I have a executable that accepts string and outputs another string. Now I need to feed a file to it as input and write output to another file. The standard command for that is like the following executable_path < input > output. Now I wrap this in python. But I get errors.
cmd = [executable_path , '<', 'tmp/input.txt', '>',
'tmp/output.txt']
p = subprocess.Popen(cmd)
p.communicate()
invalid argument: <
I also tried joining the cmd arguments:
cmd = [executable_path, ' '.join(['<', 'tmp/input.txt', '>',
'tmp/output.txt'])]
invalid argument: < tmp/input.txt > tmp/output.txt
Passing the command as string didn't work either.
p = subprocess.Popen(' '.join(cmd))
OSError: [Errno 2] No such file or directory
What am I missing here?
subprocess.Popen() has three parameters for easy redirection: stdin, stdout and stderr. By default the subprocess retains the file descriptors of your Python instance.
with open('tmp/input.txt', 'rb', 0) as in_stream, \
open('tmp/output.txt', 'wb', 0) as out_stream:
subprocess.Popen([executable_path], stdin=in_stream, stdout=out_stream)
does the same as the shell command executable_path < tmp/input.txt > tmp/output.txt.
The arguments to open() are the filename, mode and buffering. "rb" means "read binary data", "wb" means "(over)write binary data", "ab" means "append binary data".