Python's Silent Failure: When `python -m` Does Nothing (And How to Fix It)

Why You Should Care

Ever run python -m your.module and get... nothing? No errors, exit code 0, but your code never actually runs? I just spent an hour debugging this exact problem.

Here's what I learned.

The Problem

I was running a CLI command via Jenkins:

$ python -m monitoring_sdk.core.cli metabase-check --config config.yaml
$ echo $?
0
$ ls reports/
# Empty - nothing was created

Exit code 0 means success, right? But nothing happened. No output, no files, no errors. Just... silence.

The Root Cause

Missing __main__ entrypoint.

When you run python -m module.path, Python imports the module but doesn't automatically call your main() function. You need to explicitly tell it what to run.

The Quick Fix

Add one of these (or both):

Option 1: Create __main__.py (Recommended)

# monitoring_sdk/core/__main__.py
from monitoring_sdk.core.cli import main

if __name__ == "__main__":
    main()

Option 2: Add to your main file

# monitoring_sdk/core/cli.py
# ... your code ...

if __name__ == "__main__":
    main()

That's it. Problem solved.

What Actually Happens

When you run python -m monitoring_sdk.core.cli, Python:

1. Imports monitoring_sdk/core/cli.py
2. Executes module-level code (imports, decorators, function definitions)
3. Stops - it doesn't call main()
4. Exits with code 0

So you get:

• ✅ Module imported
• ✅ Functions defined
• ❌ Nothing executed
• ✅ Exit code 0 (looks like success!)

This is what I call a "silent failure" - the worst kind.

How I Debugged It

Step 1: Added logging everywhere

import logging

logger = logging.getLogger(__name__)

def metabase_check(config, output):
    logger.info("Starting command")  # This never appeared
    # ... code ...

Step 2: Checked what WAS running

$ DEBUG=true python -m monitoring_sdk.core.cli metabase-check
DEBUG - Package initialized  # Module imported
DEBUG - Monitors initialized # Module imported
# ... nothing else

Package initialization logs appeared (because __init__.py runs on import), but my function logs didn't. That's when I realized: the function was never called.

Step 3: Tried --help

$ python -m monitoring_sdk.core.cli --help
# No output at all

If even --help doesn't work, the entrypoint is definitely missing.

How Python Looks for Entrypoints

When you run python -m module.path:

1. Look for module/path/__main__.py → Run it
2. Otherwise, run module/path.py (but need if __name__ == "__main__")

Why This Bit Me (AI-Generated Code)

Full transparency: I was using AI to generate code quickly. The AI created the CLI structure perfectly - decorators, commands, options - but forgot the entrypoint.

I assumed "AI generated it, so it's complete." I didn't even check. That was my mistake.

Lesson learned: AI is great for boilerplate, but you still need to verify the basics.

The Fix in Action

After adding __main__.py:

$ python -m monitoring_sdk.core.cli metabase-check --config config.yaml
INFO - Starting card check
INFO - Card check completed: status=OK
INFO - Report generated: file=reports/summary.md
INFO - Command completed: status=OK

$ ls reports/
summary.md  # Finally!

Key Takeaways

• python -m requires an explicit entrypoint
• Exit code 0 doesn't mean your code ran
• Silent failures are hard to debug - add logging early
• Even with AI-generated code, verify the basics

The worst bugs are the simple ones you don't think to check.

If you're interested in more troubleshooting processes and decision-making in engineering, I write about them here: https://tielec.blog/