Using files as stdin and stdout for subprocess

Question

How do I replicate the following batch command using python subprocess module?

myprogram < myinput.in > myoutput.out

In other words, how do I run myprogram using the contents of myinput.in as the standard input and myoutput.out as standard output?

Solution

The following should work:

myinput = open('myinput.in')
myoutput = open('myoutput.out', 'w')
p = subprocess.Popen('myprogram.exe', stdin=myinput, stdout=myoutput)
p.wait()
myoutput.flush()

Comments

[Doo Dah]

This looks close to what I need. How do I detect if errors occurred?

[Doo Dah]

Nevermind. stderr=subprocess.PIPE, then, if stderr: print error

[flagg19]

In the line myoutput = open('myoutput.out'. 'w') parameters should be separated by a ,, not a .

[Mark Amery]

While this answer remains correct, note that subprocess.run(), added in Python 3.5, is a nicer API for the majority of subprocess use cases. It similarly takes stdin and stdout keyword arguments.