Complete Guide to File Handling in Python, CBSE Class 12

File handling is the most important chapter in CBSE Class 12 Computer Science. It carries 6-8 marks in the board exam and is also a major part of the practical exam. This guide covers everything you need to know.

Why File Handling?

Programs store data in variables, which are lost when the program ends. Files let you store data permanently on disk. Python can work with three types of files:

1. Text files - Store data as readable characters (.txt)
2. Binary files - Store data in binary format (.dat)
3. CSV files - Store tabular data as comma-separated values (.csv)

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File Modes

Mode	Description	File Exists?	File Doesn't Exist?
r	Read only (default)	Opens file	Error
w	Write only	Overwrites file	Creates new file
a	Append only	Adds to end	Creates new file
r+	Read and write	Opens file	Error
w+	Write and read	Overwrites file	Creates new file
a+	Append and read	Adds to end	Creates new file
rb	Read binary	Opens file	Error
wb	Write binary	Overwrites file	Creates new file
ab	Append binary	Adds to end	Creates new file

Exam tip: The most commonly asked modes are r, w, a, rb, and wb. Know what happens when the file does not exist for each mode.

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Part 1: Text File Handling

Opening and Closing Files

# Method 1: Manual open and close
f = open("data.txt", "r")
content = f.read()
f.close()

# Method 2: Using 'with' statement (recommended)
with open("data.txt", "r") as f:
    content = f.read()
# File automatically closes when the block ends

Always prefer the with statement. It automatically closes the file even if an error occurs. The examiner appreciates this.

Writing to a Text File

# write() - writes a string
f = open("output.txt", "w")
f.write("Hello World\n")
f.write("Python is fun\n")
f.close()

# writelines() - writes a list of strings
lines = ["Line 1\n", "Line 2\n", "Line 3\n"]
f = open("output.txt", "w")
f.writelines(lines)
f.close()

Important: write() does not add \n automatically. You must add it yourself. writelines() does not add \n between items either.

Reading from a Text File

Assume data.txt contains:

Hello World
Python is fun
CBSE Class 12

read(), Read Entire File

f = open("data.txt", "r")
content = f.read()
print(content)
f.close()

Hello World
Python is fun
CBSE Class 12

read(n), Read n Characters

f = open("data.txt", "r")
content = f.read(5)
print(content)
f.close()

Hello

readline(), Read One Line

f = open("data.txt", "r")
line1 = f.readline()
line2 = f.readline()
print(line1, end="")
print(line2, end="")
f.close()

Hello World
Python is fun

Each call to readline() reads the next line. The newline character \n is included in the returned string.

readlines(), Read All Lines as a List

f = open("data.txt", "r")
lines = f.readlines()
print(lines)
f.close()

['Hello World\n', 'Python is fun\n', 'CBSE Class 12\n']

Reading Line by Line Using a Loop

f = open("data.txt", "r")
for line in f:
    print(line, end="")
f.close()

This is the most memory-efficient way to read a file.

Appending to a File

f = open("data.txt", "a")
f.write("New line added\n")
f.close()

This adds data at the end without deleting existing content.

File Pointer, tell() and seek()

f = open("data.txt", "r")
print(f.tell())      # 0 (beginning of file)
f.read(5)
print(f.tell())      # 5 (after reading 5 characters)
f.seek(0)            # Move pointer back to beginning
print(f.tell())      # 0
f.close()

• tell() returns the current position of the file pointer
• seek(offset) moves the file pointer to a given position

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Part 2: Binary File Handling with Pickle

Binary files store data in a format that is not human-readable but is efficient for Python objects. The pickle module converts Python objects to binary (serialization) and back (deserialization).

• pickle.dump() - Write an object to a binary file (serialization/pickling)
• pickle.load() - Read an object from a binary file (deserialization/unpickling)

Writing to a Binary File

import pickle

student = {"rollno": 1, "name": "Aman", "marks": 85}

f = open("student.dat", "wb")
pickle.dump(student, f)
f.close()
print("Data written successfully")

Reading from a Binary File

import pickle

f = open("student.dat", "rb")
student = pickle.load(f)
print(student)
f.close()

{'rollno': 1, 'name': 'Aman', 'marks': 85}

Writing Multiple Records

import pickle

f = open("students.dat", "wb")

for i in range(3):
    roll = int(input("Enter roll number: "))
    name = input("Enter name: ")
    marks = float(input("Enter marks: "))
    record = [roll, name, marks]
    pickle.dump(record, f)

f.close()

Reading Multiple Records

import pickle

f = open("students.dat", "rb")
try:
    while True:
        record = pickle.load(f)
        print(record)
except EOFError:
    pass
f.close()

Important: When reading multiple records from a binary file, you must use a try-except block to catch EOFError. This tells Python to stop reading when the file ends. This pattern is asked in almost every board exam.

Searching in a Binary File

import pickle

def search_student(rollno):
    f = open("students.dat", "rb")
    found = False
    try:
        while True:
            record = pickle.load(f)
            if record[0] == rollno:
                print("Found:", record)
                found = True
                break
    except EOFError:
        pass
    if not found:
        print("Record not found")
    f.close()

search_student(2)

Updating a Record in a Binary File

import pickle

def update_marks(rollno, new_marks):
    f = open("students.dat", "rb")
    records = []
    try:
        while True:
            record = pickle.load(f)
            if record[0] == rollno:
                record[2] = new_marks
            records.append(record)
    except EOFError:
        pass
    f.close()

    f = open("students.dat", "wb")
    for record in records:
        pickle.dump(record, f)
    f.close()
    print("Record updated")

update_marks(1, 95)

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Part 3: CSV File Handling

CSV (Comma-Separated Values) files store tabular data in plain text format. Python uses the csv module for CSV file operations.

Writing to a CSV File

import csv

f = open("students.csv", "w", newline="")
writer = csv.writer(f)

writer.writerow(["RollNo", "Name", "Marks"])   # Header row
writer.writerow([1, "Aman", 85])
writer.writerow([2, "Priya", 92])
writer.writerow([3, "Rahul", 67])

f.close()

Important: Always pass newline="" when opening a CSV file for writing. Without it, extra blank lines appear between rows on Windows.

Reading from a CSV File

import csv

f = open("students.csv", "r")
reader = csv.reader(f)

for row in reader:
    print(row)

f.close()

['RollNo', 'Name', 'Marks']
['1', 'Aman', '85']
['2', 'Priya', '92']
['3', 'Rahul', '67']

Note: CSV reader returns all values as strings, even numbers.

Writing Multiple Rows at Once

import csv

data = [
    ["RollNo", "Name", "Marks"],
    [1, "Aman", 85],
    [2, "Priya", 92]
]

with open("students.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(data)

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Common Mistakes to Avoid

1. Forgetting to close the file. Use the with statement to avoid this.

2. Using w mode when you mean a mode. The w mode overwrites the entire file. Use a to add data at the end.

3. Not handling EOFError in binary files. Always use try-except EOFError when reading multiple records with pickle.

4. Forgetting newline="" in CSV. This causes blank lines on Windows.

5. Reading a file opened in write mode. If you open a file in w mode, you cannot read from it. Use r+ or w+ if you need both.

6. Not importing pickle or csv. Write import pickle or import csv at the top.

7. Confusing write() and writelines(). write() takes a single string. writelines() takes a list of strings. Neither adds \n automatically.

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Quick Revision Table

Operation	Text File	Binary File	CSV File
Module	Built-in	pickle	csv
Open mode	r, w, a	rb, wb, ab	r, w
Write	write(), writelines()	pickle.dump()	writer.writerow()
Read	read(), readline(), readlines()	pickle.load()	csv.reader()
Data format	Human-readable	Binary (not readable)	Comma-separated
Extension	.txt	.dat	.csv

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Exam-Style Question

Q: Write a function to count the number of lines in a text file "poem.txt" that start with an uppercase letter.

def count_upper_lines():
    count = 0
    with open("poem.txt", "r") as f:
        for line in f:
            if line[0].isupper():
                count += 1
    return count

print("Lines starting with uppercase:", count_upper_lines())

This is the level of question expected in the board exam. Practice writing complete functions with proper file open, read, and close operations.