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James Bennett wrote to me the other day to ask for help with a problem using coverage.py. Some code he was measuring behaved very strangely: according to coverage.py, the first half of the function was executing, but the second half wasn't. And this was happening for a whole group of functions. These were test functions for XML-producing Django views, and each parsed the XML. The call to the XML parsed was the last function executed each time, none of the lines after it were detected as run. My first thought was that the XML parsing was throwing an exception, the simplest explanation for why execution would stop in the middle of a function. It made sense, since it was a similar operation in each function exhibiting this behavior. And James had already had another problem with this same code where an unexpected exception was throwing off his tests. But James is a clever boy, and proved that exceptions were not to blame by adding asserts, print statements and so on. The lines were really being executed, but coverage.py didn't think they were. Something more interesting and unusual was at work. I broke out my old quip...