Question

If an object relies on a module that is not included with Python (like win32api, gstreamer, gui toolkits, etc.), and a class/function/method from that module may fail, what should the object do?

Here’s an example:

import guimodule  # Just an example; could be anything

class RandomWindow(object):

    def __init__(self):
        try:
            self.dialog = guimodule.Dialog()  # I might fail
        except: guimodule.DialogError:
            self.dialog = None  # This can't be right

    def update(self):
        self.dialog.prepare()
        self.dialog.paint()
        self.dialog.update()

    # ~30 more methods

This class would only be a tiny (and unnecessary, but useful) part of a bigger program.

Let’s assume we have an imaginary module called guimodule, with a class called Dialog, that may fail to instantiate. If our RandomWindow class has say, 30 methods that manipulate this window, checking if self.dialog is not None will be a pain, and will slow down the program when implemented in constantly used methods (like the update method in the example above). Calling .paint() on a NoneType (when the Dialog fails to load) will raise an error, and making a dummy Dialog class with all of the original’s methods and attributes would be absurd.

How can I modify my class to handle a failed creation of the Dialog class?

Answer 1

Rather than creating an invalid object, you should have allowed the exception raised in __init__ to propogate out so the error could be handled in an appropriate manner. Or you could have raised a different exception.

See also Python: is it bad form to raise exceptions within __init__?