Great question—let’s unpack what’s happening under the hood here, because it ties together Python’s native zip file execution behavior and setuptools’ egg system.

First, a quick reminder: .egg files are just renamed zip archives with extra packaging metadata. When you run an egg directly with python my_package.egg, Python’s core behavior for zip files kicks in: it looks for a __main__.py at the top level to execute. That’s not a setuptools quirk—it’s how Python handles running zip archives as executable scripts.

Now, when you configure the eggsecutable entry point via setuptools.installation, here’s the full flow:

• When you run setup.py bdist_egg, setuptools builds the egg and adds a bootstrap layer to it. This layer includes an auto-generated __main__.py at the top level of the egg.
• That auto-generated __main__.py is responsible for loading the setuptools entry point system, finding your eggsecutable = my_package.some_module:main_func configuration, and invoking your main_func.
• So when you run the egg, Python first finds that auto-generated __main__.py (per its native zip rules), which then hands off execution to your specified function. That’s why it appears like eggsecutable is "looking for __main__"—it’s using Python’s built-in mechanism to trigger the entry point logic.

How This Differs From Your Original Method #

• In your initial approach, you added your own custom __main__.py at the top level to run your code directly.
• With the eggsecutable entry point, setuptools manages the __main__.py for you. This is cleaner because it keeps your application code contained within your package (not floating at the top level), follows standard packaging conventions, and integrates better with other setuptools features like dependency management.

A Quick Way to Confirm #

If you unzip your generated .egg file (just rename it to .zip and extract), you’ll see:

• The auto-generated top-level __main__.py
• Your actual package directory (e.g., my_package/)
• Packaging metadata in EGG-INFO/

Open that __main__.py and you’ll see code explicitly looking for the eggsecutable entry point—this is the glue between Python’s native zip execution and your custom function.

内容的提问来源于stack exchange，提问作者betontalpfa