Place an __init__.py file in a directory to mark it as a Python package. The file can be empty. Although it is optional for package recognition, including it helps prevent directories of miscellaneous Python code from being imported as packages. Python executes it the first time the package or subpackage is loaded. section 18.3.1
Initialize a package
Suppose mathproj/__init__.py contains `print("Hello from mathproj init")` and `version = 1.03`. In a fresh Python session, `import mathproj` prints the message; `mathproj.version` then evaluates to `1.03`. Names assigned in the file become attributes of the package. section 18.3 section 18.3.2
Importing a package does not automatically load every subpackage or module beneath it. Import a nested module explicitly, or arrange the needed imports in an __init__.py file. Before a nested module loads, Python loads its parent packages and runs their initialization files. section 18.3.3 section 18.7
import mathproj.comp.numeric.n1 Hello from mathproj.comp init Hello from numeric init mathproj.comp.numeric.n1.g() version is 1.03 Called function h in module n2
The nested import prints messages from the parent subpackages’ initialization files before the module’s function is called. section 18.3.3
An __init__.py file can also define `__all__`, a list of names selected by `from package import *`. For example, `__all__ = ['comp']` selects `comp` from mathproj; it does not recursively import names from comp. Without `__all__`, a wildcard import from the package imports no names. Prefer explicit imports when the origin of names matters. section 18.4 section 10.3
