Package structure and usage in Python - Mini Project: Build & Apply

Package structure and usage

📖 Scenario: You are organizing a small Python project into packages to keep your code clean and easy to use. You will create a package with modules and use them in a main script.

🎯 Goal: Build a simple Python package with two modules and use functions from those modules in a main script.

📋 What You'll Learn

Create a package folder named mypackage with an __init__.py file

Create two modules inside mypackage: module1.py and module2.py

Write one function in each module: greet() in module1.py and farewell() in module2.py

Import and use these functions in a main script main.py

💡 Why This Matters

🌍 Real World

Organizing code into packages helps keep projects clean and makes code reusable across different programs.

💼 Career

Understanding package structure is essential for working on larger Python projects and collaborating with other developers.

1

Create package folder and modules

Create a folder named mypackage. Inside it, create an empty file named __init__.py. Then create two files inside mypackage: module1.py and module2.py. In module1.py, write a function called greet that returns the string 'Hello from module1'. In module2.py, write a function called farewell that returns the string 'Goodbye from module2'.

Python

# Create mypackage folder with __init__.py
# Create module1.py with greet() function
# Create module2.py with farewell() function

Hint

Remember, __init__.py can be empty but it tells Python this folder is a package.

2

Create main script and import functions

Create a file named main.py outside the mypackage folder. In main.py, import the greet function from mypackage.module1 and the farewell function from mypackage.module2.

Python

# Import greet from mypackage.module1
# Import farewell from mypackage.module2

Hint

Use the syntax from package.module import function to import.

3

Use imported functions in main.py

In main.py, call the greet() function and store its result in a variable named hello_message. Call the farewell() function and store its result in a variable named bye_message.

Python

from mypackage.module1 import greet
from mypackage.module2 import farewell

# Call greet() and save to hello_message
# Call farewell() and save to bye_message

Hint

Just call the functions and assign their return values to variables.

4

Print the messages

In main.py, print the variables hello_message and bye_message on separate lines.

Python

from mypackage.module1 import greet
from mypackage.module2 import farewell

hello_message = greet()
bye_message = farewell()

# Print hello_message
# Print bye_message

Hint

Use two print statements, one for each message.

Practice

1. What is the main purpose of the __init__.py file in a Python package?

easy

A. To mark a directory as a Python package

B. To execute the main program

C. To store global variables

D. To compile Python files

Solution

1. Step 1: Understand the role of __init__.py

   The __init__.py file tells Python that the directory should be treated as a package.

2. Step 2: Differentiate from other options

   It does not execute the main program, store global variables, or compile files; its main role is package identification.

3. Final Answer:

   To mark a directory as a Python package -> Option A

4. Quick Check:

   __init__.py marks packages [OK]

Hint: Remember: __init__.py makes folder a package [OK]

Common Mistakes:

• Thinking __init__.py runs main code automatically
• Confusing it with a script file
• Assuming it compiles Python files

2. Which of the following is the correct way to import the module utils from a package named mypackage?

easy

A. import mypackage.utils

B. from utils import mypackage

C. import utils.mypackage

D. from mypackage import utils

Solution

1. Step 1: Recall Python import syntax for packages

   To import a module from a package, use from package_name import module.

2. Step 2: Match syntax to options

   from mypackage import utils matches this syntax: from mypackage import utils.

3. Final Answer:

   from mypackage import utils -> Option D

4. Quick Check:

   Import module with 'from package import module' [OK]

Hint: Use 'from package import module' to import modules [OK]

Common Mistakes:

• Reversing package and module names
• Using incorrect import order
• Trying to import module as package

3. Given this package structure:

mypackage/
  __init__.py
  math_ops.py
  string_ops.py

And this code:

from mypackage import math_ops
print(math_ops.add(2, 3))

If math_ops.py contains a function def add(a, b): return a + b, what is the output?

medium

A. None

B. TypeError

C. 5

D. NameError

Solution

1. Step 1: Understand the import and function call

   The code imports math_ops from mypackage and calls math_ops.add(2, 3).

2. Step 2: Evaluate the function output

   The function add returns the sum of 2 and 3, which is 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   2 + 3 = 5 [OK]

Hint: Trace function call and return value carefully [OK]

Common Mistakes:

• Forgetting to call the function with parentheses
• Confusing module and function names
• Assuming import fails without __init__.py content

4. You have a package folder named tools with modules calc.py and format.py. You try to run:

from tools import calc
print(calc.multiply(4, 5))

But get ModuleNotFoundError. What is the most likely cause?

medium

A. Function multiply does not exist in calc.py

B. Missing __init__.py file in the tools folder

C. Syntax error in import statement

D. Python version is too old

Solution

1. Step 1: Understand ModuleNotFoundError cause

   This error often means Python does not recognize the folder as a package.

2. Step 2: Check package structure requirements

   Without __init__.py, Python won't treat 'tools' as a package, causing import failure.

3. Final Answer:

   Missing __init__.py file in the tools folder -> Option B

4. Quick Check:

   __init__.py missing causes ModuleNotFoundError [OK]

Hint: Always add __init__.py to package folders [OK]

Common Mistakes:

• Assuming function absence causes ModuleNotFoundError
• Ignoring package folder structure
• Blaming Python version without checking files

5. You want to create a package datautils with submodules clean.py and transform.py. You want users to import clean_data function directly from datautils like this:

from datautils import clean_data

Which code should you add to datautils/__init__.py to enable this?

hard

A. from .clean import clean_data

B. import clean_data from clean

C. from clean import clean_data

D. import clean.clean_data

Solution

1. Step 1: Understand relative imports in packages

   Inside __init__.py, use relative import with dot: from .clean import clean_data.

2. Step 2: Confirm correct syntax for exposing functions

   This syntax makes clean_data available directly from datautils.

3. Final Answer:

   from .clean import clean_data -> Option A

4. Quick Check:

   Use 'from .module import func' in __init__.py [OK]

Hint: Use relative import with dot in __init__.py [OK]

Common Mistakes:

• Using absolute import without dot inside __init__.py
• Wrong import syntax like 'import clean_data from clean'
• Trying to import without __init__.py setup