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Python Programming On Win32 by Mark Hammond, Andy Robinson

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Error Handling for COM Servers

When you implement a COM object, it’s often necessary to return error information to the caller. Although Python has a powerful exception mechanism, the caller of your objects is likely to be Visual Basic or Delphi, so standard Python exceptions don’t really work.

To support this, the win32com.server.exception module exposes the COMException Python object in order to raise an exception to COM. This object allows you to specify many details about the error, including the error message, the name of the application generating the error, the name of a help file in which the user can find additional information, etc. See the win32com.server.exception module for more details.

The PythonCOM framework makes the assumption that all Python exceptions other than COMException indicate a bug in your code. Thus, your object shouldn’t allow normal Python exceptions to be raised when calling your methods, but should take steps to handle these Python exceptions and translate them to an appropriate COMException.

As an example, let’s assume you want to publish a method called sqrt() that returns the square root of its argument. If you use the following code:

def sqrt(self, val):
        return math.sqrt(val)

you have a potential problem; in fact, a few of them. If you pass anything other than a positive number to your function, the code fails, and a Python exception is raised. This is considered a bug in your COM object. To improve this function, use the following code:

def sqrt(self, ...

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