#100DaysOfCode in Python Transcripts
Chapter: Days 25-27: Error handling
Lecture: Demo: Error types
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So we've stopped the errors from crashing our program
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but we can't do anything meaningful,
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we can only say, you know, that didn't work, sorry.
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Let's be a little more prescriptive here.
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So up the top,
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we need to work with the various exception types,
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so we have to import some stuff.
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So we'll say request.exceptions down here.
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Now in this area, we want to catch
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the different types of problems that can occur,
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and the way we do that is we say except:
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and then we say 'different types',
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remember when we looked at that trace back,
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the first thing is the category, or the type of error,
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and the second part after the colon was the description,
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just more details.
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So what we need to do is,
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we could have potentially multiple ones of these,
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we'll say this, we'll say,
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request.exceptions.connectionerror, okay?
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We be able to just go like this,
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and just do a little print here and say,
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so maybe we'll say something like,
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'couldn't find the server, check your network connection',
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make this really obvious like that,
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and print that out here.
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Now, it could be if this was like a web browser
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they could've typed it in wrong,
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but we've hard-coded the URL, we know that that's correct,
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so if they can't connect it's not the server doesn't exist,
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it's really that there's some problem
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getting to this one known server.
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So let's try this again with my internet off.
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We'll come down here and search for test.
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'Could not find server, check your internet connection.'
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Oh, how cool, that's way more helpful,
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we know exactly what went wrong
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when we want to handle that problem here.
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Now the network is back and we have another problem.
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If we enter no search term it's going to freak out
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and throw what's called a ValueError, and say,
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'hey that didn't work.'
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But all we get is, oh, that didn't work.
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So we could do a little bit better here,
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we could say,
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put this core exception here,
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and we could put some details like so.
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This is kind of going to be our fallback,
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we don't really know what's going on,
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so we're going to try this.
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Let's try again.
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Oh, that didn't work, 'you must specify a search term',
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and if you're just debugging it to try to figure out
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what's going on here you'd say,
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what is the actual thing that I'm getting here?
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What is this actual error?
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It's a ValueError.
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Okay.
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So now let's go add an except clause for that.
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Now notice this is gray because we're not using it,
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and when we're not, we can just leave it off like,
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as we did with the request here.
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So we don't need any details, so now,
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try to run this with nothing,
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'error, you must specify a search term.'
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Now we can really tell the user what's going on.
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We have a few more errors
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that we could potentially deal with,
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I wouldn't leave this here like that,
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and these other errors are just kind of random stuff
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that I threw in there to make the program crash.
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One is a StopIteration,
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and the other is the one that you saw at the opening,
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which is a TypeError I believe.
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So we'll just let those fall through here
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because there's not anything we can particularly do,
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they're just sort of random noise in the system
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to make sure you get some interesting crashes.
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These have real fixes,
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so we have a special message for them.
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The final important thing to see here
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in this whole try except block,
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is the order in which we specify the errors.
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If we change this around
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and we put this up here at the front,
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PyCharm's probably going to freak out
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and say, "no no no, you'll never be getting to these."
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If we have something general,
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and this is a derivative of exception,
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this is not going to work.
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It's actually just going to stop.
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The way it works is, it runs,
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if there's an error it just looks, goes, 'is the error this type, is the error this type,
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is the error that type?'
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And, if it comes along here,
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this is going to catch almost everything,
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and so, even though we have this,
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you'll see if I run it and I hit enter,
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that didn't work.
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I mean, we still did sort of print this out,
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but it's not letting us get to the part where we expect it.
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So it's super important that this goes
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from most specific to most general.
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So, here we have it;
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we've figured out what types of errors
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our program might throw,
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and then we can handle them independently based on the type.
