Python Project Entry Points and Script Management

After adopting a modern src layout, a common question arises: "How should I run my code? Where is the entry point?" This question touches on a core philosophical difference between Python and JavaScript regarding project execution.

References:

• How to structure a python project with multiple entry points - By Claude
• Python Apps the Right Way: entry points and scripts - By Chris Warrick

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1. Core Philosophy: From "Running a File" to "Running a Package"

• JavaScript (Node.js): Typically has a clear entry file (like index.js or main.js), and you can start the entire application with node index.js.
• Python: Modern Python encourages treating your project as an installable package. Running a file directly from within the package (e.g., python src/my_package/app.py) is an anti-pattern because it breaks Python's import system, leading to an ImportError.

The correct approach is to first install your package in editable mode, and then run it via its module or a defined entry point.

# In your project root (where pyproject.toml is located)
# This links your project to the current virtual environment, making it findable
pip install -e .

After installation, you have two "Pythonic" ways to run your code.

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2. Defining Entry Points

Method 1: Using the Module Runner (python -m)

The python -m <module_name> command tells the Python interpreter to find and execute module_name as a module, not as a plain script. This automatically handles path issues, allowing relative imports within the package to work correctly.

Assume your entry logic is in the src/my_package/app.py file, protected by if __name__ == "__main__":

# src/my_package/app.py
from . import helper # This is a valid relative import

def main():
    print("Application main logic starts...")
    helper.do_something()

if __name__ == "__main__":
    main()

After installing the package, you can run it like this:

python -m my_package.app

This will correctly execute the code block in app.py, and the from . import helper will work as expected.

Method 2 (Recommended): Using [project.scripts]

This is the most modern and powerful method, allowing your Python application to be called like a true native command-line tool. You can define one or more "script entry points" in your pyproject.toml file.

1. Prepare Your Function First, ensure your entry logic is encapsulated in a function that takes no arguments, like the main() function in the example above.

2. Configure in pyproject.toml Under the [project] table, add a new scripts table:

# pyproject.toml

[project]
name = "my-awesome-app"
# ... other metadata ...

[project.scripts]
my-awesome-app = "my_package.app:main"
another-tool = "my_package.tools:run_tool"

• Format: command_name = "package_name.module_name:function_name"
• my-awesome-app: This is the command that users can type directly into their terminal after installing your package.
• "my_package.app:main": This points to the actual function that should be called when the user runs the command.

3. Re-install Every time you modify the [project.scripts] section, you need to re-run pip install -e . for the scripts to take effect.

4. Run It Now, you or your users can run it directly from anywhere in the terminal:

my-awesome-app

This will directly call the main function in the my_package.app module. This approach is not only convenient but also completely hides the internal file structure of your project, providing a very professional command-line experience.

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3. Task Runners Similar to npm scripts

While [project.scripts] solves the "application entry point" problem, for other development tasks (like testing, linting, building), the Python community doesn't have a single, unified standard built into the core configuration file like npm scripts. However, there are several very popular alternatives:

Makefile (Universal and Simple)

A Makefile is a powerful, general-purpose task runner, not limited to any language. You can use it to define a series of shortcuts for common development tasks.

# Makefile
.PHONY: install test lint run

# Install all dependencies, including dev dependencies
install:
	pip install -e ".[dev]"

# Run tests
test:
	pytest

# Run the linter
lint:
	ruff check .

# Run the main program via its entry point
run:
	my-awesome-app

Developers only need to run simple commands like make test, make run, etc.

Script Features in Modern Build Tools

Some modern Python project management tools are moving closer to the npm scripts experience, allowing you to define tasks in a specific [tool.*.scripts] table in pyproject.toml.

Poetry Example:

# pyproject.toml (using Poetry)
[tool.poetry.scripts]
test = "pytest"
lint = "ruff check ."
run = "my-awesome-app"

You can then execute these tasks with poetry run test, poetry run lint, etc.

Hatch Example:

# pyproject.toml (using Hatch)
[tool.hatch.scripts]
test = "pytest"
lint = "ruff check ."
run = "my-awesome-app"

You can then execute these with hatch run test, hatch run lint, etc.

Conclusion: Although Python doesn't have a native mechanism that is a one-to-one equivalent of npm scripts, by combining [project.scripts] (for application entry points) with a task runner (like Makefile or a tool's built-in scripts), you can build a development workflow that is just as powerful, clear, and maintainable.