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Basic question: What's the most Pythonic/logical way to make my own, custom warning classes? What are the correct warning and exception classes that I should be subclassing? Motivation : The requirements for the library I'm writing specify that if a MyContainer object c contains an item x and the caller of the library tries to place a "duplicate" of x -- call it y -- into c , a warning is issued to the caller and the return value of c.my_transformation_method(x, y) is placed into c to replace x . In other words, MyContainer s will replace elements with their duplicates, but must warn the user when doing so. Based on my reading, the most flexible way to warn the caller of a library about a nonfatal action is with the warnings standard module . It allows the caller to handle the warning as it sees fit, doing anything from ignoring warnings to treating them as errors. (Note that I'm using Python 3, but I don't think that's essential to the question here.) Example: What I've done is defined the following warning subclass: class DuplicateItemWarning(UserWarning, ValueError): pass Then the add() method of MyContainer calls warnings.warn('detected duplicate', DuplicateItemWarning) when it detects an attempt to insert a duplicate item. Specific questions: Should I be subclassing UserWarning as above, or just sublcassing Warning ? It seems semantically sensible to subclass ValueError (which, in the above example, merely inserts ValueError in the MRO between Warning and Ex

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