Have you ever run a Python script that calls external programs, only to have your console flooded with unwanted output? This is a common frustration when working with Python’s subprocess module.
While the subprocess module provides powerful functionality for interacting with external programs, managing the output of these programs can sometimes be challenging.
In this comprehensive guide, I’ll explore several methods to suppress or redirect the output from subprocess calls in Python. Whether you’re developing scripts that need to run silently in the background or just want to keep your console clean while running external commands, this article will provide you with the solutions you need.
Understanding the Problem
When calling external programs using Python’s subprocess module, these programs often produce standard output (stdout) and standard error (stderr) messages. While these messages can be useful for debugging, they can also clutter your console and make it difficult to read your own program’s output.
Consider this simple example where we use the espeak text-to-speech program:
import subprocess
text = ‘Hello World.’
print(text)
subprocess.call([‘espeak’, text])
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Though the espeak command works correctly, it might produce unwanted error messages or output that clutters your console. These messages can make it difficult to see your program’s own output (the printed “Hello World.” text in this case).
Solution 1: Using Python 3.3+ with subprocess.DEVNULL
If you’re using Python 3.3 or later, the most elegant solution is to use the built-in subprocess.DEVNULL constant, which represents the null device (/dev/null on Unix or NUL on Windows).
import subprocess
text = ‘Hello World.’
print(text)
subprocess.call([‘espeak’, text], stdout=subprocess.DEVNULL, stderr=subprocess.STDOUT)
This redirects both standard output and standard error to the null device, effectively silencing all output from the subprocess.
How it works:
- stdout=subprocess.DEVNULL redirects standard output to the null device
- stderr=subprocess.STDOUT redirects standard error to the same place as standard output (which in this case is the null device)
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Solution 2: For Python 2.7 or Earlier Versions
If you’re using Python 2.7 or an earlier version that doesn’t have subprocess.DEVNULL, you can create your own null device file object:
import os
import subprocess
FNULL = open(os.devnull, ‘w’)
text = ‘Hello World.’
print(text)
subprocess.call([‘espeak’, text], stdout=FNULL, stderr=subprocess.STDOUT)
FNULL.close() # Don’t forget to close the file when you’re done
In this approach, we:
- Open the null device (os.devnull) as a writable file
- Redirect both stdout and stderr to this file
- Close the file after we’re done to avoid resource leaks
You could also use a context manager to ensure the file is properly closed:
import os
import subprocess
text = ‘Hello World.’
print(text)
with open(os.devnull, ‘w’) as FNULL:
subprocess.call([‘espeak’, text], stdout=FNULL, stderr=subprocess.STDOUT)
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Solution 3: Using subprocess.check_output()
Another approach, introduced in Python 2.7, is to use subprocess.check_output(). This function captures the output as a return value rather than displaying it:
import subprocess
text = ‘Hello World.’
print(text)
try:
output = subprocess.check_output([‘espeak’, text], stderr=subprocess.STDOUT)
# You can use ‘output’ if needed, or simply ignore it
except subprocess.CalledProcessError:
# Handle any errors that might occur during execution
pass
The benefit of this approach is that it not only suppresses the output but also makes it available for your program to use if needed. Additionally, check_output() will raise a CalledProcessError exception if the command fails, allowing you to handle errors more gracefully.
Solution 4: A Cross-Compatible Approach
If you need a solution that works across different Python versions, you might consider this more portable approach:
import subprocess
from subprocess import PIPE, STDOUT
# Try to import DEVNULL from subprocess if available (Python 3.3+)
try:
from subprocess import DEVNULL
except ImportError:
# Fall back to using os.devnull if DEVNULL is not available
import os
DEVNULL = open(os.devnull, ‘wb’)
text = ‘Hello World.’
print(text)
# For Python 3, encode the text if passing to stdin
if hasattr(text, ‘encode’):
text = text.encode(‘utf-8’)
p = subprocess.Popen([‘espeak’], stdin=PIPE, stdout=DEVNULL, stderr=STDOUT)
p.communicate(text)
# Close DEVNULL if it’s a file object we created
if hasattr(DEVNULL, ‘close’):
DEVNULL.close()
This approach has the advantage of being compatible with both Python 2 and 3, handling text encoding differences, and properly managing resources.
Understanding Redirection Options
When redirecting subprocess output, it’s important to understand the different options available:
- Redirecting stdout only: stdout=subprocess.DEVNULL
- Only the standard output is suppressed; error messages will still appear
- Redirecting stderr only: stderr=subprocess.DEVNULL
- Only error messages are suppressed; standard output will still appear
- Redirecting both independently: stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL
- Both standard output and error messages are suppressed independently
- Redirecting stderr to stdout: stderr=subprocess.STDOUT
- Error messages go to the same place as standard output
- Often combined with redirection of stdout: stdout=subprocess.DEVNULL, stderr=subprocess.STDOUT
- Capturing output: stdout=subprocess.PIPE
- Output is captured rather than displayed
- Must be used with p.communicate() to avoid potential deadlocks
Best Practices and Considerations
When suppressing subprocess output, keep these best practices in mind:
1. Resource Management
Always close file handles when you’re done with them to avoid resource leaks. The best approach is to use context managers (the with statement) whenever possible:
with open(os.devnull, ‘w’) as devnull:
subprocess.call([‘espeak’, text], stdout=devnull, stderr=subprocess.STDOUT)
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2. Error Handling
Even if you’re suppressing output, you should still handle potential errors from the subprocess. The return code from subprocess.call() or exceptions from subprocess.check_output() provide valuable information:
try:
result = subprocess.call([‘espeak’, text], stdout=subprocess.DEVNULL, stderr=subprocess.STDOUT)
if result != 0:
print(f”Command failed with return code {result}”)
except Exception as e:
print(f”An error occurred: {e}”)
3. Selective Suppression
Consider whether you need to suppress both stdout and stderr. In some cases, you might want to keep error messages visible while suppressing normal output:
# Suppress only stdout, keep stderr visible
subprocess.call([‘espeak’, text], stdout=subprocess.DEVNULL)
# Suppress only stderr, keep stdout visible
subprocess.call([‘espeak’, text], stderr=subprocess.DEVNULL)
4. When Not to Suppress Output
While suppressing output can keep your console clean, it can also hide valuable information. During development or when debugging, consider keeping the output visible or redirecting it to a log file instead of discarding it completely.
Advanced Techniques
Redirecting to a Log File
Instead of completely discarding the output, you might want to redirect it to a log file:
with open(‘subprocess_log.txt’, ‘w’) as logfile:
subprocess.call([‘espeak’, text], stdout=logfile, stderr=subprocess.STDOUT)
This approach maintains a record of the subprocess output while keeping your console clean.
Using the subprocess.run() Function (Python 3.5+)
In Python 3.5 and later, the subprocess.run() function provides a more convenient interface:
import subprocess
text = ‘Hello World.’
print(text)
completed_process = subprocess.run([‘espeak’, text],
stdout=subprocess.DEVNULL,
stderr=subprocess.DEVNULL,
check=False)
The run() function returns a CompletedProcess object that contains information about the completed process, including return code and captured output (if any).
Frequently Asked Questions
How do I suppress output only if the command succeeds?
You can use a try-except block with check_output():
try:
subprocess.check_output([‘espeak’, text], stderr=subprocess.STDOUT)
except subprocess.CalledProcessError as e:
# Display the output only if there was an error
print(f”Command failed with output: {e.output.decode()}”)
Can I temporarily suppress output and then restore it?
Yes, you can redirect output and then restore it. This is more complex but can be done by temporarily redirecting system stdout and stderr:
import sys
import os
import subprocess
# Save the original stdout and stderr
original_stdout = sys.stdout
original_stderr = sys.stderr
# Redirect to devnull
with open(os.devnull, ‘w’) as devnull:
sys.stdout = devnull
sys.stderr = devnull
# Run your subprocess
subprocess.call([‘espeak’, ‘Hello World.’])
# Restore original stdout and stderr
sys.stdout = original_stdout
sys.stderr = original_stderr
print(“This will appear in the console”)
How do I suppress output when using shell=True?
When using shell=True, the same redirection techniques apply:
import subprocess
subprocess.call(‘espeak “Hello World”‘,
shell=True,
stdout=subprocess.DEVNULL,
stderr=subprocess.STDOUT)
Why am I still seeing output even after redirecting to DEVNULL?
This could happen for several reasons:
- The program might be bypassing standard output/error channels
- You might be redirecting only stdout but not stderr (or vice versa)
- If using Popen with stdout=PIPE or stderr=PIPE, you must call communicate()
Make sure you’re redirecting both stdout and stderr:
subprocess.call([‘espeak’, text],
stdout=subprocess.DEVNULL,
stderr=subprocess.DEVNULL)
Conclusion
Suppressing output from subprocess calls in Python is a common requirement when creating clean, user-friendly scripts. By using the techniques described in this article, you can control what output is displayed, captured, or discarded, making your Python scripts more professional and easier to use.
Whether you’re using the modern subprocess.DEVNULL approach in Python 3 or creating your own null device file in Python 2, the principles remain the same. With proper error handling and resource management, you can create robust scripts that interact with external programs without cluttering your console.
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