Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 10: Movie Search App
Lecture: Catching errors with try except
0:00 We have our network working again, we searched for cats and dogs,
0:03 we should get things about cats and dogs, but if we turn off it's going to crash.
0:09 Not so great.
0:11 Another thing maybe we want to do is we might want to check
0:14 if I just enter nothing instead of actually trying to go and search for nothing
0:19 that I don't even know what that means
0:21 maybe we also want to validate this input and return that as an error as well.
0:24 remember, in normal methods,
0:27 you theoretically could return like false or none or something like that to say no,
0:32 but in these initializers, you can't,
0:36 there is no way to communicate an error other than an exception,
0:39 that said, for both methods and initializers,
0:42 often exceptions are the preferred way to do things in Python.
0:46 So let's come down here and well do a little test
0:49 and we can say something like this,
0:51 if not search text or not search text.strip(),
0:56 we need this first test here because if search text comes in as none,
1:01 this will actually give us an exception trying to call it strip on none, ok.
1:06 So either it's empty or it's not entirely empty but it's white space,
1:11 then we want to basically cause some kind of error,
1:15 we want to report an error back
1:17 and we can say raise ValueError, it's one possible option here,
1:22 we can say so let's try this, if I go down here and I hit cats,
1:25 I should get NetworkError, if I come down here
1:29 and I hit empty ValueError you must specify a search string,
1:33 ok, so we are no longer going to be sort of sending off meaningless searches
1:37 and in fact we can't create a movie client in a malformed state,
1:42 we'll never be able to even allocate and initialize an object
1:46 instead we'll raise this ValueError, ok.
1:48 Now, in both cases, it didn't really matter what we do, our app is crashing,
1:52 we'd like to just go no that didn't work, keep going,
1:55 so the way we do that in Python is we come down here
1:59 and we use what's called a try block so we say try:,
2:02 and here is all the code we would like to interact with
2:04 and if anywhere along the way,
2:06 if this allocation this initialization threw us an exception
2:10 because of here or the perform search those an exception
2:13 because the network is down or something like this,
2:15 regardless of where the error happens,
2:17 we'd like to immediately start running there
2:20 and go deal with the error so we can say except:, and for now,
2:22 we'll just say print('YIKES, THAT SUCKED'), ok,
2:26 so if we try this, notice there is a little warning here
2:29 but this should help let's go hit enter, yikes, that didn't work,
2:31 search for something else, oh, that didn't work, turn the network,
2:36 if I come down here search for cats now, hey, it worked,
2:39 so it's no longer crashing, that's really good
2:41 but you'll see that PyCharm is warning us about this
2:44 and we should improve it, it says, you are doing too broad of a catch,
2:49 you are just catching everything
2:51 and there is no meaningful information here giving back about it,
2:54 let's do that next.