Refactor so tests don't shell out to docker_rerun.

docker_rerun.py

     return [func for (name, func) in functions() if name.startswith('modify_')]
 
 
-def main():
+def main(argv, out):
     """Script entry point."""
     parser = argparse.ArgumentParser(description='Reruns docker containers ' \
                                                  'with different parameters.')
     for mod in mods:
         mod(parser=parser)
 
-    args = parser.parse_args()
+    args = parser.parse_args(argv)
     container_info = docker_inspect(args.container, 'container')
     image_info = docker_inspect(container_info['Config']['Image'], 'image')
     container = Container(container_info, image_info)
 
     if args.dry_run:
         print('Performing dry run for container %s. The following would be ' \
-              'executed:' % args.container)
+              'executed:' % args.container, file=out)
         for command in commands:
-            print(' '.join(command))
+            print(' '.join(command), file=out)
     else:
-        print('Re-running container %s...' % args.container)
+        print('Re-running container %s...' % args.container, file=out)
         for command in commands:
             subprocess.check_call(command)
 
 
 if __name__ == "__main__":
-    main()
+    main(sys.argv[1:], sys.stdout)

test.py

 #!/usr/bin/python3
 
+import docker_rerun
 import subprocess
+from io import StringIO
 from nose.tools import with_setup
 
 
 @with_setup(setup_each, teardown_each)
 def check(command, args=[], expected=None):
     _run(command)
-    output = subprocess.check_output(['./docker_rerun.py', '--dry-run', 'test123'] + args)
-    output = output.decode('utf-8').strip().splitlines()
+    output = StringIO()
+    docker_rerun.main(['--dry-run', 'test123'] + args, output) 
+    output = output.getvalue().splitlines()
     expected = ' '.join(expected or command)
     assert output[3] == expected, 'Expected "%s" but got "%s"' % (expected, output[3])