How to Suppress All Output When Calling Executables in Python: A Step-by-Step Guide

Table of Contents#

1. Why Suppress Output from Executables?
2. Prerequisites
3. Understanding Output Streams: stdout vs. stderr
4. Method 1: Using subprocess.DEVNULL (Python 3.3+)
   • 4.1 Basic Example: Suppress All Output
   • 4.2 Suppressing stdout Only or stderr Only
   • 4.3 Merging Streams and Suppressing Together
5. Method 2: Compatibility with Python <3.3 (Using os.devnull)
6. Advanced Scenarios
   • 6.1 Running Executables in the Background
   • 6.2 Checking Exit Codes While Suppressing Output
7. Troubleshooting: Output Still Appears?
8. Best Practices
9. Conclusion
10. References

Why Suppress Output from Executables?#

Before diving into the "how," let’s clarify the "why." You might want to suppress output when:

• The executable is a background task (e.g., a cleanup script or scheduled job) where console output is irrelevant.
• The executable is noisy (e.g., verbose debug logs) and clogs your application’s logs or terminal.
• You need to run the executable silently in a production environment where unnecessary output wastes resources.
• The output is irrelevant to your workflow (e.g., a tool that prints "Success!" but you only care about its exit code).

Prerequisites#

To follow this guide, you’ll need:

• Python 3.3 or newer (for subprocess.DEVNULL; we’ll cover older versions separately).
• Basic familiarity with Python and the command line.
• An external executable to test with (e.g., ls (Linux/macOS), dir (Windows), or a custom script).

Understanding Output Streams: stdout vs. stderr#

Before suppressing output, it’s critical to understand the two primary output streams:

• stdout (Standard Output): Used for regular program output (e.g., print("Hello") in Python).
• stderr (Standard Error): Used for error messages and diagnostics (e.g., print("Error!", file=sys.stderr) in Python).

Most executables use both streams. To fully suppress output, you must redirect both stdout and stderr. Failing to redirect stderr is a common mistake—you’ll still see error messages even if stdout is suppressed!

Method 1: Using subprocess.DEVNULL (Python 3.3+)#

Python 3.3 introduced subprocess.DEVNULL, a special value that represents the "null device" (a system-specific black hole where output is discarded). This is the simplest and recommended way to suppress output.

4.1 Basic Example: Suppress All Output#

To suppress both stdout and stderr, pass stdout=subprocess.DEVNULL and stderr=subprocess.DEVNULL to subprocess.run().

Example:#

Suppose we run the ls command (Linux/macOS) or dir (Windows), which lists directory contents to stdout. Let’s suppress its output:

import subprocess
 
# Suppress stdout and stderr for 'ls' (Linux/macOS)
subprocess.run(["ls"], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
 
# For Windows, use ["dir", "/b"] instead:
# subprocess.run(["dir", "/b"], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL, shell=True)

Explanation:

• subprocess.run() is the primary function for running external commands in Python 3.5+.
• stdout=subprocess.DEVNULL: Redirects stdout to the null device.
• stderr=subprocess.DEVNULL: Redirects stderr to the null device.
• shell=True (Windows only for dir): Required for shell-builtin commands like dir (use cautiously—see "Best Practices" below).

4.2 Suppressing stdout Only or stderr Only#

Sometimes, you may want to suppress one stream but keep the other. For example:

• Suppress stdout but keep stderr to debug errors.
• Suppress stderr but log stdout.

Example 1: Suppress stdout, Keep stderr#

import subprocess
 
# Run 'ls non_existent_file' (will fail, sending an error to stderr)
result = subprocess.run(
    ["ls", "non_existent_file"],  # Invalid file to trigger an error
    stdout=subprocess.DEVNULL,    # Suppress stdout
    stderr=subprocess.PIPE        # Capture stderr (or omit to show in console)
)
 
# Print the captured stderr (if needed)
print("Error message:", result.stderr.decode("utf-8"))

Example 2: Suppress stderr, Keep stdout#

import subprocess
 
# Run 'echo "Hello"' and suppress stderr (though 'echo' rarely uses stderr)
result = subprocess.run(
    ["echo", "Hello"],
    stdout=subprocess.PIPE,       # Capture stdout
    stderr=subprocess.DEVNULL     # Suppress stderr
)
 
print("Output:", result.stdout.decode("utf-8"))  # Prints "Hello"

4.3 Merging Streams and Suppressing Together#

Some executables mix output between stdout and stderr. To treat them as a single stream (and suppress both), merge stderr into stdout with stderr=subprocess.STDOUT, then redirect the merged stream to DEVNULL:

import subprocess
 
# Merge stderr into stdout, then suppress the merged stream
subprocess.run(
    ["noisy_executable"],
    stdout=subprocess.DEVNULL,
    stderr=subprocess.STDOUT  # Merge stderr into stdout
)

This ensures all output (stdout + stderr) is discarded.

Method 2: Compatibility with Python <3.3 (Using os.devnull)#

For Python versions older than 3.3 (e.g., 2.7), subprocess.DEVNULL doesn’t exist. Instead, use os.devnull, which returns the path to the system’s null device (e.g., /dev/null on Linux/macOS, NUL on Windows).

Steps:#

1. Import os and subprocess.
2. Open the null device in write mode.
3. Pass the file handle to stdout and stderr.

Example:#

import os
import subprocess
 
# Open the null device
with open(os.devnull, "w") as fnull:
    # Suppress stdout and stderr
    subprocess.run(["ls"], stdout=fnull, stderr=fnull)

Why with? The with statement ensures the file handle is closed automatically, even if an error occurs.

Advanced Scenarios#

6.1 Running Executables in the Background#

To run an executable in the background (asynchronously) and suppress output, use subprocess.Popen instead of subprocess.run(). Popen starts the process and returns immediately, allowing your Python script to continue running.

Example:#

import subprocess
 
# Start 'long_running_task' in the background, suppress output
process = subprocess.Popen(
    ["long_running_task"],
    stdout=subprocess.DEVNULL,
    stderr=subprocess.DEVNULL
)
 
# Your script continues running while 'long_running_task' executes...
print("Task started in background!")
 
# Later, wait for the process to finish (optional)
process.wait()
print("Task finished with exit code:", process.returncode)

6.2 Checking Exit Codes While Suppressing Output#

Even when suppressing output, you’ll often want to check if the executable succeeded. The subprocess.run() function returns a CompletedProcess object with a returncode attribute:

• returncode=0: Success.
• returncode!=0: Failure (specific codes vary by executable).

Example:#

import subprocess
 
result = subprocess.run(
    ["ls", "non_existent_file"],  # Will fail
    stdout=subprocess.DEVNULL,
    stderr=subprocess.DEVNULL
)
 
if result.returncode == 0:
    print("Executable succeeded!")
else:
    print(f"Executable failed with exit code: {result.returncode}")

Output:

Executable failed with exit code: 2  # 'ls' returns 2 for "file not found"

Troubleshooting: Output Still Appears?#

If output persists despite redirection, check these common issues:

Issue 1: Only Redirected stdout (Forgot stderr)#

You redirected stdout but not stderr. Error messages will still appear.

Fix: Add stderr=subprocess.DEVNULL (or merge stderr into stdout with stderr=subprocess.STDOUT).

Issue 2: The Executable Bypasses Streams#

Rarely, executables may write directly to the terminal (bypassing stdout/stderr). For example, some low-level tools use system calls like write(1, ...) instead of standard libraries.

Fix: This is hard to suppress. Use nohup (Linux/macOS) or start /b (Windows) to detach the process from the terminal, but this is system-specific.

Issue 3: Permissions Error for DEVNULL#

On rare occasions, you may get a PermissionError when accessing the null device. This is usually due to a misconfigured system.

Fix: Verify the null device path with os.devnull and ensure your user has write access.

Best Practices#

1. Always Redirect Both Streams: To fully suppress output, redirect both stdout and stderr.
2. Avoid shell=True Unless Necessary: Using shell=True runs the command through the system shell, which can introduce security risks (e.g., shell injection). Use shell=False (the default) and pass the command as a list (e.g., ["ls", "-l"] instead of "ls -l").
3. Check Exit Codes: If the executable’s success matters, always check result.returncode.
4. Use DEVNULL for Python 3.3+: It’s cleaner and more readable than manually opening os.devnull.

Conclusion#

Suppressing output from external executables in Python is straightforward with subprocess.DEVNULL (Python 3.3+) or os.devnull (older versions). By redirecting both stdout and stderr to the null device, you can run executables silently, keeping your logs clean and your workflow efficient. Remember to check exit codes for success/failure and avoid shell=True for security.

References#

• Python subprocess Module Documentation
• Python os.devnull Documentation
• PEP 324: PEP proposing the subprocess module
• Null Device (Wikipedia)