Handling a File Error

As already pointed out in the exceptions lesson, Python contains a vast set of built-in exceptions. One important class has to do with file handling. The example presented in the lesson will help to review file handling methods and put them in the context of handling exceptions.

Adding Arguments to Exceptions

Sometimes it is handy to add arguments to exceptions in order to give a better context to users. With the example of the average, let's first define a more complex exception:

class MyException(BaseException):
    pass

class NonPositiveIntegerError(MyException):
    def __init__(self, x, y):
        super(NonPositiveIntegerError, self).__init__()
        if x<=0 and y<=0:
            self.msg = 'Both x and y are negative: x={}, y={}'.format(x, y)
        elif x<=0:
            self.msg = 'Only x is negative: x={}'.format(x)
        elif y<=0:
            self.msg = 'Only y is negative: y={}'.format(y)

    def __str__(self):
        return self.msg


def average(x, y):
    if x<=0 or y<=0:
        raise NonPositiveIntegerError(x, y)
    return (x+y)/2

try:
    average(1, -1)
except MyException as e:
    print(e)

What you can see is that the exception takes two arguments, x and y and it generates a message based on them. They can be both negative or only one of them is negative. It doesn't only give you that information, but it actually displays the value that gave problems. This is very handy to understand what went wrong exactly. The most important part is at the end of the class: the __str__ method. This method is responsible for what appears on the screen when you do print(e) in the except block. In this case, we are just returning the message generated within the __init__, but many developers choose to generate the message in this method, based on the parameters passed at the beginning.