line

157
00:12:59.860 --> 00:13:06.850
13 where we set the price to $12.75 so we've been
using dot notation extensively in our

158
00:13:06.850 --> 00:13:08.420
programs in this course

159
00:13:08.420 --> 00:13:12.370
when you seen that when we've used the
append method to add items to a list for example

160
00:13:12.370 --> 00:13:18.440
also all the TK widgets are objects
that have .pack and grid methods amongst

161
00:13:18.440 --> 00:13:23.010
many others to allow them to be placed
in the window manager so lets add another

162
00:13:23.010 --> 00:13:27.750
example in this program so I'm going to start typing and put on some code on line 18

163
00:13:27.750 --> 00:13:30.940
gonna type...

164
00:13:42.620 --> 00:13:55.779
.....

165
00:13:55.779 --> 00:14:08.040
....and you can see here models kenwood equals$12.75 and Hamilton equals $14.55 so it's

166
00:14:08.040 --> 00:14:12.459
retrieved the values out of each
instance of the 2 kettle objects that

167
00:14:12.459 --> 00:14:19.060
we've created so the parameters in the format
method are the make price data attributes of

168
00:14:19.060 --> 00:14:23.000
the Kenwood and Hamilton objects
another bit of jargon here the

169
00:14:23.000 --> 00:14:27.070
attributes so we've become used to
talking about variables but when a

170
00:14:27.070 --> 00:14:31.399
variable is bound to an instance of a
class then it's referred to as a data

171
00:14:31.399 --> 00:14:36.110
attribute in Python now their other
object-oriented programming terms for

172
00:14:36.110 --> 00:14:40.480
the same things you might find variable 
attributes also called fields in

173
00:14:40.480 --> 00:14:45.970
languages such as Java or data members
in C++ now I'll introduced yet another term

174
00:14:45.970 --> 00:14:49.690
shortly probably in the next video one
that's been borrowed from the small talk

175
00:14:49.690 --> 00:14:55.319
language but lets end this video here and in then next
video we'll continue our discussion about

176
00:14:55.319 --> 00:14:56.839
objects see you in the next video