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Stop Fighting Python's Relative Imports: Meet `run-main` for Smoother Module Execution & Debugging

#python #devtools #opensource #productivity

Ever found yourself wrestling with ImportError: attempted relative import with no known parent package when trying to run a Python module directly? You're not alone! This is a common frustration for Python developers, especially when working with larger projects structured into packages.

But what if there was a simple way to bypass this headache and run your modules smoothly, with relative imports working just as you'd expect?

Enter run-main! 🚀

What is run-main?

run-main is a lightweight Python utility designed to simplify the execution and debugging of individual Python modules (.py files), particularly those that use relative imports. It elegantly mimics the behavior of python -m <package.module> but for single files, ensuring your module runs with the correct package context.

Think of it as your friendly helper that says, "Don't worry about the import paths, I'll handle that for you!"

The Problems It Solves

run-main directly addresses several common pain points in Python development:

  1. 💥 Relative Import Errors:
    When you run a Python file directly from within a package (e.g., python my_package/my_module.py), Python often can't resolve relative imports (like from . import utils). run-main loads your module in a way that establishes the correct package context, making those pesky ImportErrors disappear.

  2. 🎯 Debugger Misdirection:
    Sometimes, when an error occurs during a module's import phase, standard import mechanisms might wrap the original exception. This can lead your debugger to stop at the import call itself, not the actual line causing the error. run-main promotes a "fast-fail" approach, allowing original errors to surface directly, so your debugger points you right to the source of the problem.

  3. ⚙️ IDE Configuration Overhead:
    While IDEs like VS Code offer "Python: Module" debug configurations, they often require you to hardcode the module path for each file. run-main allows you to use a generic debug configuration (e.g., using ${file} in VS Code) to debug any compatible module with a single setup.

Super Quick Start

Getting started with run-main is incredibly easy:

1. Install 

pip install run-main
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2. Prepare Your Module

Define a _main() function in your Python file (e.g., your_module.py):

# your_module.py
# Replace 'if __name__ == "__main__":' with this for seamless relative imports!
def _main(*args):
    print(f"Hello from _main in {__file__}!")
    # from . import your_sibling_module # Relative imports now work!
    # from ..package import another_module # And these too!
    if args:
        print(f"Received arguments: {args}")

# Optional: For direct execution (though not recommended for solving relative import issues)
# if __name__ == "__main__":
#     import sys
#     _main(*sys.argv[1:])
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The _main() function becomes your run-main-aware entry point.

3. Run 

run-main path/to/your_module.py arg1 arg2
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4. (Optional) Quick Debug in VS Code

Add this minimal configuration to your .vscode/launch.json:

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Python: Debug current file with run-main",
            "type": "debugpy",
            "request": "launch",
            "module": "run_main", // run-main is executed as a module
            "args": ["${file}"], // Passes the current file to run-main
            "console": "integratedTerminal",
            "cwd": "${workspaceFolder}"
        }
    ]
}
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Now, just open the Python file containing _main() and press F5 to debug!

Why Choose run-main?

  • Effortless Module Execution: Run any .py file with a _main() as if it's the main program.
  • Correct Relative Import Handling: Say goodbye to ImportErrors caused by incorrect package context.
  • "Fast-Fail" Debugging: Errors surface directly from your module, not wrapped in obscure import errors.
  • Simplified IDE Debugging: Use a single, reusable debug configuration for any compatible file.
  • Argument Passing: Easily pass command-line arguments to your _main() function.

How It Works (Briefly)

When you execute run-main path/to/your_module.py, it intelligently converts the file path into a Python module import path (e.g., path.to.your_module). It then dynamically loads and executes your module's _main() function in a way that ensures the Python interpreter correctly identifies its package context. This is similar to how python -m works, but tailored for the convenience of running individual files.

Join the Effortless Path!

Ready to simplify your Python module execution and debugging?

We welcome feedback, issues, and contributions. If run-main helps you, let us know!

Happy Pythoning!

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