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Python uses exceptions as its primary method of handling errors.

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We can raise our own exceptions in the functions and modules that we write.

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Let's take a look at how we do that.

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Make a working copy of exceptions.py, call it exceptions-working.py, and we'll go

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ahead and open our working copy, and here we have our simple print the lines of

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text from this file. Go ahead and run that.

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Now, let's say that we want it to write a function that will open a file and

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return its lines of text.

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It's pretty easy little function, call it readfile, and we'll pass it a filename

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and open(filename), like that, and we'll return the readlines method from the

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filehandle and that's actually an Iterator and it allow us to do this. We can

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now get rid off that and here we can say readfile lines.text. Save that and run it

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and there it works exactly like we expect.

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Now, let's say that we misspell lines.text here. We'll get our same error

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message that we got before, IOError, and of course we can handle that in a try block like this.

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And there we have our error handling, our exception catching,

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and there we get our error message like that, but let's say that we have another

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condition that we want to raise a different kind of an error for.

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We could check the filename for example, using the endswith method of the string

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object to see if it ends with .TXT. And in that case, we'll just go ahead and do

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what we normally do, an else. We're going to raise an exception.

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We're going to use the raise statement, we're going to raise a ValueError and we're going to

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give it a little message, "Filename must end with .TXT," all right and then give

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it a different kind of a name here.

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We save this and run it.

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Now, we have an unhandled exception.

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We get exactly the same kind of error reporting that Python does when it raises

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its own exceptions and we see it gives us the trace back.

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It's just an unhandled exception.

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That's what Python does with an unhandled exception.

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And so, we can handle that exception with a separate except, except ValueError

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as e and we can print bad filename and the message.

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So now when we save it and run it, we get this nice little error message here.

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So that's how we raise an exception in Python.

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You'll notice it's very simple. We use the raise keyword and we give it the exception.

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Python has predefined a number of exceptions in its library, pretty much

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anything you are going to need, and on your screen is the URL where you can find

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that list with a lot of descriptions of them.

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In this way you don't need to define your own and your programs will work more

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consistently with the rest of the Python universe, by using the exception names

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that are already defined.