subprocess-support.rst

Subprocess support

Normally coverage writes the data via a pretty standard atexit handler. However, if the subprocess doesn't exit on its own then the atexit handler might not run. Why that happens is best left to the adventurous to discover by waddling though the Python bug tracker.

pytest-cov supports subprocesses and multiprocessing, and works around these atexit limitations. However, there are a few pitfalls that need to be explained.

If you use multiprocessing.Pool

In pytest-cov 2.6 and older a multiprocessing finalizer is automatically registered. The finalizer will only run reliably if the pool is closed. If you use multiprocessing.Pool.terminate or the context manager API (__exit__ will just call terminate) then the workers can get SIGTERM and then the finalizers won't run or complete in time. Thus you need to make sure your multiprocessing.Pool gets a nice and clean exit:

from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':
    p = Pool(5)
    try:
        print(p.map(f, [1, 2, 3]))
    finally:
        p.close()  # Marks the pool as closed.
        p.join()   # Waits for workers to exit.

In pytest-cov 2.7 a SIGTERM handler is also automatically registered if multiprocessing is used. Thus you can use the convenient context manger API:

from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':
    with Pool(5) as p:
        print(p.map(f, [1, 2, 3]))

If you use multiprocessing.Process

There's similar issue when using the Process objects. Don't forget to use .join():

from multiprocessing import Process

def f(name):
    print('hello', name)

if __name__ == '__main__':
    p = Process(target=f, args=('bob',))
    try:
        p.start()
    finally:
        p.join()  # necessary so that the Process exists before the test suite exits (thus coverage is collected)

If you got custom signal handling

pytest-cov 2.6 has a rudimentary pytest_cov.embed.cleanup_on_sigterm you can use to register a SIGTERM handler that flushes the coverage data.

pytest-cov 2.7 adds a pytest_cov.embed.cleanup_on_signal function and changes the implementation to be more robust: the handler will call the previous handler (if you had previously registered any), and is re-entrant (will defer extra signals if delivered while the handler runs).

For example, if you reload on SIGHUP you should have something like this:

import os
import signal

def restart_service(frame, signum):
    os.exec( ... )  # or whatever your custom signal would do
signal.signal(signal.SIGHUP, restart_service)

try:
    from pytest_cov.embed import cleanup_on_signal
except ImportError:
    pass
else:
    cleanup_on_signal(signal.SIGHUP)

Note that both cleanup_on_signal and cleanup_on_sigterm will run the previous signal handler.

Alternatively you can do this:

import os
import signal

try:
    from pytest_cov.embed import cleanup
except ImportError:
    cleanup = None

def restart_service(frame, signum):
    if cleanup is not None:
        cleanup()

    os.exec( ... )  # or whatever your custom signal would do
signal.signal(signal.SIGHUP, restart_service)

If you use Windows

On Windows you can register a handler for SIGTERM but it doesn't actually work. However you can have a working handler for SIGBREAK:

import os
import signal

def shutdown(frame, signum):
    # your app's shutdown or whatever
signal.signal(signal.SIGBREAK, shutdown)

try:
    from pytest_cov.embed import cleanup_on_signal
except ImportError:
    pass
else:
    cleanup_on_signal(signal.SIGBREAK)

Note that SIGBREAK is tricky:

• you need to deliver signal.CTRL_BREAK_EVENT
• it gets delivered to the whole process group, and that can have unforeseen consequences