Package structure and usage in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of importing a submodule

Consider this package structure:

mypackage/
  __init__.py
  module_a.py
  subpackage/
    __init__.py
    module_b.py

In module_b.py, there is a function def greet(): return 'Hello from B'.

What is the output of this code?

from mypackage.subpackage import module_b
print(module_b.greet())

Python

from mypackage.subpackage import module_b
print(module_b.greet())

A ModuleNotFoundError

B AttributeError

C ImportError

D Hello from B

💡 Hint

Think about how Python imports functions from submodules inside packages.

Importing the submodule module_b from mypackage.subpackage works if the package structure is correct and greet() is defined there. Calling module_b.greet() returns the string as expected.

❓ Predict Output

intermediate

Effect of __init__.py on package import

Given this package structure:

mypackage/
  __init__.py
  utils.py

and __init__.py contains:

from .utils import helper

and utils.py contains:

def helper():
    return 'Helping'

What is the output of this code?

import mypackage
print(mypackage.helper())

Python

import mypackage
print(mypackage.helper())

A Helping

B AttributeError

C ImportError

D NameError

💡 Hint

Check what __init__.py exposes when importing the package.

The __init__.py imports helper from utils and makes it available as mypackage.helper. So calling mypackage.helper() works and returns 'Helping'.

🔧 Debug

advanced

Why does this import fail?

You have this package structure:

mypackage/
  __init__.py
  tools.py

In tools.py you have:

def tool():
    return 'Tool here'

You run this code from outside the package folder:

from mypackage import tool
print(tool())

But it raises an error. What is the error?

Python

from mypackage import tool
print(tool())

A AttributeError

B ImportError

C ModuleNotFoundError

D NameError

💡 Hint

Check what names are exposed by mypackage when importing.

The package mypackage does not expose tool directly because __init__.py does not import tool. So from mypackage import tool raises ImportError.

📝 Syntax

advanced

Correct relative import syntax inside a package

Inside mypackage/subpackage/module_c.py, you want to import func from mypackage/module_a.py.

Which import statement is correct?

A from ..module_a import func

B from .module_a import func

C import module_a.func

D from mypackage.module_a import func

💡 Hint

Think about how many levels up you need to go to reach module_a.

Since module_c.py is inside subpackage, to import from module_a.py in the parent package, you use two dots .. to go up one level: from ..module_a import func.

🚀 Application

expert

Number of modules loaded after import

Given this package structure:

mypackage/
  __init__.py
  alpha.py
  beta.py
  gamma/
    __init__.py
    delta.py

Where:
- __init__.py in mypackage imports alpha and gamma
- __init__.py in gamma imports delta

What is the total number of distinct modules loaded into memory after running:

import mypackage

Python

import mypackage

A 3

B 1

C 4

D 2

💡 Hint

Count the modules explicitly imported during the package import.

Importing mypackage runs its __init__.py, which imports alpha and gamma. Importing gamma runs its __init__.py, which imports delta. So modules loaded are: mypackage, alpha, gamma, and delta. Total 4 modules.

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