Thursday, July 11, 2013

Python subprocess, Popen and PIPE

Typically when using Python's subprocess we use PIPEs to communicate with the process. However, it turns out, PIPEs suck when the data gets even slightly large (somewhere in the vicinity of 16K). You can verify this by running the following test code:
from subprocess import Popen, PIPE
import argparse, time

def execute(n):
  p = Popen(['python', 'test.py', '-n', str(n)], stdin=PIPE, stdout=PIPE, stderr=PIPE)
  p.wait()
  return p.stdout.read().splitlines()

if __name__ == "__main__":
  parser = argparse.ArgumentParser()
  parser.add_argument('-n', type=int)
  args = parser.parse_args()
  if args.n is not None:
    print '0'*args.n
  else:
    for n in [10,100,1000,10000,12000,16000,16200, 16500]:
      t0 = time.clock()
      execute(n)
      print n, time.clock() - t0
The output is
10 0.001219
100 0.001254
1000 0.001162
10000 0.001362
12000 0.001429
16000 0.001305
16200 0.00121
(Hangs after this)
The way to handle this is to generate a temporary file and let the process write to the file. A detailed note can be found here.
def execute_long(n):
  with open('query.txt','w') as stdout:
    p = Popen(['python', 'test.py', '-n', str(n)], stdin=PIPE, stdout=stdout, stderr=PIPE)
    p.wait()
  with open('query.txt','r') as stdout:
    return stdout.read().splitlines()

if __name__ == "__main__":
  parser = argparse.ArgumentParser()
  parser.add_argument('-n', type=int)
  args = parser.parse_args()
  if args.n is not None:
    print '0'*args.n
  else:
    for n in [10,100,1000,10000,12000,16000,16200, 16500]:
      t0 = time.clock()
      execute_long(n)
      print n, time.clock() - t0
10 0.001601
100 0.001263
1000 0.001272
10000 0.001404
12000 0.001419
16000 0.001333
32000 0.001445
64000 0.001692
128000 0.001763

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