Learning

Python Packages

What is a Package?

A package is a directory containing an __init__.py file. It allows you to group related modules together.

python

my_project/
├── main.py
└── ecommerce/              # The PACKAGE
    ├── __init__.py         # Makes 'ecommerce' a package
    ├── products.py
    ├── cart.py
    └── utils/
        ├── __init__.py     # Makes 'utils' a SUBPACKAGE
        └── formatting.py

The init.py File

python

# This runs when the package is imported
print('E-commerce package loaded!')

# Define package-level variables
VERSION = '2.0'

# Control what gets imported with 'from ecommerce import *'
__all__ = ['products', 'cart']

# Provide convenient imports at the package level
from .products import get_product
from .cart import ShoppingCart

Now users can do:

python

undefined

Importing from Packages

python

import ecommerce.products
ecommerce.products.get_product(1)

# Import specific item from module
from ecommerce.products import get_product
get_product(1)

# Import from subpackage
from ecommerce.utils.formatting import format_currency

# Relative imports (INSIDE the package only)
# In ecommerce/cart.py:
from .products import get_product  # Same package
from ..core import database        # Parent package

Namespace Packages (Python 3.3+)

You CAN have packages without __init__.py (called namespace packages). Useful for splitting a large package across multiple directories.

python

# Both directories contribute to 'shared_pkg'
/site-packages/shared_pkg/module_a.py
/another_path/shared_pkg/module_b.py

Recommendation: Always use __init__.py for your own projects. Namespace packages are mainly for library vendors.

init.py Patterns

python

def __getattr__(name):
    if name == 'heavy_module':
        from . import heavy_module
        return heavy_module
    raise AttributeError(f'module {__name__!r} has no attribute {name!r}')

# Pattern 2: Submodule import shortcut
# __init__.py
from .database import db
from .models import User, Product

# Now: from myapp import db, User

Key Rules

•A directory is a package ONLY if it contains __init__.py (unless using implicit namespace packages)
•Use __init__.py to expose a clean, simplified API and hide internal module structure
•Relative imports (from . import x) ONLY work inside packages — they fail if you run the file as a script directly
•Use 'from .module import func' in __init__.py so users don't need to know your internal file structure
•Run packages as scripts using: python -m mypackage.mymodule

Your Task

Simulate a package structure using dictionaries and strings. Create a dict `package_structure` representing an `ecommerce` package with `__init__` code, `products.py` code, and `cart.py` code. Write the `__init__.py` string so that it imports `get_product` from `products` and `ShoppingCart` from `cart` using relative imports. Print the structure clearly.

Learning

Python Packages

What is a Package?

A package is a directory containing an __init__.py file. It allows you to group related modules together.

python

my_project/
├── main.py
└── ecommerce/              # The PACKAGE
    ├── __init__.py         # Makes 'ecommerce' a package
    ├── products.py
    ├── cart.py
    └── utils/
        ├── __init__.py     # Makes 'utils' a SUBPACKAGE
        └── formatting.py

The init.py File

python

# This runs when the package is imported
print('E-commerce package loaded!')

# Define package-level variables
VERSION = '2.0'

# Control what gets imported with 'from ecommerce import *'
__all__ = ['products', 'cart']

# Provide convenient imports at the package level
from .products import get_product
from .cart import ShoppingCart

Now users can do:

python

undefined

Importing from Packages

python

import ecommerce.products
ecommerce.products.get_product(1)

# Import specific item from module
from ecommerce.products import get_product
get_product(1)

# Import from subpackage
from ecommerce.utils.formatting import format_currency

# Relative imports (INSIDE the package only)
# In ecommerce/cart.py:
from .products import get_product  # Same package
from ..core import database        # Parent package

Namespace Packages (Python 3.3+)

You CAN have packages without __init__.py (called namespace packages). Useful for splitting a large package across multiple directories.

python

# Both directories contribute to 'shared_pkg'
/site-packages/shared_pkg/module_a.py
/another_path/shared_pkg/module_b.py

Recommendation: Always use __init__.py for your own projects. Namespace packages are mainly for library vendors.

init.py Patterns

python

def __getattr__(name):
    if name == 'heavy_module':
        from . import heavy_module
        return heavy_module
    raise AttributeError(f'module {__name__!r} has no attribute {name!r}')

# Pattern 2: Submodule import shortcut
# __init__.py
from .database import db
from .models import User, Product

# Now: from myapp import db, User

Key Rules

•A directory is a package ONLY if it contains __init__.py (unless using implicit namespace packages)
•Use __init__.py to expose a clean, simplified API and hide internal module structure
•Relative imports (from . import x) ONLY work inside packages — they fail if you run the file as a script directly
•Use 'from .module import func' in __init__.py so users don't need to know your internal file structure
•Run packages as scripts using: python -m mypackage.mymodule

Your Task