WEBVTT

1
00:00:01.930 --> 00:00:05.939
so as I mentioned at the end of the
previous video theirs a fundamental problem

2
00:00:05.939 --> 00:00:11.430
with the code for the account class that
we've created and although fixing it is

3
00:00:11.430 --> 00:00:16.490
easy there's more than one way to do so
so I'm gonna cover that later but so I

4
00:00:16.490 --> 00:00:19.600
will let you see if you can spot the problem until we
get to that part

5
00:00:19.600 --> 00:00:24.350
of this course so now that we have a program
that is using the class to maintain an

6
00:00:24.350 --> 00:00:28.560
account balance it's easy to see
hopefully easy to see how the class

7
00:00:28.560 --> 00:00:33.879
encapsulates the balance attributes and the methods
to manipulate it without any client of

8
00:00:33.879 --> 00:00:38.270
the class having to worry about the
details now a client is anything

9
00:00:38.270 --> 00:00:43.160
that uses a particular class or this
class so clients don't have to worry about the

10
00:00:43.160 --> 00:00:47.170
implementation details they just need to
call the deposit method to put more money

11
00:00:47.170 --> 00:00:52.390
in and the withdrawal method to take
money out so one advantage provided

12
00:00:52.390 --> 00:00:56.710
by encapsulation is that we can change
the way that the methods are implemented

13
00:00:56.710 --> 00:01:01.489
as long as we don't change their
signature in a way that is it that is

14
00:01:01.489 --> 00:01:05.610
incompatible with the current signature
so if we do that any client code will

15
00:01:05.610 --> 00:01:10.409
continue to work now if you remember back
from the lectures on functions that a

16
00:01:10.409 --> 00:01:15.130
signature is the definition of the name
and parameters of a function as well as

17
00:01:15.130 --> 00:01:21.430
any return value and applies to methods as well so to demonstrate that what I'll do is I'll keep the bank

18
00:01:21.430 --> 00:01:25.540
orders as happy by including a transaction
log to track the details of all deposits

19
00:01:25.540 --> 00:01:31.170
and withdrawals now will start by just
tracking deposits because the same code

20
00:01:31.170 --> 00:01:34.979
will ultimately be used for withdrawals
and you know rather than duplicating the

21
00:01:34.979 --> 00:01:38.570
code I'll use that as an opportunity to
introduce class methods

22
00:01:39.250 --> 00:01:43.549
and first I'll add a list to the classes
data attributes and modify the deposit

23
00:01:43.549 --> 00:01:47.689
method to add the transaction details to
the list the details to be log will be

24
00:01:47.689 --> 00:01:51.390
using the date and time of the
transaction also the amount so we'll need

25
00:01:51.390 --> 00:01:55.790
to import the date time module and remember from the videos in section eleven that

26
00:01:55.790 --> 00:02:01.200
to we really should install local time
so we'll also used to pytz module so

27
00:02:01.200 --> 00:02:06.619
that we can log our location and where UTC
time so lets go ahead and start the changes and

28
00:02:06.619 --> 00:02:09.530
the first we need to do is import those
2

29
00:02:09.530 --> 00:02:16.990
modules  so....and remember if you having

30
00:02:16.990 --> 00:02:21.560
problems with pytz getting that to work we did
install that in a previous sections so go

31
00:02:21.560 --> 00:02:22.610
back and check that out

32
00:02:22.610 --> 00:02:27.630
as its not installed usually by default
with the Python installation alright so what we're

33
00:02:27.630 --> 00:02:32.400
going to do is we need to add the list to
the class attributes so that we can store

34
00:02:32.400 --> 00:02:34.060
the log and we'll do that in our

35
00:02:34.060 --> 00:02:39.989
init method so we'll come down here to the init
and....

36
00:02:39.989 --> 00:02:47.920
....and what we need to do is in

37
00:02:47.920 --> 00:02:52.360
the deposit method we need to add the
call that is going to append the amount

38
00:02:52.360 --> 00:02:57.930
of the deposit with the UTC time so
we'll do that by typing...

39
00:02:57.930 --> 00:03:02.920
...

40
00:03:05.350 --> 00:03:19.230
....

41
00:03:19.230 --> 00:03:26.730
....and attribute called transaction list has been created in

42
00:03:26.730 --> 00:03:32.030
the init method and that sent to an empty
list initially and in the deposit method

43
00:03:32.030 --> 00:03:37.489
we then appended the transaction details
to the list which is the date and the

44
00:03:37.489 --> 00:03:42.239
amount in this case now I'll come back to how the dates is calculated in a

45
00:03:42.239 --> 00:03:47.430
minute because their have been a quite a few video since the ones on dates and times in Python

46
00:03:48.410 --> 00:03:52.459
and I have chose to store the details as a tuple containing the date time and the amount

47
00:03:52.459 --> 00:03:56.329
so I'll add the method to print out the
transaction log after the show balance

48
00:03:56.329 --> 00:04:03.080
method so let's go ahead and do that so...

49
00:04:03.080 --> 00:04:18.660
...

50
00:04:18.660 --> 00:04:20.390
....

51
00:04:20.390 --> 00:04:35.030
...

52
00:04:35.030 --> 00:05:15.020
....

53
00:05:16.280 --> 00:05:27.070
.....and that should be it
so before I make any changes to the main

54
00:05:27.070 --> 00:05:32.220
program down here lets just run it to make
sure that it still works even though I've

55
00:05:32.220 --> 00:05:36.540
made changes to the implementation of course
of the deposit method by adding the call

56
00:05:36.540 --> 00:05:44.710
to add the transaction log so if I just run that so it still works but the

57
00:05:44.710 --> 00:05:49.400
class now has got additional
functionality the ability to display a

58
00:05:49.400 --> 00:05:53.580
transaction list so any existing code
doesn't break as a result of the changes

59
00:05:53.580 --> 00:05:57.610
we made to the deposit method bit if we want
to we can also print out the transaction

60
00:05:57.610 --> 00:06:03.900
list by adding that to our main so we can
come down here after the withdrawal....

61
00:06:03.900 --> 00:06:07.820
...and if we run that

62
00:06:08.880 --> 00:06:17.900
....you can see $1000 deposited on the date of the transaction in
UTC time and in the local time which is

63
00:06:17.900 --> 00:06:23.280
works out to 8:32 in the morning Australian
time so before I move on you might be wondering

64
00:06:23.280 --> 00:06:29.520
about the for loop in line 30 or on line 30
where the code iterates through the

65
00:06:29.520 --> 00:06:34.510
transaction list so we created the
transaction list to contain tuples and in

66
00:06:34.510 --> 00:06:38.690
the videos on tuples I showed you that you can assign a value to several variables in a

67
00:06:38.690 --> 00:06:42.350
single assignment statement by using a
tuple and that's all it's really

68
00:06:42.350 --> 00:06:46.450
happening in the for loop instead of just assigning a value to a single variable for

69
00:06:46.450 --> 00:06:48.010
each value in the list

70
00:06:48.010 --> 00:06:53.120
Python unpacks the tuple and assigns the
value to both data and amount at the same

71
00:06:53.120 --> 00:06:53.600
time

72
00:06:53.600 --> 00:06:58.230
ok so our class now logs deposits and I
could do the same thing in the

73
00:06:58.230 --> 00:07:02.000
withdrawal method to log withdrawals as
well and to do that that would involve

74
00:07:02.000 --> 00:07:07.600
duplicating the code that creates a date
and time with time zone information so its

75
00:07:07.600 --> 00:07:12.030
probably better to create a function to
do that instead now I could do that by

76
00:07:12.030 --> 00:07:15.870
adding another method to the class but
the method wouldn't be acting on any

77
00:07:15.870 --> 00:07:19.270
data attributes and there's no point
including it in the namespace of every

78
00:07:19.270 --> 00:07:23.960
single instance when a function in the
would do the job just fine so

79
00:07:23.960 --> 00:07:28.900
it's a good place to introduce static
methods which is the same term used in

80
00:07:28.900 --> 00:07:34.000
Java and C++ so what i'll do is i'll
start by creating it as a method which I

81
00:07:34.000 --> 00:07:39.900
put right at the start of the class
so we are going

82
00:07:39.900 --> 00:07:47.040
put it above the init so....

83
00:07:47.040 --> 00:07:58.380
....

84
00:07:58.380 --> 00:08:01.200
...

85
00:08:01.200 --> 00:08:10.880
.....so two things firstly
you notice I've use a underscore at the start when defining

86
00:08:10.880 --> 00:08:16.180
the method and also we've got a warning so IntelliJ has detected that the

87
00:08:16.180 --> 00:08:18.940
self parameter isn't use and suggest
that the method

88
00:08:18.940 --> 00:08:25.320
can be implemented as a static method
instead...so detecting

89
00:08:25.320 --> 00:08:30.420
a method which may safely be made static so you can just
repeat what I said it's detected that the

90
00:08:30.420 --> 00:08:34.789
self parameter isn't being used in the
method that was created and its

91
00:08:34.789 --> 00:08:39.390
suggesting that this method can be
implemented as static method instead a

92
00:08:39.390 --> 00:08:43.400
static method is shared by all instances
of the class in the same way that the

93
00:08:43.400 --> 00:08:47.700
power_source class
attribute of our kettle class was shared

94
00:08:47.700 --> 00:08:52.770
by all instances so making the method
static is pretty simple we just remove the

95
00:08:52.770 --> 00:08:57.360
self parameter and put an annotation
before the method definition so if I

96
00:08:57.360 --> 00:09:05.700
removed self after we delete that parameter we
also have to put annotation so above the method we put...

97
00:09:05.700 --> 00:09:11.000
...and notice how the warnings disappeared

98
00:09:11.000 --> 00:09:16.270
and if we hold the Ctrl key down or command
on a Mac and hover over the annotation

99
00:09:16.270 --> 00:09:22.190
you can see that it's coming up and showing its a class and as I said everything in

100
00:09:22.190 --> 00:09:27.670
Python is an object so if we now click while
holding down control or command on this

101
00:09:27.670 --> 00:09:34.250
annotation you see definitions showing
up here in the built ins module that useful

102
00:09:34.250 --> 00:09:37.640
to remember again that everything is an
object but unless your experience in object

103
00:09:37.640 --> 00:09:40.530
oriented programming I wouldn't worry
too much about this implementation

104
00:09:40.530 --> 00:09:47.730
details just yet so the current time method
uses the date time class or the date time

105
00:09:47.730 --> 00:09:52.680
module to get the current date and time
in UTC format and then uses the pytz

106
00:09:52.680 --> 00:09:57.190
module to localize it so that it stores time zone information for the current location

107
00:09:57.190 --> 00:10:04.520
and then on line 10 it returns the localize UTC date time so we looked at pytz and

108
00:10:04.520 --> 00:10:08.780
time zones in section eleven and here I'm
just making sure that I store UTC time

109
00:10:08.780 --> 00:10:12.530
but have the ability to also display the
times in the local time that the

110
00:10:12.530 --> 00:10:17.420
transaction happen and I've also
named the current underscore time method

111
00:10:17.420 --> 00:10:21.460
with the leading underscore to indicate
to anyone using this class that this

112
00:10:21.460 --> 00:10:25.930
method isn't really part of the normal
usage of the class now this conventional

113
00:10:25.930 --> 00:10:30.400
the convention is that names starting
with an underscore are non-public

114
00:10:30.400 --> 00:10:34.910
even though if you remember theirs nothing
in Python that enforces this so the account

115
00:10:34.910 --> 00:10:39.660
class is concerned with managing bank
accounts not with dates and times so

116
00:10:39.660 --> 00:10:42.690
although clients can call the current
underscore time method if they want to

117
00:10:42.690 --> 00:10:47.250
the underscore makes it clear that this
method isn't intended to be used outside

118
00:10:47.250 --> 00:10:51.630
of the class now if someone who does use
the current time method from outside our

119
00:10:51.630 --> 00:10:56.370
account class they run the risk that we may
remove it in later version or change

120
00:10:56.370 --> 00:10:59.550
it to remove to return something
completely different in the future

121
00:10:59.550 --> 00:11:02.700
starting the name again with an
underscore is the Python way of

122
00:11:02.700 --> 00:11:06.270
informing them of this they used the
method at their own risk

123
00:11:06.970 --> 00:11:12.660
ok so now we've implemented this static
method we can go back to our deposit

124
00:11:12.660 --> 00:11:17.330
method and used that static method
instead of performing the conversion in

125
00:11:17.330 --> 00:11:22.370
places as we currently got on line 22 so
come down and make a change to that so let's

126
00:11:22.370 --> 00:11:23.420
make the change here

127
00:11:23.420 --> 00:11:29.090
delete all this code now in the transaction_list.append

128
00:11:29.090 --> 00:11:34.720
then brackets that will only be a matter of now
adding an extra set of brackets and

129
00:11:34.720 --> 00:11:37.720
....

130
00:11:39.060 --> 00:11:45.750
...so that's all we need to do now so its

131
00:11:45.750 --> 00:11:51.160
now calling the static method that we
defined on line 8 returning or adding the

132
00:11:51.160 --> 00:11:57.620
returned date and then also as it was
doing before appending the amount and we

133
00:11:57.620 --> 00:12:03.700
can go back to our withdrawal method and
in the code that's confirmed that

134
00:12:03.700 --> 00:12:10.070
we're able to do the withdraw after we deducted
the amount we put....

135
00:12:10.070 --> 00:12:26.660
....

136
00:12:26.660 --> 00:12:31.490
...so if we run this to confirm its running you can see now we've got our code working

137
00:12:31.490 --> 00:12:35.040
quite nicely we got our deposit and
withdrawal by showing nicely in it's

138
00:12:35.040 --> 00:12:39.070
formatted as it should be and obviously
we could do a bit of work if we wanted to

139
00:12:39.070 --> 00:12:41.480
do to fix these dates up but we will leave them for now

140
00:12:41.480 --> 00:12:45.430
so the point here is that despite the
additional functionality that the class

141
00:12:45.430 --> 00:12:49.620
now has the program still runs fine
without any modification to the main

142
00:12:49.620 --> 00:12:54.500
part in other words we didn't modify
anything here in the code from line

143
00:12:54.500 --> 00:12:59.160
46 onwards that's still as it was before
and its still working despite us making

144
00:12:59.160 --> 00:13:04.519
these changes in the various class
methods and looking at lines

145
00:13:04.519 --> 00:13:10.070
22 and 28 you notice
that when we are calling our static method were

146
00:13:10.070 --> 00:13:14.750
using the class name then the name of the
method so we're not using self in other

147
00:13:14.750 --> 00:13:20.100
words again that's because the current
time method is a static or class method now you

148
00:13:20.100 --> 00:13:24.690
can call class method on an instance as I
said some in other words I can change that and

149
00:13:24.690 --> 00:13:33.389
just make this say here....and that's will
work just as well as but theirs no advantage to

150
00:13:33.389 --> 00:13:37.790
doing so and in actual fact
performance slightly suffers because

151
00:13:37.790 --> 00:13:41.870
Python will first attempt to find the
method in the instances namespace and

152
00:13:41.870 --> 00:13:47.000
when it fails it then checks the class name space so if you know you got a static methods best to leave it

153
00:13:47.000 --> 00:13:50.949
there just a to say that little bit of
time and to make it clear also in the

154
00:13:50.949 --> 00:13:55.649
code that it is a static method so some
ways needed to tell the difference

155
00:13:55.649 --> 00:13:59.190
between deposits and withdrawals and
negating the amount is a quick way to do

156
00:13:59.190 --> 00:14:04.800
this you could as an alternative
add a transaction type item to the tuple if you wanted a tuple

157
00:14:04.800 --> 00:14:08.680
if you wanted to do that so I'm going to end the video
here and then the next video we're

158
00:14:08.680 --> 00:14:12.760
going to talk a little bit more about
the transactions and start a mini

159
00:14:12.760 --> 00:14:14.260
challenge so see you in the next video