Question

I have a class where I create a file object in the constructor. This class also implements a finish() method as part of its interface and in this method I close the file object. The problem is that if I get an exception before this point, the file will not be closed. The class in question has a number of other methods that use the file object. Do I need to wrap all of these in a try finally clause or is there a better approach?

Thanks,

Barry

Answer 1

You could make your class a context-manager, and then wrap object creation and use of that class in a with-statement. See PEP 343 for details.

To make your class a context-manager, it has to implement the methods __enter__() and __exit__(). __enter__() is called when you enter the with-statement, and __exit__() is guaranteed to be called when you leave it, no matter how.

You could then use your class like this:

with MyClass() as foo:
    # use foo here

If you acquire your resources in the constructor, you can make __enter__() simply return self without doing anything. __exit__() should just call your finish()-method.