1
00:00:00,000 --> 00:00:03,200
Code that errors out is likely to kill your program.

2
00:00:03,280 --> 00:00:05,170
That's not good for the end user.

3
00:00:05,180 --> 00:00:09,240
That's not good for you as a programmer, because then someone

4
00:00:09,250 --> 00:00:11,810
else is going to complain to you, which honestly gets a little bit

5
00:00:11,820 --> 00:00:13,070
annoying after a little while.

6
00:00:13,070 --> 00:00:17,300
So we might as well catch what could be a problem and fix it beforehand.

7
00:00:17,380 --> 00:00:22,430
So luckily we have this 'try-except-finally' keywords,

8
00:00:22,440 --> 00:00:23,690
these three keywords here.

9
00:00:24,110 --> 00:00:28,380
Now, what if you're executing some code and it could error

10
00:00:28,380 --> 00:00:30,300
out for multiple reasons.

11
00:00:30,300 --> 00:00:36,720
So let's 'try: 1/0', and we could 'except' this

12
00:00:36,730 --> 00:00:44,020
'Exception as e:', and we can 'print(type(e))', and we can also

13
00:00:44,020 --> 00:00:45,700
print what the error actually is.

14
00:00:45,700 --> 00:00:50,100
Now, this is actually a really nice little, not really a hack,

15
00:00:50,100 --> 00:00:53,200
but it's a nice little way of doing things that I like to do.

16
00:00:53,200 --> 00:00:56,700
So I don't actually know all of Python's internal errors.

17
00:00:56,700 --> 00:00:59,600
There's a lot of them, and it's pretty hard for the average

18
00:00:59,600 --> 00:01:00,700
person to know all of them.

19
00:01:00,920 --> 00:01:04,280
So what I can do, is I can 'try' something, and then 'except' the

20
00:01:04,280 --> 00:01:08,000
'Exception', and if I don't know what that is, this will tell me what it is.

21
00:01:08,020 --> 00:01:09,520
So let's go ahead and run this.

22
00:01:09,530 --> 00:01:12,540
And this says, 'try: 1/0'.

23
00:01:12,540 --> 00:01:16,300
The exception was a 'class'.

24
00:01:16,370 --> 00:01:21,540
So 'e', we did 'print(type(e))'. 'class ZeroDivisionError'.

25
00:01:22,200 --> 00:01:26,400
And the error is, actually the message itself is, "division by zero".

26
00:01:26,400 --> 00:01:30,810
So now we can do this, throw 'ZeroDivisionError' in there

27
00:01:30,900 --> 00:01:37,700
and say, 'print("Cannot divide by zero")'.

28
00:01:38,900 --> 00:01:43,100
And just like that, we now have a nice way of handling this.

29
00:01:43,170 --> 00:01:45,080
So it's going to 'try' to do some code,

30
00:01:45,080 --> 00:01:46,400
the code does not work,

31
00:01:46,400 --> 00:01:50,600
we can catch this, this very specific error, and we can handle it.

32
00:01:50,600 --> 00:01:54,400
And if it's anything else, it will print what it is.

33
00:01:54,400 --> 00:01:59,200
So let's go ahead, comment this out and let's add a dictionary,

34
00:01:59,600 --> 00:02:01,600
and it's going to be a 'person'.

35
00:02:01,600 --> 00:02:04,400
And in here there's going to be, I don't know, a 'course'

36
00:02:04,400 --> 00:02:08,199
that they're enrolled in called "Python for Everybody".

37
00:02:08,199 --> 00:02:11,600
I don't know who's heard of that course, but sounds pretty

38
00:02:11,699 --> 00:02:14,800
good, doesn't it? Maybe. Bad humor.

39
00:02:14,860 --> 00:02:22,320
Anyways. We could try to say 'print(person)', and then I don't

40
00:02:22,320 --> 00:02:25,800
know their favorite food, 'fav_food', because maybe we didn't know

41
00:02:25,800 --> 00:02:27,900
that there isn't a 'fav_food' key in here.

42
00:02:27,900 --> 00:02:31,800
Let's go ahead and execute this, and we get a 'KeyError'.

43
00:02:31,800 --> 00:02:34,740
Now, this one says the dictionary key does not exist, and

44
00:02:34,740 --> 00:02:37,500
the 'KeyError' is actually the key itself.

45
00:02:37,500 --> 00:02:39,800
So now we can 'except' this as well.

46
00:02:40,100 --> 00:02:46,800
'except KeyError: print("Missing key")', and we don't know what tht is,

47
00:02:46,800 --> 00:02:48,800
so let's do 'KeyError as e:',

48
00:02:48,800 --> 00:02:51,400
[no audio]

49
00:02:51,400 --> 00:02:52,400
and let's rerun this.

50
00:02:52,420 --> 00:02:54,910
And it says "Missing key: fav_food".

51
00:02:56,100 --> 00:02:58,200
Now, that's cool that we can change these all together, but

52
00:02:58,200 --> 00:03:04,800
it's not really a realistic example, I guess of trying and catching something.

53
00:03:04,880 --> 00:03:08,250
I mean, it's sort of realistic, but let's do something else

54
00:03:08,250 --> 00:03:10,400
a little more bound to reality.

55
00:03:10,460 --> 00:03:13,280
So let's try to get some user input.

56
00:03:13,290 --> 00:03:14,600
We want two numbers.

57
00:03:14,600 --> 00:03:17,700
We want 'num1 = input(

58
00:03:18,740 --> 00:03:25,030
"Enter the first number: ", and 'num2' is going to be the

59
00:03:25,040 --> 00:03:28,930
exact same thing, but it is going to be, "Enter the second

60
00:03:29,080 --> 00:03:33,500
number: ". Now, what we're going to 'try' here? Well, we actually don't

61
00:03:33,500 --> 00:03:34,500
need to 'try' that at all.

62
00:03:34,500 --> 00:03:36,500
[no audio]

63
00:03:36,500 --> 00:03:38,700
We know that that's not going to fail on us.

64
00:03:39,500 --> 00:03:45,600
But what we want to 'try' here, is let's say casting these numbers,

65
00:03:45,650 --> 00:03:47,800
because we know these are going to come back as strings.

66
00:03:47,800 --> 00:03:51,500
So let's cast these as floats in case someone types 7.5.

67
00:03:51,500 --> 00:03:53,600
We want to be able to support that.

68
00:03:53,620 --> 00:03:58,300
So let's do 'num1 = float(num1)'.

69
00:03:58,700 --> 00:04:00,400
And let's do the same thing with 'num2'.

70
00:04:00,400 --> 00:04:02,600
[no audio]

71
00:04:02,600 --> 00:04:06,400
And let's just go ahead and throw a general exception in here 'as e'

72
00:04:06,400 --> 00:04:08,700
and so we can figure out what it is.

73
00:04:09,800 --> 00:04:15,200
We're going to 'print(type(e))', and then we're going to 'print(e)'.

74
00:04:15,200 --> 00:04:17,600
[no audio]

75
00:04:17,600 --> 00:04:19,200
Now, there could be an error in here.

76
00:04:19,200 --> 00:04:23,100
Someone could type in a string that cannot be turned into a float.

77
00:04:23,899 --> 00:04:26,800
But let's assume that it does work, and the program needs

78
00:04:26,800 --> 00:04:29,100
to continue. So we want a new total in here.

79
00:04:29,100 --> 00:04:30,800
We can say the 'total',

80
00:04:30,800 --> 00:04:32,700
actually, no, we want the division.

81
00:04:32,700 --> 00:04:37,800
So let's say 'div = num1/num2',

82
00:04:39,500 --> 00:04:40,700
and let's try this out.

83
00:04:40,700 --> 00:04:43,700
"Enter the first number: ", let's do 10,

84
00:04:43,700 --> 00:04:45,700
and the second number is going to be 20.

85
00:04:45,700 --> 00:04:50,100
And what we should have done was also printed 'div'.

86
00:04:50,500 --> 00:04:51,500
Let's try that again.

87
00:04:51,500 --> 00:04:55,700
10; 20; 10 / 20 is in fact 0.5.

88
00:04:55,730 --> 00:04:56,650
So that worked out well.

89
00:04:56,930 --> 00:05:00,600
But what if we run this again,

90
00:05:00,600 --> 00:05:04,000
and first number is 10 and someone spells out the word 20, "twenty".

91
00:05:04,000 --> 00:05:05,770
Users can be weird that way.

92
00:05:07,200 --> 00:05:09,000
We get this thing called a 'ValueError:

93
00:05:09,000 --> 00:05:11,200
could not convert the string to float'.

94
00:05:11,200 --> 00:05:14,260
So we now have one that we can 'except' in here.

95
00:05:14,260 --> 00:05:17,500
We have a particular exception that we can catch in here rather.

96
00:05:17,500 --> 00:05:20,500
So except ValueError:

97
00:05:20,900 --> 00:05:25,200
Print whatever the error is. We could print it or we just say

98
00:05:25,200 --> 00:05:29,600
"There was a value error WOOPS".

99
00:05:30,380 --> 00:05:31,520
Let's try that again.

100
00:05:31,530 --> 00:05:33,290
10, 20,

101
00:05:33,600 --> 00:05:36,800
and our program no longer errors out on us.

102
00:05:37,800 --> 00:05:40,500
What if someone says 10 and 0?

103
00:05:42,800 --> 00:05:45,000
Well, now we have a 'ZeroDivisionError'.

104
00:05:45,080 --> 00:05:46,360
We can catch that one as well.

105
00:05:46,500 --> 00:05:48,600
'except ZeroDivisionError:

106
00:05:48,600 --> 00:05:53,800
print("Cannot divide by zero")'.

107
00:05:53,890 --> 00:05:56,820
Simple as that. 10, 0,

108
00:05:57,390 --> 00:05:59,160
and it says, "Cannot divide by zero".

109
00:05:59,170 --> 00:06:02,520
So now essentially, we're on our way to creating a calculator

110
00:06:02,530 --> 00:06:04,680
program. Let's take a look at one more.

111
00:06:04,690 --> 00:06:08,450
Let's say there is a typo in here.

112
00:06:08,460 --> 00:06:11,210
Now, this is actually a bad example for a 'try-except',

113
00:06:11,210 --> 00:06:13,010
but this will show us one more error in here.

114
00:06:13,060 --> 00:06:15,550
So let's do a typo of 'num3',

115
00:06:15,560 --> 00:06:17,020
and let's say that was just a typo.

116
00:06:17,020 --> 00:06:20,500
We meant to say 'num2', but accidentally typed 'num3', that happens.

117
00:06:20,700 --> 00:06:23,800
So your first number is going to be, I don't know, 19

118
00:06:23,800 --> 00:06:25,100
The second one is going to be 56.

119
00:06:25,300 --> 00:06:26,980
And we get a 'NameError' in here.

120
00:06:26,980 --> 00:06:30,430
'num3' is not defined. So we can now go ahead and catch

121
00:06:30,500 --> 00:06:31,400
this as well.

122
00:06:32,600 --> 00:06:38,700
'print("There was an undefined variable somewhere")'.

123
00:06:39,400 --> 00:06:42,300
We could say 'as e', and put that in there.

124
00:06:42,360 --> 00:06:43,730
Let's go ahead and try this again.

125
00:06:44,900 --> 00:06:46,500
10, 34.

126
00:06:46,880 --> 00:06:48,650
"There was an undefined variable

127
00:06:48,650 --> 00:06:50,500
somewhere. name 'num3' is not defined".

128
00:06:50,540 --> 00:06:52,130
Okay, that's pretty cool.

129
00:06:52,140 --> 00:06:55,730
And this is actually a better example of something that

130
00:06:55,760 --> 00:06:58,490
is a little more likely to happen in a Python program because

131
00:06:58,500 --> 00:07:01,310
you will probably be accepting user input to some degree,

132
00:07:01,500 --> 00:07:05,100
and you need to be able to cast data.

133
00:07:05,100 --> 00:07:06,900
So we got to turn the string into a number.

134
00:07:06,900 --> 00:07:10,300
You need to be able to, maybe not check for typos necessarily,

135
00:07:10,320 --> 00:07:13,770
but maybe you don't know what those numbers are going to be,

136
00:07:13,780 --> 00:07:16,230
and if one of those numbers ends up being 0 because

137
00:07:16,240 --> 00:07:19,170
you have a malicious user, you need to be able to catch that

138
00:07:19,170 --> 00:07:20,300
'ZeroDivisionError'.

139
00:07:20,340 --> 00:07:22,920
And if it's anything else, what I like to do is just print

140
00:07:22,930 --> 00:07:25,900
the type and the error so that I can create an exception

141
00:07:25,910 --> 00:07:26,980
for it as I go along.

142
00:07:26,980 --> 00:07:29,700
Now, there's one more thing we can do in here. Two more things

143
00:07:29,720 --> 00:07:32,950
actually. We can do an 'else' statement, and we can do a 'finally'

144
00:07:32,950 --> 00:07:37,390
statement. So 'print', in an 'else' statement if there are no

145
00:07:37,390 --> 00:07:41,010
exceptions, so no exceptions were raised,

146
00:07:41,020 --> 00:07:42,360
this worked out perfectly,

147
00:07:42,360 --> 00:07:46,600
we can say, "Huzzah, success!

148
00:07:48,100 --> 00:07:54,200
We divided and ", and let's turn this

149
00:07:54,230 --> 00:07:55,570
into an f-string.

150
00:07:56,600 --> 00:07:58,700
And 'finally' will always run.

151
00:07:58,700 --> 00:08:02,900
"This will always run no matter what".

152
00:08:02,900 --> 00:08:04,900
[no audio]

153
00:08:04,900 --> 00:08:06,700
Let's try our little program here.

154
00:08:06,760 --> 00:08:07,960
Let's do 75.

155
00:08:07,960 --> 00:08:12,700
Divided by 25 is 3.0. That's correct.

156
00:08:12,700 --> 00:08:14,400
It says, "Huzzah success!

157
00:08:14,400 --> 00:08:17,100
We divided 75 by 25".

158
00:08:17,400 --> 00:08:22,200
And then the 'finally', that kicked in and said, "This will always

159
00:08:22,200 --> 00:08:24,600
run no matter what". And so a 'try-

160
00:08:24,600 --> 00:08:29,200
except-finally' block is a lot like an 'if-elif-else' statement

161
00:08:29,280 --> 00:08:31,680
but it has a slightly different purpose.

162
00:08:31,690 --> 00:08:35,190
So we're not trying to separate our code into different paths,

163
00:08:35,200 --> 00:08:38,100
which is what 'if-else' statement does.

164
00:08:38,710 --> 00:08:41,950
What we're trying to do, is perform some sort of logic,

165
00:08:41,960 --> 00:08:45,100
and if the program has a problem with that, we need to catch

166
00:08:45,110 --> 00:08:47,470
the program's error, not the user's error.

167
00:08:47,480 --> 00:08:50,710
We don't want to direct, we don't want to direct the user flow necessarily.

168
00:08:50,830 --> 00:08:54,880
We want to catch the program error, and when it errors, then

169
00:08:54,890 --> 00:08:58,260
we can sort of move the user into the right direction.

170
00:08:58,260 --> 00:09:01,400
We can make certain assumptions, or we could try things again.

171
00:09:01,460 --> 00:09:06,170
So let's go ahead and let's create a loop that will run forever

172
00:09:06,170 --> 00:09:09,700
until it can successfully divide a number. So let's do

173
00:09:09,740 --> 00:09:14,050
'while True: num1 = input()', we're just going

174
00:09:14,060 --> 00:09:17,140
to ask for number one, and 'num2' is also going to be

175
00:09:17,140 --> 00:09:18,900
input, we're going to ask for a second number.

176
00:09:18,900 --> 00:09:22,800
'num3' variable is going to be 'num1 / num2'.

177
00:09:23,900 --> 00:09:28,500
Now, this is where we would put our 'try' block because we

178
00:09:28,500 --> 00:09:30,800
know that this is likely to run into an error.

179
00:09:32,200 --> 00:09:34,900
Then we could 'except', and in that exception

180
00:09:35,100 --> 00:09:38,400
if it fails for any reason, we could say, "Nope.

181
00:09:38,800 --> 00:09:39,800
Try again".

182
00:09:39,860 --> 00:09:42,820
And let's put a 'print' statement in here.

183
00:09:42,900 --> 00:09:47,500
'print("Make a successful division")'.

184
00:09:47,500 --> 00:09:49,500
[no audio]

185
00:09:49,500 --> 00:09:50,800
Let's go ahead and run this.

186
00:09:50,800 --> 00:09:52,700
[no audio]

187
00:09:52,700 --> 00:09:53,900
It's going to ask for the first number,

188
00:09:54,000 --> 00:09:56,400
10; second number is going to be 0.

189
00:09:57,000 --> 00:09:58,000
Says, "Nope".

190
00:09:58,000 --> 00:09:59,880
Okay, going to ask for that first number again.

191
00:10:00,060 --> 00:10:01,350
10, 10.

192
00:10:02,500 --> 00:10:04,400
Nope. That's not going to work either.

193
00:10:04,480 --> 00:10:05,910
Now, why is that not working?

194
00:10:06,100 --> 00:10:09,500
Well, we said remember, this comes back to loops,

195
00:10:09,500 --> 00:10:12,400
we said, while this is True, this is always True, this will

196
00:10:12,400 --> 00:10:13,500
never, ever change.

197
00:10:13,500 --> 00:10:17,500
So because we know if this is going to cause an error, the

198
00:10:17,600 --> 00:10:20,020
code below it is not going to work in the 'try' block. It's

199
00:10:20,020 --> 00:10:24,600
going to automatically jump out, and jump into the first 'except',

200
00:10:24,600 --> 00:10:26,880
in which case we can say 'break'.

201
00:10:27,600 --> 00:10:31,200
And this cell is actually still trying to run.

202
00:10:31,220 --> 00:10:33,800
Okay. So that was just running infinitely.

203
00:10:33,800 --> 00:10:36,300
So I just went up to 'Kernel', went into 'Interrupt'.

204
00:10:36,300 --> 00:10:40,200
So what I'm going to do here is now rerun this with 'break'.

205
00:10:40,200 --> 00:10:42,200
[no audio]

206
00:10:42,200 --> 00:10:43,400
It's going to ask me for the first number,

207
00:10:43,430 --> 00:10:44,320
10; second number,

208
00:10:44,330 --> 00:10:46,520
let's do 20, and it didn't work.

209
00:10:46,530 --> 00:10:48,680
10 and 10, also not going to work.

210
00:10:48,690 --> 00:10:51,560
Now, we don't know why this is going to work, so, and this is

211
00:10:51,560 --> 00:10:54,000
starting to get, to get a little more realistic here.

212
00:10:54,020 --> 00:10:56,000
Let's go ahead and hit 'Kernel', 'Interrupt'.

213
00:10:56,010 --> 00:11:03,530
And let's 'except' this as an 'Exception as e', and 'print(e)', or

214
00:11:03,530 --> 00:11:07,800
'print(type(e))', and 'print(e)' as well.

215
00:11:07,820 --> 00:11:14,240
And all 'e' is, is a variable for 'Exception' here, and we can

216
00:11:14,250 --> 00:11:15,650
run that inside of our 'except' block.

217
00:11:15,650 --> 00:11:17,100
So let's go ahead and rerun this.

218
00:11:17,100 --> 00:11:19,900
10, 20. 'TypeError'.

219
00:11:20,900 --> 00:11:21,900
Guess what?

220
00:11:21,920 --> 00:11:25,280
You cannot divide a string by string.

221
00:11:25,500 --> 00:11:27,400
Let's go ahead. Interrupt this.

222
00:11:29,300 --> 00:11:30,600
Let's cast this as a 'float'.

223
00:11:31,300 --> 00:11:32,600
Let's cast this as a 'float'.

224
00:11:33,300 --> 00:11:34,300
Let's rerun it.

225
00:11:35,100 --> 00:11:36,100
10, 20.

226
00:11:37,300 --> 00:11:38,300
Cool. That worked.

227
00:11:38,900 --> 00:11:39,900
10, 0.

228
00:11:40,500 --> 00:11:42,700
Nope. Got a division error in there.

229
00:11:42,780 --> 00:11:44,520
Okay. Let's try that again.

230
00:11:45,000 --> 00:11:46,000
10, 1.

231
00:11:46,000 --> 00:11:47,600
And it worked, dnd then it exited.

232
00:11:47,660 --> 00:11:51,800
So that's a little more of a real life use case of 'try and

233
00:11:51,810 --> 00:11:55,660
except' in a loop that's essentially supposed to run forever

234
00:11:55,660 --> 00:11:56,800
until it's told not to.

235
00:11:57,400 --> 00:12:00,100
Anyways, that is 'try and except'.

236
00:12:00,100 --> 00:12:01,200
If you would like,

237
00:12:01,200 --> 00:12:02,400
and I would like you to,

238
00:12:02,400 --> 00:12:05,200
if you have a couple of minutes, go ahead and give this a shot.

239
00:12:05,210 --> 00:12:08,780
Create a 'try' block, cause some sort of error on purpose.

240
00:12:08,790 --> 00:12:10,610
You can even 'try: 1/0'.

241
00:12:10,610 --> 00:12:11,800
That's a common one.

242
00:12:11,800 --> 00:12:15,300
And then 'except' that particular 'exception',

243
00:12:15,900 --> 00:12:19,300
and then also run an 'else' statement and a 'finally' statement.