FAQ and other help

Frequently asked questions

Q: Why are some of my files not measured?

Coverage.py has a number of mechanisms for deciding which files to measure and which to skip. If your files aren’t being measured, use the --debug=trace option, also settable as [run] debug=trace in the settings file, or as COVERAGE_DEBUG=trace in an environment variable.

This will write a line for each file considered, indicating whether it is traced or not, and if not, why not. Be careful though: the output might be swallowed by your test runner. If so, a COVERAGE_DEBUG_FILE=/tmp/cov.out environment variable can direct the output to a file instead to ensure you see everything.

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 are my function definitions marked as run when I haven’t tested them?

The def and class lines in your Python file are executed when the file is imported. Those are the lines that define your functions and classes. They run even if you never call the functions. It’s the body of the functions that will be marked as not executed if you don’t test them, not the def lines.

This can mean that your code has a moderate coverage total even if no tests have been written or run. This might seem surprising, but it is accurate: the def lines have actually been run.

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

If this happens, it’s because coverage.py has 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 CTracer: available or CTracer: unavailable. If it says unavailable, 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 Python discussion forums. If you ping me (@nedbat), there’s a higher chance I’ll see the post.

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.