Unit-4: Python File Operations

1. Introduction to File Operations

Why File Operations?

Persistent storage - Data survives program termination
Large data handling - Process data too big for memory
Data sharing - Share data between programs
Configuration - Store settings and preferences

File Types

Text Files - Human-readable (.txt, .csv, .json)
Binary Files - Machine-readable (.exe, .bin, .dat)

File Operations Flow

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

2. Opening and Closing Files

open() Function

Syntax:

file_object = open(filename, mode)

File Modes

Mode	Description	Creates if Not Exists
`'r'`	Read (default)	No (Error)
`'w'`	Write (overwrites)	Yes
`'a'`	Append (adds to end)	Yes
`'x'`	Exclusive create	No (Error if exists)
`'r+'`	Read + Write	No
`'w+'`	Write + Read	Yes
`'a+'`	Append + Read	Yes
`'rb'`	Read binary	No
`'wb'`	Write binary	Yes

Basic File Operations

# Open file for reading
file = open("sample.txt", "r")

# Perform operations
content = file.read()
print(content)

# Close file
file.close()

Using with Statement (Recommended)

# Automatically closes file
with open("sample.txt", "r") as file:
    content = file.read()
    print(content)
# File is automatically closed here

Advantages of with:

Automatic file closing
Exception safe
Cleaner code

3. Writing Files

write() Method

Writes string to file

# Write mode - overwrites existing content
with open("output.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("This is Python file handling.\n")
    file.write("Writing to files is easy!")

print("File written successfully!")

output.txt content:

Hello, World!
This is Python file handling.
Writing to files is easy!

writelines() Method

Writes list of strings to file

lines = [
    "Line 1\n",
    "Line 2\n",
    "Line 3\n"
]

with open("output.txt", "w") as file:
    file.writelines(lines)

print("Multiple lines written!")

output.txt content:

Line 1
Line 2
Line 3

Append Mode

# Append mode - adds to end of file
with open("output.txt", "a") as file:
    file.write("Line 4\n")
    file.write("Line 5\n")

print("Lines appended!")

output.txt content (after append):

Line 1
Line 2
Line 3
Line 4
Line 5

Write Examples

Example 1: Write Numbers

# Write numbers 1 to 10
with open("numbers.txt", "w") as file:
    for i in range(1, 11):
        file.write(f"{i}\n")

print("Numbers written to file")

numbers.txt:

Example 2: Write Dictionary Data

student = {
    "name": "Alice",
    "age": 20,
    "grade": "A"
}

with open("student.txt", "w") as file:
    for key, value in student.items():
        file.write(f"{key}: {value}\n")

student.txt:

name: Alice
age: 20
grade: A

Example 3: Write List to CSV

data = [
    ["Name", "Age", "Grade"],
    ["Alice", "20", "A"],
    ["Bob", "22", "B"],
    ["Charlie", "21", "A"]
]

with open("students.csv", "w") as file:
    for row in data:
        file.write(",".join(row) + "\n")

students.csv:

Name,Age,Grade
Alice,20,A
Bob,22,B
Charlie,21,A

4. Reading Files

read() Method

Reads entire file as string

# Read entire file
with open("sample.txt", "r") as file:
    content = file.read()
    print(content)
    print(f"Type: {type(content)}")  # <class 'str'>

Read specific number of characters:

with open("sample.txt", "r") as file:
    # Read first 10 characters
    first_10 = file.read(10)
    print(first_10)
    
    # Read next 10 characters
    next_10 = file.read(10)
    print(next_10)

readline() Method

Reads single line from file

with open("sample.txt", "r") as file:
    # Read first line
    line1 = file.readline()
    print("Line 1:", line1)
    
    # Read second line
    line2 = file.readline()
    print("Line 2:", line2)
    
    # Read third line
    line3 = file.readline()
    print("Line 3:", line3)

Example: Read Until Empty

with open("sample.txt", "r") as file:
    line = file.readline()
    while line:
        print(line.strip())  # strip() removes \n
        line = file.readline()

readlines() Method

Reads all lines into list

with open("sample.txt", "r") as file:
    lines = file.readlines()
    print(f"Total lines: {len(lines)}")
    print(f"Type: {type(lines)}")  # <class 'list'>
    
    # Print each line
    for line in lines:
        print(line.strip())

Iterating Through File

Most Pythonic way:

with open("sample.txt", "r") as file:
    for line in file:
        print(line.strip())

Comparison of Read Methods

Method	Returns	Use Case
`read()`	Entire file as string	Small files, need all content
`read(n)`	n characters	Process specific bytes
`readline()`	Single line	Process line by line
`readlines()`	List of lines	Need all lines as list
`for line in file`	Iterator	Large files, line-by-line

Read Examples

Example 1: Count Words

with open("sample.txt", "r") as file:
    content = file.read()
    words = content.split()
    print(f"Total words: {len(words)}")

Example 2: Count Lines

with open("sample.txt", "r") as file:
    lines = file.readlines()
    print(f"Total lines: {len(lines)}")

Example 3: Count Characters

with open("sample.txt", "r") as file:
    content = file.read()
    print(f"Total characters: {len(content)}")
    
    # Without spaces
    no_spaces = content.replace(" ", "").replace("\n", "")
    print(f"Characters (no spaces): {len(no_spaces)}")

Example 4: Find Specific Word

search_word = "Python"

with open("sample.txt", "r") as file:
    line_num = 0
    for line in file:
        line_num += 1
        if search_word in line:
            print(f"Found '{search_word}' at line {line_num}")
            print(f"Content: {line.strip()}")

Example 5: Read CSV File

# Assuming students.csv exists
with open("students.csv", "r") as file:
    # Skip header
    header = file.readline()
    print("Header:", header.strip())
    print("\nStudents:")
    
    for line in file:
        data = line.strip().split(",")
        name, age, grade = data
        print(f"Name: {name}, Age: {age}, Grade: {grade}")

Output:

Header: Name,Age,Grade

Students:
Name: Alice, Age: 20, Grade: A
Name: Bob, Age: 22, Grade: B
Name: Charlie, Age: 21, Grade: A

5. File Pointer and seek()

File Pointer

Current position in file
Moves as you read/write
Can be controlled using seek() and tell()

tell() Method

Returns current position

with open("sample.txt", "r") as file:
    print(f"Initial position: {file.tell()}")  # 0
    
    file.read(10)
    print(f"After reading 10 chars: {file.tell()}")  # 10
    
    file.read(5)
    print(f"After reading 5 more: {file.tell()}")  # 15

seek() Method

Move file pointer to specific position

Syntax:

file.seek(offset, whence)

Parameters:

offset: Number of bytes to move
whence: Reference point
- 0: Start of file (default)
- 1: Current position
- 2: End of file

with open("sample.txt", "r") as file:
    # Read first 10 characters
    print(file.read(10))
    
    # Move pointer to beginning
    file.seek(0)
    
    # Read first 10 again
    print(file.read(10))
    
    # Move to position 20
    file.seek(20)
    print(file.read(10))

seek() Examples

Example 1: Read File Multiple Times

with open("sample.txt", "r") as file:
    print("First read:")
    print(file.read())
    
    # Reset pointer to beginning
    file.seek(0)
    
    print("\nSecond read:")
    print(file.read())

Example 2: Read Specific Portion

with open("sample.txt", "r") as file:
    # Skip first 20 characters
    file.seek(20)
    
    # Read next 30 characters
    content = file.read(30)
    print(content)

Example 3: Read Last N Characters

with open("sample.txt", "r") as file:
    # Move to end
    file.seek(0, 2)
    file_size = file.tell()
    
    # Move to last 50 characters
    file.seek(file_size - 50)
    
    last_50 = file.read()
    print(last_50)

6. File Operations Examples

Example 1: Copy File

# Copy content from source to destination
with open("source.txt", "r") as source:
    content = source.read()

with open("destination.txt", "w") as dest:
    dest.write(content)

print("File copied successfully!")

Example 2: Merge Files

# Merge multiple files into one
files_to_merge = ["file1.txt", "file2.txt", "file3.txt"]

with open("merged.txt", "w") as output:
    for filename in files_to_merge:
        with open(filename, "r") as input_file:
            content = input_file.read()
            output.write(content)
            output.write("\n")  # Separator

print("Files merged successfully!")

Example 3: Count Vowels in File

vowels = "aeiouAEIOU"
vowel_count = 0

with open("sample.txt", "r") as file:
    content = file.read()
    for char in content:
        if char in vowels:
            vowel_count += 1

print(f"Total vowels: {vowel_count}")

Example 4: Word Frequency

word_count = {}

with open("sample.txt", "r") as file:
    for line in file:
        words = line.lower().split()
        for word in words:
            # Remove punctuation
            word = word.strip('.,!?;:"')
            if word:
                word_count[word] = word_count.get(word, 0) + 1

# Display top 5 frequent words
sorted_words = sorted(word_count.items(), key=lambda x: x[1], reverse=True)
print("Top 5 frequent words:")
for word, count in sorted_words[:5]:
    print(f"{word}: {count}")

Example 5: Replace Word in File

# Replace all occurrences of a word
old_word = "Python"
new_word = "Java"

with open("sample.txt", "r") as file:
    content = file.read()

# Replace
new_content = content.replace(old_word, new_word)

# Write back
with open("sample.txt", "w") as file:
    file.write(new_content)

print(f"Replaced '{old_word}' with '{new_word}'")

Example 6: Student Database (CRUD Operations)

import os

def add_student(name, roll, grade):
    with open("students.txt", "a") as file:
        file.write(f"{roll},{name},{grade}\n")
    print("Student added successfully!")

def view_students():
    if not os.path.exists("students.txt"):
        print("No students found!")
        return
    
    print("\n{:<10} {:<20} {:<10}".format("Roll No", "Name", "Grade"))
    print("-" * 40)
    
    with open("students.txt", "r") as file:
        for line in file:
            roll, name, grade = line.strip().split(",")
            print(f"{roll:<10} {name:<20} {grade:<10}")

def search_student(roll):
    found = False
    with open("students.txt", "r") as file:
        for line in file:
            r, name, grade = line.strip().split(",")
            if r == roll:
                print(f"\nRoll: {r}, Name: {name}, Grade: {grade}")
                found = True
                break
    
    if not found:
        print("Student not found!")

def delete_student(roll):
    lines = []
    found = False
    
    with open("students.txt", "r") as file:
        for line in file:
            r = line.strip().split(",")[0]
            if r != roll:
                lines.append(line)
            else:
                found = True
    
    if found:
        with open("students.txt", "w") as file:
            file.writelines(lines)
        print("Student deleted successfully!")
    else:
        print("Student not found!")

# Usage
add_student("Alice", "101", "A")
add_student("Bob", "102", "B")
add_student("Charlie", "103", "A")

view_students()
search_student("102")
delete_student("102")
view_students()

Output:

Student added successfully!
Student added successfully!
Student added successfully!

Roll No    Name                 Grade     
----------------------------------------
101        Alice                A         
102        Bob                  B         
103        Charlie              A         

Roll: 102, Name: Bob, Grade: B
Student deleted successfully!

Roll No    Name                 Grade     
----------------------------------------
101        Alice                A         
103        Charlie              A

Example 7: Log File Analysis

# Analyze log file
error_count = 0
warning_count = 0
info_count = 0

with open("app.log", "r") as file:
    for line in file:
        if "ERROR" in line:
            error_count += 1
        elif "WARNING" in line:
            warning_count += 1
        elif "INFO" in line:
            info_count += 1

print("Log Summary:")
print(f"Errors: {error_count}")
print(f"Warnings: {warning_count}")
print(f"Info: {info_count}")

Example 8: CSV to Dictionary

# Read CSV and convert to list of dictionaries
students = []

with open("students.csv", "r") as file:
    # Read header
    header = file.readline().strip().split(",")
    
    # Read data
    for line in file:
        values = line.strip().split(",")
        student = dict(zip(header, values))
        students.append(student)

# Display
for student in students:
    print(student)

Output:

{'Name': 'Alice', 'Age': '20', 'Grade': 'A'}
{'Name': 'Bob', 'Age': '22', 'Grade': 'B'}
{'Name': 'Charlie', 'Age': '21', 'Grade': 'A'}

Example 9: Filter Lines

# Copy only lines containing specific word
keyword = "Python"

with open("source.txt", "r") as source:
    with open("filtered.txt", "w") as dest:
        for line in source:
            if keyword in line:
                dest.write(line)

print(f"Lines containing '{keyword}' copied to filtered.txt")

Example 10: File Statistics

import os

def file_stats(filename):
    if not os.path.exists(filename):
        print("File not found!")
        return
    
    with open(filename, "r") as file:
        content = file.read()
        lines = content.split("\n")
        words = content.split()
        chars = len(content)
        
        # Count non-empty lines
        non_empty_lines = sum(1 for line in lines if line.strip())
    
    # File size
    size = os.path.getsize(filename)
    
    print(f"File: {filename}")
    print(f"Size: {size} bytes")
    print(f"Lines: {non_empty_lines}")
    print(f"Words: {len(words)}")
    print(f"Characters: {chars}")

file_stats("sample.txt")

Output:

File: sample.txt
Size: 1024 bytes
Lines: 25
Words: 150
Characters: 1024

7. Exception Handling with Files

Common File Exceptions

# FileNotFoundError
try:
    with open("nonexistent.txt", "r") as file:
        content = file.read()
except FileNotFoundError:
    print("Error: File not found!")

# PermissionError
try:
    with open("/system/protected.txt", "w") as file:
        file.write("data")
except PermissionError:
    print("Error: Permission denied!")

# IOError
try:
    with open("sample.txt", "r") as file:
        content = file.read()
except IOError:
    print("Error: I/O operation failed!")

Complete Error Handling

def safe_file_read(filename):
    try:
        with open(filename, "r") as file:
            content = file.read()
            return content
    except FileNotFoundError:
        print(f"Error: {filename} not found!")
        return None
    except PermissionError:
        print(f"Error: No permission to read {filename}!")
        return None
    except Exception as e:
        print(f"Unexpected error: {e}")
        return None

# Usage
content = safe_file_read("sample.txt")
if content:
    print(content)

8. Binary File Operations

Writing Binary Files

# Write binary data
data = bytes([65, 66, 67, 68, 69])  # ASCII codes for ABCDE

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

print("Binary file written!")

Reading Binary Files

# Read binary data
with open("binary.bin", "rb") as file:
    data = file.read()
    print(data)  # b'ABCDE'
    print(list(data))  # [65, 66, 67, 68, 69]

Copy Binary File (e.g., Image)

# Copy image file
with open("source.jpg", "rb") as source:
    with open("destination.jpg", "wb") as dest:
        dest.write(source.read())

print("Image copied successfully!")

9. Working with os Module

import os

# Check if file exists
if os.path.exists("sample.txt"):
    print("File exists!")

# Get file size
size = os.path.getsize("sample.txt")
print(f"File size: {size} bytes")

# Rename file
os.rename("old_name.txt", "new_name.txt")

# Delete file
os.remove("temp.txt")

# Get current directory
print(f"Current directory: {os.getcwd()}")

# List files in directory
files = os.listdir(".")
print("Files:", files)

# Create directory
os.mkdir("new_folder")

# Check if path is file or directory
print(os.path.isfile("sample.txt"))  # True
print(os.path.isdir("folder"))       # True

🚀 Quick Reference Summary

File Modes

Mode	Read	Write	Create	Truncate
`'r'`	✓	✗	✗	✗
`'w'`	✗	✓	✓	✓
`'a'`	✗	✓	✓	✗
`'r+'`	✓	✓	✗	✗
`'w+'`	✓	✓	✓	✓
`'a+'`	✓	✓	✓	✗

Read Functions

Function	Returns	Moves Pointer
`read()`	Entire file	End
`read(n)`	n characters	n positions
`readline()`	One line	Next line
`readlines()`	List of lines	End

Write Functions

Function	Parameter	Description
`write()`	String	Write string
`writelines()`	List	Write list of strings

File Pointer

file.tell()        # Get current position
file.seek(0)       # Move to beginning
file.seek(0, 2)    # Move to end

📝 Exam Important Questions

Explain different file opening modes with examples.
Write a program to copy content from one file to another.
Demonstrate the difference between read(), readline(), and readlines().
Write a program to count number of lines, words, and characters in a file.
Explain file pointer manipulation using seek() and tell().
Write a program to merge two files into a third file.
Create a program for student record management (add, view, search, delete).
Explain the importance of closing files and advantages of with statement.
Write a program to find and replace a word in a file.
Create a program to:
- Read a CSV file
- Calculate average of numeric column
- Write result to new file

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Unit-4: Python File Operations

1. Introduction to File Operations

Why File Operations?

File Types

File Operations Flow

2. Opening and Closing Files

open() Function

File Modes

Basic File Operations

Using with Statement (Recommended)

3. Writing Files

write() Method

writelines() Method

Append Mode

Write Examples

Example 1: Write Numbers

Example 2: Write Dictionary Data

Example 3: Write List to CSV

4. Reading Files

read() Method

readline() Method

readlines() Method

Iterating Through File

Comparison of Read Methods

Read Examples

Example 1: Count Words

Example 2: Count Lines

Example 3: Count Characters

Example 4: Find Specific Word

Example 5: Read CSV File

5. File Pointer and seek()

File Pointer

tell() Method

seek() Method

seek() Examples

Example 1: Read File Multiple Times

Example 2: Read Specific Portion

Example 3: Read Last N Characters

6. File Operations Examples

Example 1: Copy File

Example 2: Merge Files

Example 3: Count Vowels in File

Example 4: Word Frequency

Example 5: Replace Word in File

Example 6: Student Database (CRUD Operations)

Example 7: Log File Analysis

Example 8: CSV to Dictionary

Example 9: Filter Lines

Example 10: File Statistics

7. Exception Handling with Files

Common File Exceptions

Complete Error Handling

8. Binary File Operations

Writing Binary Files

Reading Binary Files

Copy Binary File (e.g., Image)

9. Working with os Module

🚀 Quick Reference Summary

File Modes

Read Functions

Write Functions

File Pointer

📝 Exam Important Questions