How to collect coverage for C extensions in Python¶
Collecting code coverage data on the C code that makes up a Python extension module is not quite as straightforward as with a regular C program.
As with a normal C project,
we have to compile our code with coverage instrumentation.
and then run
python3 setup.py build_ext.
build_ext can rebuild a source file
even if the current object file is up to date.
If multiple extension modules share the same source code file,
gcov will get confused by the different timestamps
and report inaccurate coverage.
It is nontrivial to adapt the
build_ext process to avoid this.
Instead, we can use the
ccache utility to make the compilation lazy
(works best on Unix systems).
Before we invoke the
build_ext step, we first
export CC="ccache gcc".
Ccache works well but isn’t absolutely perfect,
see the ccache manual for caveats.
A shell session might look like this:
# Set required env vars export CFLAGS="--coverage" export CC="ccache gcc" # clear out build files so we get a fresh compile rm -rf build/temp.* # contains old .gcda, .gcno files rm -rf build/lib.* # rebuild extensions python3 setup.py build_ext --inplace # possibly --force # run test command i.e. pytest # run gcovr rm -rf coverage; mkdir coverage gcovr --filter src/ --print-summary --html-details -o coverage/index.html