Files I/O, Exceptions and Modules

File handling, exception handling, and modules are three very important parts of Python.
Almost every real project uses them. File handling helps you store and read data. Exceptions help you handle errors safely. Modules help you organize your code into smaller reusable parts.

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Files in Python

Files are used to store data permanently. When your program ends, variables are lost, but files keep the data saved on disk. Python can work with many types of files like text files, binary files, CSV files, logs, and more.

The basic process of working with files is:

1. Open the file
2. Perform operations (read/write)
3. Close the file

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File Modes

Mode	Meaning
r	Read file (must exist)
w	Write (overwrite file)
a	Append (add at end)
x	Create new file
b	Binary mode
t	Text mode
+	Read and write

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Opening Files

You can open a file using open() and manually close it.

file = open("notes.txt", "r", encoding="utf-8")
content = file.read()
print(content)
file.close()

Explanation:

• open() opens the file
• read() reads content
• close() releases the file

Problem: If an error happens before close(), the file may remain open, which can cause bugs or memory issues.

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Using with

The with statement automatically handles closing the file, even if an error occurs.

with open("notes.txt", "r", encoding="utf-8") as file:
    content = file.read()
    print(content)

Why use with:

• Automatically closes file
• Safer and cleaner
• Prevents resource leaks

graph TD Start --> Open["Open file"] Open --> Work["Read or write data"] Work --> Exit["Exit block"] Exit --> Close["File closes automatically"]

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Reading Files

Reading means getting data from a file into your program.

Read full file

with open("notes.txt", "r", encoding="utf-8") as file:
    content = file.read()

Read line by line

with open("notes.txt", "r", encoding="utf-8") as file:
    for line in file:
        print(line.strip())

Line-by-line reading is better for large files.

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Writing and Appending

Writing replaces the file content, while appending adds new content.

# Write
with open("notes.txt", "w", encoding="utf-8") as file:
    file.write("First line\n")

# Append
with open("notes.txt", "a", encoding="utf-8") as file:
    file.write("Second line\n")

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Binary Files

Binary files store data as raw bytes instead of text. These are used for images, videos, PDFs, etc.

Writing binary file

data = b"Hello"

with open("data.bin", "wb") as file:
    file.write(data)

Reading binary file

with open("data.bin", "rb") as file:
    content = file.read()
    print(content)

Caution

• Use rb for reading
• Use wb for writing
• Do not use encoding in binary mode

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Object Files

Sometimes you want to save Python objects like lists or dictionaries directly into a file. This is called serialization.

Using pickle

import pickle

data = {"name": "Sahil", "age": 21}

# Save object
with open("data.pkl", "wb") as file:
    pickle.dump(data, file)

# Load object
with open("data.pkl", "rb") as file:
    loaded = pickle.load(file)
    print(loaded)

Explanation:

• dump() → store object
• load() → retrieve object

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Using pathlib

pathlib is a modern way to handle file paths. It is easier and cleaner.

from pathlib import Path

path = Path("data") / "notes.txt"
print(path)

It works correctly on all operating systems.

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Exceptions in Python

Exceptions are errors that occur while a program is running. Instead of crashing the program, Python allows you to handle these errors.

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Common Exceptions

Exception	Meaning
FileNotFoundError	File not found
ZeroDivisionError	Division by zero
ValueError	Invalid value
TypeError	Wrong type
IndexError	Invalid index
KeyError	Missing key

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try / except / else / finally

try:
    number = int("42")
except ValueError:
    print("Invalid number")
else:
    print("Success:", number)
finally:
    print("Always runs")

Explanation:

• try → code that may fail
• except → runs if error occurs
• else → runs if no error
• finally → always runs

graph TD Start --> Try Try -->|No error| Else Try -->|Error| Except Else --> Finally Except --> Finally Finally --> End

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Raising Exceptions

You can create errors manually when something is wrong.

def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    return age

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Custom Exceptions

Custom exceptions make your program more clear.

class InvalidAmountError(Exception):
    pass


def withdraw(balance, amount):
    if amount > balance:
        raise InvalidAmountError("Not enough balance")
    return balance - amount

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Modules in Python

A module is a Python file that contains reusable code. It helps you organize your program and avoid repeating code.

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Import Styles

import math
import math as m
from math import sqrt
from math import sqrt as square_root

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Creating Your Own Module

helpers.py

def clean_text(text):
    return text.strip().lower()

main.py

from helpers import clean_text

print(clean_text(" Hello "))

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__name__ == __main__

This allows a file to act both as a script and a module.

def main():
    print("Running directly")


if __name__ == "__main__":
    main()

This block runs only when the file is executed directly.

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Import Flow

graph TD Script --> Import Import --> Load Load --> Cache["Loaded once"] Cache --> Use

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Packages in Python

A package is a folder that contains multiple modules. It helps organize large projects.

Example structure:

project/
│
├── main.py
├── utils/
│   ├── __init__.py
│   ├── helpers.py
│   └── parser.py

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Importing from Packages

from utils.helpers import clean_text

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Relative Imports (., ..)

Relative imports are used inside packages to refer to files based on location.

Current directory (.)

from .helpers import clean_text

This means: import from the same folder.

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Parent directory (..)

from ..config import settings

This means: go one folder up, then import.

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Understanding with structure

project/
│
├── app/
│   ├── main.py
│   └── utils/
│       └── helper.py

Inside helper.py:

from ..main import something

Explanation:

• .. → go to parent folder (app)
• then import main.py

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Absolute vs Relative Imports

Type	Example	Meaning
Absolute	from utils.helpers import clean_text	Full path
Relative	from .helpers import clean_text	Based on current file

Absolute imports are easier to read. Relative imports are useful inside packages.