Capturing Program Output: Beyond Naïve Solutions

In Python scripting, capturing program output for further processing is a common need. While naïve solutions may seem straightforward, they often fall short. Consider the following script that writes to stdout:

# writer.py
import sys

def write():
    sys.stdout.write("foobar")

Attempting to capture the output using the following code fails:

# mymodule.py
from writer import write

out = write()
print(out.upper())

To effectively capture the output, a more robust solution is required. One approach involves modifying the system's stdout stream:

import sys
from cStringIO import StringIO

# Redirect stdout to a StringIO object
backup = sys.stdout
sys.stdout = StringIO()

# Perform the write operation
write()

# Retrieve and restore stdout
out = sys.stdout.getvalue()
sys.stdout.close()
sys.stdout = backup

# Process the captured output
print(out.upper())

Context Manager for Python 3.4 :

For Python 3.4 and later, a simpler and more concise solution is available using the contextlib.redirect_stdout context manager:

from contextlib import redirect_stdout
import io

f = io.StringIO()
# Redirect stdout to f using the context manager
with redirect_stdout(f):
    help(pow)

# Retrieve captured output from f
s = f.getvalue()

This elegant approach simplifies the output capturing process, making it easier to handle in your Python scripts.