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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Now we have our error handling in place, our app can fail a search
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either because the network is down or for other reasons,
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and we do catch it, but we have no information about it here,
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it just says hey, that didn't work, sorry, something like that,
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so we can add a little bit of extra information here to carry that error over,
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and we can work with it, so we could say except exception as
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and then we name a variable, the format is a little bit like context manager,
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so except exception as x, and so this x is going to have all the details,
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let's just say it like this, so now we can print this out
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and we can see what the details are, so if we come over here
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and turn off the wi-fi, I'll search for network— boom, that didn't work
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http connection pool, let's see we got some kind of error
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we'll figure out what that is in a second,
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but how do you think users are going to feel about this error,
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they are going to love it, they think this is a great error message
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that really tells them what's going on—
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no, they're going to hate it, so we need to do something a little more specific,
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the first thing I want to look at is just what is the type really quick of that error,
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the actual type, so the type is a request.exception.connection_error
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so let's put this back for a second, and go up here to the top and import that;
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ok, so now we'll be able to over here and we can have multiple except blocks
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so watch this, we come up here and we create another one
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we can say request.exception.connection_error
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as this should be named differently, say ce, something like that
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so for this one, we could change this and we could say
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something to the effect of error, your network is down,
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or host unreachable or something like that.
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Now, we could put in details here,
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but really I feel like that message already carries over enough of the information,
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so let's just try this again with the network off,
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I search for J — error, your network is down;
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notice it didn't run this 'Yikes, that didn't work',
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the way it works is we do the try block and if there is an error
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Python will start looking from top to bottom to say
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the thing that is the error is it one of these types,
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the very first one that matches it's going to go with that.
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So the ce thing, this is also an exception
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because this is a base type in terms of inheritance,
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it does match this one, so the first one it finds it stops and it uses that
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so this means that you need the most specific ones first
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and these more general ones have to go at the end
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because it's just going to go top to bottom and stop as soon as it finds one.
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Now notice here we're using the x and we're putting the details here
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but in this one, we're not, like we're not actually using the ce
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and PyCharm has it grayed, so we can omit this like so.
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Now there's another problem, let me put that back just for a second
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so I can show you the details, ok let's run this,
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now when the wi-fi was off, we had a particular problem,
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like if I do this it crashes, but let's turn the wi-fi on.
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Now if I search for 'runner', it should work,
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but if I search for nothing, if I just hit enter, it crashes, Yikes, that didn't work
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client 404, this was not found, what is going on here?
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So if you go look at this, notice the url structure has to have an api search
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and then the keyword, if the keyword's not there,
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you just get 404 because hey, I don't know what to do with that.
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All right, so that is a problem, we need to somehow take that error
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which was not a connection error, it was a different type, it was,
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I think some sort of raise for status thing, yeah http error, more general one.
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What we want to do is we want to actually go over to this movie service here
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and say not even let them try to call this, like why are we doing this,
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if this data is invalid we should check that,
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so we can come over here and we can do this,
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we can say if not search text, so if it's completely like if it's equal to none,
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we want to test this or, not search text.strip
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so if they just put like space or new line or something like that;
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if that's the case, we want to communicate an error back to them as well,
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we don't even want to let this code run, we just want to say no that failed.
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Now we could return false, but that's very tricky to check,
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how do you indicate empty result versus false, and things like that,
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so what we should use here, just like request
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some things like that are, we should use exceptions.
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In this case, we want to create an exception, not catch one,
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so we'll say raise, and probably the best thing to say is
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value error, something like search text is required, something to that effect, okay.
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Now if we run our code, we have our network on,
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and if we search for 'cats', we get results,
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if we search for nothing we get Yikes that didn't work value error,
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so let's go back and add another specific error message for value error,
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so we go over here, say value error
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and here we'll say something to the effect of search text is required
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let's clean this up a little bit, like this.
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Excellent, so if there is a value error, we give them this message,
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if there's a network error, we give them this message,
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otherwise we'll just say unexpected error, details like that
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and let's not put the type, there we go,
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so I feel like this is pretty good, let's give it a try—
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network is on, we search for 'runner', do we get something, yes,
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we search for nothing— error, search text is required,
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network turned off, what do we get— error, your network is down;
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search for nothing— error, search text is required,
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turn the network back on, last time—
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and we're back in business, how about that?
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We're able to use the type of error to actually catch it,
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and handle different cases differently,
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and we're also able to raise these errors
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and indicate some kind of problem
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before it cascades into something non obvious
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like the http error, like why does that mean that the search text is empty,
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let's be more proactive and make that really obvious
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in a way that we can deal with it specifically.
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Alright, so that's error handling in Python.
