WEBVTT

1
00:00:01.960 --> 00:00:07.259
so continuing on from our previous video
so we ended up talking about variables

2
00:00:07.259 --> 00:00:11.620
but saying that when a variable is
bound to an instance of a class in

3
00:00:11.620 --> 00:00:16.430
Python its referred to as a data attribute now their other object orientated

4
00:00:16.430 --> 00:00:21.079
terms  for the same thing so you might
find variable attributes also called

5
00:00:21.079 --> 00:00:27.119
fields in languages such as Java or data
members in C++ and I'll introduced yet

6
00:00:27.119 --> 00:00:28.360
another term shortly

7
00:00:28.360 --> 00:00:32.250
one borrowed from the small talk
language but I over simplified things

8
00:00:32.250 --> 00:00:37.140
here and now we will be talking
about different kinds of methods and

9
00:00:37.140 --> 00:00:42.570
attributes later but that's the basic
jargon relating to classes which you

10
00:00:42.570 --> 00:00:46.589
should probably become familiar with
now before I summarise these terms

11
00:00:46.589 --> 00:00:50.320
there's another way to use a string
replacement fields that is useful when

12
00:00:50.320 --> 00:00:55.999
dealing with classes so because Kenwood
and Hamilton are objects we can specify

13
00:00:55.999 --> 00:01:00.269
their attributes in the replacement
fields and let's have a look at that

14
00:01:00.269 --> 00:01:04.850
to see how we can do that so we are gonna add
that on line 20 we type....

15
00:01:11.010 --> 00:01:25.160
....

16
00:01:26.970 --> 00:01:40.370
...so here because Kenwood and

17
00:01:40.370 --> 00:01:44.260
Hamilton are objects we can specify
their attributes in the replacement

18
00:01:44.260 --> 00:01:48.590
fields so if we run this just to confirm
we should see the identical results the

19
00:01:48.590 --> 00:01:57.780
print out as you can see their exactly the same but looking at
the code I will argue that line 20 is easy to read because

20
00:01:57.780 --> 00:02:02.090
you can see immediately which attributes
are being used at each point in the

21
00:02:02.090 --> 00:02:06.500
string and of course on line 18 you couldn't see
that because there wasn't anything in the

22
00:02:06.500 --> 00:02:13.980
braces so just to summarize where we are at just gonna paste in that code in their now paste in those

23
00:02:13.980 --> 00:02:19.830
comments to be more precise so a class is a
template for creating objects all

24
00:02:19.830 --> 00:02:23.250
objects created using the same class
will have the same characteristics

25
00:02:23.250 --> 00:02:30.660
object is an instance of a class instantiate we didn't used the word instantiate but

26
00:02:30.660 --> 00:02:36.010
that is how the process of creating an
instance of a class method is a function

27
00:02:36.010 --> 00:02:41.269
defined in a class and attribute is
a variable bound to an instance of a

28
00:02:41.269 --> 00:02:46.690
class so as you can see here a class is a
template for creating objects the

29
00:02:46.690 --> 00:02:50.110
template defines the attributes and
methods that all instances of the class

30
00:02:50.110 --> 00:02:52.880
will have now although this is not
true

31
00:02:52.880 --> 00:02:57.250
of other object oriented programming
languages in Python a class is also

32
00:02:57.250 --> 00:03:02.370
exactly the same thing as a type other
languages might refer to Kingwood in

33
00:03:02.370 --> 00:03:07.459
our sample program is being
of type kettle but they also have

34
00:03:07.459 --> 00:03:13.550
types that are not classes but in Python every type is a class which is why we

35
00:03:13.550 --> 00:03:17.610
were able to call the add method of our int object at the start of the previous

36
00:03:17.610 --> 00:03:18.530
video

37
00:03:18.530 --> 00:03:22.830
an object is another name for an
instance of a class then once the class

38
00:03:22.830 --> 00:03:27.280
has been defined it is the actual
instances that are used in a program to

39
00:03:27.280 --> 00:03:30.940
store the values of the attributes and
perform actions on them and you saw

40
00:03:30.940 --> 00:03:35.580
instances being created on line 9 and
line 16 on the screen so when

41
00:03:35.580 --> 00:03:39.709
creating a new object from a class
definition the terms uses instantiating

42
00:03:39.709 --> 00:03:44.510
an object and that's just a fancy word
for creating an instance of a class but

43
00:03:44.510 --> 00:03:49.350
you should be familiar with that term
now method is a function as bound to an

44
00:03:49.350 --> 00:03:53.310
instance of a class now in the Tkinter
videos you might recall we created

45
00:03:53.310 --> 00:03:57.730
instances of the back label and canvas
classes and we used their grid and pack

46
00:03:57.730 --> 00:04:03.300
methods to place them on the screen and
finally an attribute is a variable that

47
00:04:03.300 --> 00:04:07.870
is bound to an instance of a class so
the kettle variable and kenwood can keep

48
00:04:07.870 --> 00:04:12.290
track of the values of its making
price without affecting the attributes

49
00:04:12.290 --> 00:04:18.489
of the Hamilton object which has got its own values for those attributes so as I

50
00:04:18.489 --> 00:04:22.360
hinted at earlier this is a slight
over-simplification in the last two

51
00:04:22.360 --> 00:04:26.260
definitions and i will be explaining a bit more
about that a little bit later on in the

52
00:04:26.260 --> 00:04:30.229
section but we have to walk here before we
can run so those definitions will give

53
00:04:30.229 --> 00:04:34.210
us something to work with while we get
the hang of classes and object oriented

54
00:04:34.210 --> 00:04:39.240
programming in general so continuing on now with our kettle class at the moment

55
00:04:39.240 --> 00:04:44.180
it has 2 data attributes make and price and a single method called init

56
00:04:44.180 --> 00:04:47.360
__init__ which you can see is

57
00:04:47.360 --> 00:04:52.090
showing on line three and if you look at
that lit looks like it's sets the values for

58
00:04:52.090 --> 00:04:55.090
the attributes but also looks a little
strange

59
00:04:55.660 --> 00:05:00.419
now as I explain what's going on here I'm gonna be
referring to quite a few things whos names

60
00:05:00.419 --> 00:05:05.220
start with and end with 2 underscores and
it's going to get a lil bit tedious

61
00:05:05.220 --> 00:05:08.710
if I keep saying __ all the
time so I'm just going to use the name

62
00:05:08.710 --> 00:05:12.370
of the object and you can see on the
screen that its got underscores around

63
00:05:12.370 --> 00:05:16.050
it but I will review this use of
underscores and how it relates to

64
00:05:16.050 --> 00:05:19.720
classes later on but just keep in mind
that time when we refer to something for

65
00:05:19.720 --> 00:05:23.110
example on line 3 as init I'm referring to __init__

66
00:05:23.110 --> 00:05:28.310
now we looked at
most of the users of underscore around

67
00:05:28.310 --> 00:05:30.300
variable names in the previous section

68
00:05:30.300 --> 00:05:35.210
and theirs a little bit more to add to that
once we get more into classes now this

69
00:05:35.210 --> 00:05:35.940
class

70
00:05:35.940 --> 00:05:40.610
obviously doesn't do a great deal and
theirs not really a lot going on and it's

71
00:05:40.610 --> 00:05:44.000
not really a lot you can do with a
kettle other than switching it on and wait but

72
00:05:44.000 --> 00:05:49.210
I will add a method to do that before looking at
more complicated classes now the Python

73
00:05:49.210 --> 00:05:51.509
style guide pep8 by the way

74
00:05:51.509 --> 00:05:55.590
recommends just single blank line
between methods inside a class which is

75
00:05:55.590 --> 00:06:00.110
different to the recommendation and
there should be two blank lines between

76
00:06:00.110 --> 00:06:05.330
top level functions in a module so if you
use 2 blanks lines then IntelliJ will show

77
00:06:05.330 --> 00:06:09.050
warning and personally I like to leave 2
blank lines after class definitions

78
00:06:09.050 --> 00:06:13.599
but this is not covered in the style guide
and you won't get a warning unless you leave more than two blank

79
00:06:13.599 --> 00:06:14.930
lines and

80
00:06:14.930 --> 00:06:17.930
I'm gonna add a second method now

81
00:06:19.580 --> 00:06:30.870
and if we start this....and notice as I press the left bracket

82
00:06:30.870 --> 00:06:36.099
IntelliJ automatically added the self parameter
so as I mentioned functions that are bound to a

83
00:06:36.099 --> 00:06:40.830
class called methods and main difference
between a method and a function is the

84
00:06:40.830 --> 00:06:45.690
presence of this self parameter that's been
added automatically just go ahead and

85
00:06:45.690 --> 00:06:57.960
finish this method....and just a reminder if I go to and add that as a

86
00:06:57.960 --> 00:07:02.879
second add 2 lines between methods we get
the warning from IntelliJ 2

87
00:07:02.879 --> 00:07:04.080
blanks lines so if your in a

88
00:07:04.080 --> 00:07:10.639
class only have one line between methods
to remove that warning and you might be

89
00:07:10.639 --> 00:07:14.500
wondering what is the self parameter well so there's nothing really special

90
00:07:14.500 --> 00:07:19.500
about the name self or the parameter and
you have called it anything you wanted avoiding

91
00:07:19.500 --> 00:07:24.449
name conflicts with other objects of course
but unlike to key word this in Java for

92
00:07:24.449 --> 00:07:29.210
example self is just the name of a
parameter the important thing is the

93
00:07:29.210 --> 00:07:33.210
presence of the parameter not what its
called so having said that the

94
00:07:33.210 --> 00:07:37.259
convention is that it's called self and
although this is not enforced in any way

95
00:07:37.259 --> 00:07:41.310
it's not a good idea to change the
convention is so well established that

96
00:07:41.310 --> 00:07:43.460
tools may expect to be called self and may not

97
00:07:43.460 --> 00:07:48.720
work if you use another name in fact
as we have just seen IntelliJ automatically use the

98
00:07:48.720 --> 00:07:50.139
name self for it

99
00:07:50.139 --> 00:07:54.120
when we are defining the method just now so what is
self

100
00:07:54.120 --> 00:07:59.080
well the answer is that it's a reference
to the instance of the class now if you

101
00:07:59.080 --> 00:08:03.970
used to C++ or Java then you can think
of it as equivalent to this and it's

102
00:08:03.970 --> 00:08:08.039
used to refer to instance variables and
you could see that we're doing that

103
00:08:08.039 --> 00:08:14.590
on lines four five and six and again on
line 9 and although we've use many methods in the early

104
00:08:14.590 --> 00:08:18.210
lectures in this course we've never had to
specify an argument for the self

105
00:08:18.210 --> 00:08:22.460
parameter and this is because Python
supplies the value automatically when

106
00:08:22.460 --> 00:08:26.120
calling the method now I will talked about this
a little bit more in a minute but first

107
00:08:26.120 --> 00:08:38.270
let's call the method and check to see
if it is a kettle or not so we type....

108
00:08:38.270 --> 00:08:50.130
....and notice that again when I typed that....

109
00:08:50.130 --> 00:08:55.640
....you can see the definition shows the
self parameter but when I press enter it automatically

110
00:08:55.640 --> 00:08:59.540
there's no parameter passed in the
brackets and if we just confirm that....

111
00:08:59.540 --> 00:09:11.329
...and if we run this first
time we ran it on line 33 was false

112
00:09:11.329 --> 00:09:16.230
that's because up here on line six we defined it as false second time after

113
00:09:16.230 --> 00:09:21.779
calling Hamilton.switch_on the variable the attribute in this case for

114
00:09:21.779 --> 00:09:27.770
our hamilton instance was set to true in other words
the value of the hamiltons instance on

115
00:09:27.770 --> 00:09:31.490
started off as false because that's what it
was initialized to in the constructor

116
00:09:31.490 --> 00:09:36.740
and calling the switch-on method on line
34 set to true which is the value of

117
00:09:36.740 --> 00:09:42.260
course we are getting when we print out the value for the second time on line 35 now the

118
00:09:42.260 --> 00:09:47.079
term constructor by the way refers to a
special method that is executed when an

119
00:09:47.079 --> 00:09:51.890
instance of a class is created or
constructed in Python this is the init

120
00:09:51.890 --> 00:09:54.480
method that you can see defined on line 3

121
00:09:54.480 --> 00:09:59.459
and as you saw when I typed in line 34
we don't provide a value for the

122
00:09:59.459 --> 00:10:03.300
self parameter when calling the switch_on method when you call a method

123
00:10:03.300 --> 00:10:04.600
on an instance

124
00:10:04.600 --> 00:10:08.860
Python automatically provides a reference to
the instance as the first parameter to

125
00:10:08.860 --> 00:10:13.410
the method now we can see this in a bit more
detail by not using and instance but

126
00:10:13.410 --> 00:10:18.160
calling the method the class itself
instead so we come down here and put this

127
00:10:18.160 --> 00:10:27.089
on lines 37 we can type....

128
00:10:27.089 --> 00:10:39.339
...so if we run that you can see that we got the value true

129
00:10:39.339 --> 00:10:43.250
and of course Hamilton initially when we created the instance of the

130
00:10:43.250 --> 00:10:48.769
object was then constructor made it the value false that by calling this method in an

131
00:10:48.769 --> 00:10:53.389
alternative ways you can see they're
using the class definition initially you

132
00:10:53.389 --> 00:10:57.260
can see what's happening there and the
parameters has been passed so instead of

133
00:10:57.260 --> 00:11:01.720
using an instance of kettle to call a
method we've use the class itself because

134
00:11:01.720 --> 00:11:06.139
there is no instance here the value for self
has to be specified and failing to do so

135
00:11:06.139 --> 00:11:11.589
would give an error so the two forms have
exactly the same effect and as you can see

136
00:11:11.589 --> 00:11:16.269
when we ran the program kettle switch on
ended up returning true so we can see

137
00:11:16.269 --> 00:11:20.800
that it was updated correctly so lets go
back to the definition again I just want to

138
00:11:20.800 --> 00:11:28.630
go back and delete this out...just to confirm that we may have miss this the first and when I add the bracket not the parameter

139
00:11:28.630 --> 00:11:32.410
showing up there as self so its asking for
that parameter in this case and I can

140
00:11:32.410 --> 00:11:37.260
change it too kenwood but again just to confirm if we do the

141
00:11:37.260 --> 00:11:46.160
alternative syntax......so when I do that

142
00:11:46.160 --> 00:11:50.440
noting that the self parameter
is showing at the top of the pop up but when

143
00:11:50.440 --> 00:11:55.560
I press enter notice how it didn't ask for the
parameter in this case because it knows

144
00:11:55.560 --> 00:11:58.510
we've already got an instance that we're working
on

145
00:11:58.510 --> 00:12:02.460
so the implementation of classes in Python can create and some unexpected

146
00:12:02.460 --> 00:12:03.850
behavior

147
00:12:03.850 --> 00:12:08.080
partly because it's a mix of modular
three and C++ with some small talk thrown

148
00:12:08.080 --> 00:12:12.200
in for good measure and once again if you are
learning object oriented programming for

149
00:12:12.200 --> 00:12:16.080
the first time using Python then you'll
probably be quite happy with the way

150
00:12:16.080 --> 00:12:20.440
things work that's not to say you won't have to concentrate but there shouldn't be

151
00:12:20.440 --> 00:12:27.370
any surprises but for C++ and Java programmers it might be a good idea to put what you already know to the back

152
00:12:27.370 --> 00:12:31.610
of your mind and be prepared to learn a
new way of thinking about things when it

153
00:12:31.610 --> 00:12:34.710
comes to classes and object-oriented
programming in Python

154
00:12:35.710 --> 00:12:39.740
now just like everything else in Python
classes are dynamic and it can be modified

155
00:12:39.740 --> 00:12:43.930
after their created now to see this in
action and reinforce my previous comment

156
00:12:43.930 --> 00:12:47.990
about surprises for C++ and Java
programmers I'm going to add a measure

157
00:12:47.990 --> 00:12:52.730
for the keyword kettles power and
kilowatts let's go ahead and do that so

158
00:12:52.730 --> 00:13:00.030
come down here to line 41 a bit more
space so we can see.....

159
00:13:00.030 --> 00:13:18.370
....

160
00:13:24.270 --> 00:13:28.750
so this case now the Kenwood instance
of the kettle class now has another

161
00:13:28.750 --> 00:13:34.770
data attribute called power with the
value 1.5 now remember that variables

162
00:13:34.770 --> 00:13:38.610
in Python come into existence the first
time they're assigned a value and the

163
00:13:38.610 --> 00:13:43.050
same thing is true for instance
variables which is the small talk name for data

164
00:13:43.050 --> 00:13:48.440
attributes so here I created a data attribute for the Kenwood object called power

165
00:13:48.440 --> 00:13:55.240
and I gave it a value of 1.5 so the term instance
variable is useful because it describes

166
00:13:55.240 --> 00:14:00.810
power as a variable that is bound to an
instance of the kettle class so as a

167
00:14:00.810 --> 00:14:07.050
result the following code I'm gonna add
shouldn't be surprising.....

168
00:14:07.050 --> 00:14:22.590
...and if we run this we get an error message attribute kettle object has not attribute power so if I close that

169
00:14:22.590 --> 00:14:27.500
down again and that's because the Hamilton
instance doesn't have a power attribute

170
00:14:27.500 --> 00:14:32.470
because if you're not created one by
assigning a value to it on line 43 we added

171
00:14:32.470 --> 00:14:37.240
an instance variable for the Kenwood
instance of the kettle class but of course we didn't

172
00:14:37.240 --> 00:14:41.010
do that for the Hamilton instance and
that's why we got an error when we run

173
00:14:41.010 --> 00:14:41.810
the program

174
00:14:41.810 --> 00:14:46.580
alright so I'm going to finish this
video here now in the next video we're going to talk more

175
00:14:46.580 --> 00:14:50.160
about objects and find a bit more
about instance variables

176
00:14:50.660 --> 00:14:56.310
and continue on generally speaking in our discussion
and in our journey finding out more

177
00:14:56.310 --> 00:14:59.350
about object oriented programming in Python see you in the next video