Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 10: Movie Search App
Lecture: Using try except to handle specific errors

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0:00 We've seen that we can stop the errors from crAshing our app
0:04 and sending out a kind of meaningless error message,
0:07 let's try to improve this.
0:09 So I can actually say I would like to handle specific types of errors,
0:12 and first of all, before I even get to that, like what was the problem,
0:16 I can't even tell them here what went wrong,
0:19 I can start by saying except exception as x:,
0:23 this tells us what type of exception we want to catch
0:26 and then this is kind of like our context manager with block
0:29 creates the object that we re going to store
0:32 so I can come down here and say format, I can put the x here,
0:35 so now I have a little better information, so if I hit nothing,
0:38 it says Yikes, that didn't work, you must specify a string.
0:42 Turn off the network, it says Yikes, that wasn't good,
0:44 your http connection, da da da, what do you think users will feel
0:48 when they see that message, will they feel joy,
0:51 will they be like wow I love this program, it really communicates well with me,
0:55 like no, this is not a great message,
0:58 so let's change this, now we kind of need to know
1:01 what type that is and this is coming from requests
1:04 so let's just print out really quick the type here,
1:07 so when we enter nothing we get ValueError, when we enter something,
1:11 but the network is down we get request.exceptions.ConnectionError,
1:15 so let's handle these differently.
1:18 Let me import this type up here, well say import,
1:21 so we'll import request.exceptions
1:23 and down here I can say another except like I can say except
1:27 request.exceptions.ConnectionError,
1:29 there is a connection error that's built in in Python
1:32 and the built in to this is a different one, as let's say ce,
1:34 and then let me just say pass for a moment
1:37 then I could say except ValueError as ve: pass
1:42 and then I can have this down here,
1:44 so we could have sort of three levels of error handling,
1:47 we should log this or do other things,
1:49 but let's just print out some message,
1:51 well say cannot search your network is down.
1:54 Or host unreachable, I mean, there is it's not just your network right,
1:57 it could be something in the middle,
1:59 over here we could say print('ERROR: Your search string is invalid: {})
2:04 and give us some more info, and down here we could finally just say
2:08 let's take away this little type thing that was just temporary,
2:11 I will just say ERROR, and we don't really know why, right,
2:14 but here in these cases we know what connection error means,
2:17 that means that you couldn't search your network is down.
2:20 We know a ValueError means that means that string you entered is invalid
2:23 now notice this is great and it's not used,
2:26 if in this case we are not actually using it, we could,
2:29 but if we are not we can omit this section.
2:31 Ok, and it's just the exception type,
2:34 the error type that we are kind of get down here to drive this
2:36 so let's try this again, enter nothing, error you search string is invalid, let's fix that,
2:41 ERROR: Your search string is invalid, You must specify a search string,
2:46 now if I enter cats remember network is still down,
2:49 ERROR: Cannot search your network is down.
2:52 So here is how we use try except blocks to make our code reliable
2:57 and differentiate on particular errors, right,
3:00 so we try to do some search, it's possible that there is a ValueError
3:05 because we've given it some invalid data here.
3:08 That will throw the exception,
3:09 raise the exception and it will come in tests
3:11 and it says is the type derived from,
3:14 is it a connection error from request
3:16 or is it a more specific derived class, answer is no,
3:19 and it'll say is it derived from more exactly ValueError, yes,
3:22 and so it will run this code and this is it, if it's none of those,
3:26 it will run this exception,
3:28 now exception is not the most base type but there are certain things
3:32 that you don't want to catch that are below exception,
3:35 things like control c right, if you catch,
3:39 if you just did a straight up except
3:41 like here we could say, for a moment, like this,
3:47 so if I come down here and I hit this,
3:49 I get some error if I search for cats it should work,
3:52 the network is back, see I am trying to stop the program, it says no,
3:57 you will not stop this, right, so this is actually this is bad, bad news,
4:02 you don't want to catch too broad of an exception, right,
4:05 but here is a pretty good place to go.
4:08 Right, let's run our app one more time just to make sure it works,
4:11 ok let's search for capital c boom, there is our results, capital c,
4:17 now if we enter nothing we should get the error,
4:19 if we enter x we should be out of here.
4:22 That's error handling in Python.