FAQ and other help

Frequently asked questions

Q: Why do unexecutable lines show up as executed?

Usually this is because you’ve updated your code and run coverage.py on it again without erasing the old data. Coverage.py records line numbers executed, so the old data may have recorded a line number which has since moved, causing coverage.py to claim a line has been executed which cannot be.

If old data is persisting, you can use an explicit coverage erase command to clean out the old data.

Q: Why do the bodies of functions show as executed, but the def lines do not?

This happens because coverage.py is started after the functions are defined. The definition lines are executed without coverage measurement, then coverage.py is started, then the function is called. This means the body is measured, but the definition of the function itself is not.

The same thing can happen with the bodies of classes.

To fix this, start coverage.py earlier. If you use the command line to run your program with coverage.py, then your entire program will be monitored. If you are using the API, you need to call coverage.start() before importing the modules that define your functions.

Q: My decorator lines are marked as covered, but the “def” line is not. Why?

Different versions of Python report execution on different lines. Coverage.py adapts its behavior to the version of Python being used. In Python 3.7 and earlier, a decorated function definition only reported the decorator as executed. In Python 3.8 and later, both the decorator and the “def” are reported. If you collect execution data on Python 3.7, and then run coverage reports on Python 3.8, there will be a discrepancy.

Q: Can I find out which tests ran which lines?

Yes! Coverage.py has a feature called Dynamic contexts which can collect this information. Add this to your .coveragerc file:

dynamic_context = test_function

and then use the --contexts option when generating an HTML report.

Q: How is the total percentage calculated?

Coverage.py counts the total number of possible executions. This is the number of executable statements minus the number of excluded statements. It then counts the number of those possibilities that were actually executed. The total percentage is the actual executions divided by the possible executions.

As an example, a coverage report with 1514 statements and 901 missed statements would calculate a total percentage of (1514-901)/1514, or 40.49%.

Branch coverage extends the calculation to include the total number of possible branch exits, and the number of those taken. In this case the specific numbers shown in coverage reports don’t calculate out to the percentage shown, because the number of missing branch exits isn’t reported explicitly. A branch line that wasn’t executed at all is counted once as a missing statement in the report, instead of as two missing branches. Reports show the number of partial branches, which is the lines that were executed but did not execute all of their exits.

Q: Coverage.py is much slower than I remember, what’s going on?

Make sure you are using the C trace function. Coverage.py provides two implementations of the trace function. The C implementation runs much faster. To see what you are running, use coverage debug sys. The output contains details of the environment, including a line that says either tracer: CTracer or tracer: PyTracer. If it says PyTracer then you are using the slow Python implementation.

Try re-installing coverage.py to see what happened and if you get the CTracer as you should.

Q: Isn’t coverage testing the best thing ever?

It’s good, but it isn’t perfect.

Q: Where can I get more help with coverage.py?

You can discuss coverage.py or get help using it on the Testing In Python mailing list.

Bug reports are gladly accepted at the GitHub issue tracker.

I can be reached in a number of ways, I’m happy to answer questions about using coverage.py.


Coverage.py was originally written by Gareth Rees. Since 2004, Ned Batchelder has extended and maintained it with the help of many others. The change history has all the details.