Packages

https://docs.python.org/3/reference/import.html

PEP 420 - Implicit Namespace Packages [a regular package (with an __init__.py) vs a namespace package]

INTRODUCTION

Python has a concept of 'packages' to help organize modules and provide a naming hierarchy. You can think of packages as the directories on a file system and modules as files within directories.

It’s important to keep in mind that all packages are modules, but not all modules are packages [a package contains a '__path__' attribute].

Python defines two types of packages, 'regular packages' and 'namespace packages'.

REGULAR PACKAGES

Regular packages are traditional packages typically implemented as a directory containing an '__init__.py' file.

When a regular package is imported, this '__init__.py' file is implicitly executed, and the objects it defines are bound to names in the package’s namespace. The '__init__.py' file can contain the same Python code that any other module can contain, and Python will add some additional attributes to the module when it is imported.

# Files in the current directory.

script.py      # the main module
dir1/__init__.py       # required, dir1 is a package
dir1/module1.py
dir1/dir2/__init__.py   # required, dir2 is a subpackage
dir1/dir2/module2.py

# Possible statements in script.py

import dir1
import dir1.module1
import dir1.dir2.module2

from dir1.module1 import name1
from dir1.dir2.module2 import *

# Possible statements in module1.py

import dir2.module2
from dir2.module2 import name2

NAMESPACE PACKAGES

A namespace package is a composite of various 'portions', where each portion contributes a subpackage to the parent package. Portions may reside in different locations on the file system. Portions may also be found in zip files, on the network, or anywhere else that Python searches during import. Namespace packages may or may not correspond directly to objects on the file system; they may be virtual modules that have no concrete representation.

SYMPY

https://www.sympy.org/

https://github.com/sympy/sympy
[GitHub account: sympy, GitHub repo: sympy]

SymPy is the Python Library for Symbolic Mathematics.

sympy/          # main directory
sympy/__init__.py   # not empty

sympy/core/
sympy/core/__init__.py   # not empty
sympy/core/basic.py

sympy/core/tests/
sympy/core/tests/__init__.py   # empty
sympy/core/tests/test_basic.py

sympy/combinatorics/
sympy/combinatorics/__init__.py   # not empty
sympy/combinatorics/permutations.py
# from sympy.core.basic import Atom

sympy/combinatorics/tests/
sympy/combinatorics/tests/__init__.py   # empty
sympy/combinatorics/tests/test_permutations.py
# from sympy.combinatorics.permutations import Permutation