When dealing with open files, Python has the with syntax that makes sure the file closes when leaving the block - regardless of exceptions etc.
with open('foo.txt') as f:
foo = f.read()
Since processes are resources too, I was wondering: is something similar possible or recommended when using Popen? For example, should Popen.kill(); Popen.communicate() be run in a finally clause - assuming I don't mind blocking until the process finishes?
Starting from Python 3.2 Popen is a context manager.
from the docs:
Popen objects are supported as context managers via the with statement: on exit, standard file descriptors are closed, and the process is waited for.
This should do pretty much what you want.
This is the relevant part from subprocess.py from the standard lib
in Python 3.4:
def __enter__(self):
return self
def __exit__(self, type, value, traceback):
if self.stdout:
self.stdout.close()
if self.stderr:
self.stderr.close()
if self.stdin:
self.stdin.close()
# Wait for the process to terminate, to avoid zombies.
self.wait()
Now you can do in Python 2.7
from subprocess import Popen
class MyPopen(Popen):
def __enter__(self):
return self
def __exit__(self, type, value, traceback):
if self.stdout:
self.stdout.close()
if self.stderr:
self.stderr.close()
if self.stdin:
self.stdin.close()
# Wait for the process to terminate, to avoid zombies.
self.wait()
if __name__ == '__main__':
with MyPopen(['ls']) as p:
print(p)
try...finally
subprocess32 module that you can install on Python 2.7 (it provides other improvements too. Though it might not work on non-POSIX systems).For 2.7 you could also use the @contextlib.contextmanager:
import contextlib
@contextlib.contextmanager
def manage_process(process):
try:
yield process
finally:
for stream in [process.stdout, process.stdin, process.stderr]:
if stream:
stream.close()
process.wait()
e.g:
with manage_process(Popen(['ls'])) as p:
print(p)
You can add just two custom methods to any class to implement comptability with with statement.
class CustomObject(object):
def __enter__(self):
""" This method execudes when entering block. """
return thing_you_want_to_use
def __exit__(self, type, value, traceback):
""" This method execudes on block exit. """
# Tear things down.
__enter__and__exit__magic methods to usewith.