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Exceptions are a primary method of handling errors in Python.

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So let's take a look at how we handle exceptions.

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We will make a working copy of exceptions.py, call it exceptions-working.py.

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We will open our working copy and here we have a very simple little script

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that reads lines of text from a file and there is the text file there and it

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prints them on the screen.

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Go ahead and we will run this and there is our lines of text.

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You'll notice, we did this in different way this time.

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Instead of using end = a blank string, ctually using the strip method in

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the string itself and that just strips any trailing new lines from the end of the string.

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It's another way to do it, using the string method rather than using the print

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function to resolve that discrepancy.

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Now if I had typed the name wrong or if I am referring to a file that doesn't

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exist and I save this and I run it,

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you'll notice that we get one of Python's lovely little error messages and it

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has the Traceback (most recent call last).

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It's doing what it can to help you to find the error in your code and then down

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at the bottom here, it says IOError and it has a message.

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This IOError before the colon is actually the name of the exception.

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That's the exception that's being raised by the Python interpreter and then

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after the colon is its error message.

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Now I can trap that exception.

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And then I could do something different with it in my code.

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So here is how I do that.

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I use try and I am just going to put that line there in the try and then I use

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except like this, and I can print a message.

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I can say, "could not open the file."

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And then I don't want this to run, so I can put that in the else.

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Put the else there and so now what I have, I save this and run it.

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I will just get this little message, "could not open the file, come back tomorrow"

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and it doesn't try to read the lines in it.

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And if the typo is not there, I save that and run it, now I get the actual lines of text.

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Now this will actually catch any error at all, because I just said except like that.

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If I want to just kept that particular error, I say except IOError, and if I

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save that, well let's go ahead and just type this again.

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Save that, run it, then I get that error.

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So it's sometimes useful to go ahead and let Python give you that error message,

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because that's the easy way to find out which error is going to raise.

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I can also get the error message itself. I can say as e:

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like this and then instead of come back tomorrow, I can give it that error

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message like this and then save and run and it says, "Could not open the file:

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Error 2 No such file or directory:".

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And there we have something it's actually useful for the user.

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Now we can actually if we want to, we can put all of this code up here in

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the try and it doesn't hurt anything to do it that way and then we don't

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need an else clause.

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Save that and run it.

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We are still getting our error message.

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Let's go ahead and cut that out and we see that it works the same way.

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So in the event that you have more than one line of code in your try clause,

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execution will stop after the error is raised.

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So the error gets raised here and then it will go right to handling the error

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in the except clause.

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So this line won't get run when the exception happens.

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So if we have this misspelling in here, you save it and run it, we get the error

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message and it doesn't try to use the file handle that it never got, because as

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soon as that exception is raised, which happens in this open function, then the

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exception clause is run right away.

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So you'll see this pattern commonly where a number of lines of code will be put

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in a try block and then in fact sometimes there will be several different except

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clauses as well, listing out a number of different errors that could be raised.

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So this is how exceptions work in Python and this is how you handle exceptions

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in your code using try and except.