python-debug-execution

Python Debug Execution Pattern

When executing Python scripts that may fail, use this pattern to surface clear error information and diagnose issues effectively.

Core Technique

1. Execute with Full Error Output

Always run Python scripts with stderr redirected to stdout and echo the exit code:

python3 script.py 2>&1 ; echo Exit code: $?

Why this works:

• 2>&1 captures both stdout and stderr, ensuring tracebacks are visible
• echo Exit code: $? reveals the actual exit status for debugging
• Avoids opaque "command failed" errors that hide the real cause

2. Verify Working Directory in Script

Before any file operations in Python scripts, add working directory verification:

import os

# At the start of your script or before file operations
print(f"Current working directory: {os.getcwd()}")

# For debugging, also list directory contents
print(f"Directory contents: {os.listdir('.')}")

Why this works:

• File not found errors often stem from incorrect working directory assumptions
• Makes path-related failures immediately diagnosable
• Confirms the execution context matches expectations

Complete Debugging Workflow

Step 1: Add Diagnostic Code to Script

#!/usr/bin/env python3
import os
import sys

def main():
    # Diagnostic: verify execution context
    print(f"Working directory: {os.getcwd()}")
    print(f"Python version: {sys.version}")
    print(f"Directory listing: {os.listdir('.')}")
    
    # Your actual logic here
    # ...

if __name__ == "__main__":
    main()

Step 2: Execute with Full Error Capture

python3 script.py 2>&1 ; echo Exit code: $?

Step 3: Analyze Output

Look for:

• Traceback messages (indicate code errors)
• Exit code (0 = success, non-zero = failure)
• Working directory confirmation
• Missing file/directory errors

Common Failure Patterns

Symptom	Likely Cause	Debug Clue
FileNotFoundError	Wrong working directory	Check os.getcwd() output
ModuleNotFoundError	Missing dependencies	Traceback shows import path
PermissionError	File access issues	Traceback shows file path
Silent failure (exit 0, no output)	Logic bug, not crash	Add print statements