java,python,subprocess,popen,communicate , 'Communicate' in Python does not work


'Communicate' in Python does not work

Question:

Tag: java,python,subprocess,popen,communicate

I'm trying to write a python program to test a java program that takes input from stdin using Scanner.

All other posts point to using communicate with popen, but for me it absolutely does not work. When i run my python program, it just calls popen and then stops while the java program waits for input. I wrote a print statement after popen to check. It never prints.

Its very simple. I just want to give this program that waits for input some input.

here is the code:

import os.path, subprocess
from subprocess import PIPE

p = subprocess.Popen(['java', 'Main'], stdin=PIPE, stdout=PIPE)
print 'after subprocess' #this never get's printed
output = p.communicate(input='5 5 4 3 2 1'.encode())[0]
print output

Answer:

Without more information (like some sample Java code) it's hard to be sure, but I'll bet the problem is that the Java code is waiting for a complete line, and you haven't sent one.

If so, the fix is simple:

output = p.communicate(input='5 5 4 3 2 1\n'.encode())[0]

As a side note, why exactly are you calling encode on that string? It's already encoded in whatever character set your source code uses. So, when you call encode, it has to first decode that to Unicode. And then, because you didn't pass an argument to encode, it's going to encode it to your default character set (sys.getdefaultencoding()), which doesn't seem any more likely to match what the Java code is expecting than what you already have. It's rarely worth calling encode with an argument, and you should almost* never call it on a str, only a unicode.

* In case you're wondering, the exception is when you're using a handful of special codecs like hex or gzip. In Python 3, they decided that the occasional usefulness of those special cases was nowhere near as much as the frequent bug-magnet of calling encode on already-encoded strings, so they took it out of the language.


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