What __init__.py does in a package

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Before Python 3.3, a directory needed __init__.py to be treated as a package at all. When you write 'import mypackage', Python executes mypackage/__init__.py. You can leave it empty (just a marker file) or use it to set up the public API - importing submodules, defining __all__, or running initialization code. A common pattern is to re-export the most-used names so callers can write 'from mypackage import MyClass' instead of 'from mypackage.core import MyClass'.