Binary File Handling in Python (Pickle Module)

Prerequisites: Text file handling, lists, dictionaries, exception handling

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1. What Are Binary Files?

• Binary files store data in binary format (bytes), not human-readable text., They can store any Python object: lists, dictionaries, tuples, class objects., Binary files are not readable in a text editor, they appear as garbled characters., They are platform-independent (no line-ending translation like text files)., Faster to read/write for complex data structures.

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2. The pickle Module

The pickle module converts Python objects to byte streams and back.

• Pickling (Serialization): Converting a Python object to a byte stream and writing it to a file.
• Unpickling (Deserialization): Reading a byte stream from a file and converting it back to a Python object.

import pickle

Key Functions

Function	Purpose
pickle.dump(object, file)	Write (pickle) an object to a binary file
pickle.load(file)	Read (unpickle) an object from a binary file

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3. File Modes for Binary Files

Mode	Description
"rb"	Read binary
"wb"	Write binary (creates new / overwrites)
"ab"	Append binary
"rb+"	Read and write binary (file must exist)
"wb+"	Write and read binary (overwrites)
"ab+"	Append and read binary

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4. Writing to Binary Files: dump

import pickle

# Writing a single object
data = [10, 20, 30, 40, 50]
with open("data.dat", "wb") as f:
 pickle.dump(data, f)
print("Data written to file.")

Writing Multiple Objects

import pickle

with open("multi.dat", "wb") as f:
 pickle.dump([1, 2, 3], f) # First object
 pickle.dump("Hello", f) # Second object
 pickle.dump({"a": 1, "b": 2}, f) # Third object

print("Multiple objects written.")

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5. Reading from Binary Files: load

import pickle

with open("data.dat", "rb") as f:
 data = pickle.load(f)
 print(data)

# Output: [10, 20, 30, 40, 50]

Reading Multiple Objects

import pickle

with open("multi.dat", "rb") as f:
 obj1 = pickle.load(f) # [1, 2, 3]
 obj2 = pickle.load(f) # 'Hello'
 obj3 = pickle.load(f) # {'a': 1, 'b': 2}
 print(obj1)
 print(obj2)
 print(obj3)

# Output:
# [1, 2, 3]
# Hello
# {'a': 1, 'b': 2}

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6. Handling EOFError When Reading

When reading a binary file, you don't know how many objects are stored. Calling load after all objects are read raises EOFError.

Pattern: Read All Objects Until EOFError

import pickle

with open("multi.dat", "rb") as f:
 try:
 while True:
 obj = pickle.load(f)
 print(obj)
 except EOFError:
 print("--- End of file reached ---")

# Output:
# [1, 2, 3]
# Hello
# {'a': 1, 'b': 2}
# --- End of file reached ---

This pattern is essential for exams and is used in almost every binary file reading program.

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7. Difference: Text Files vs Binary Files

Feature	Text File	Binary File
Module	Built-in (no import)	import pickle
Mode	"r", "w", "a"	"rb", "wb", "ab"
Write method	write, writelines	pickle.dump
Read method	read, readline, readlines	pickle.load
Data type	Strings only	Any Python object
Readability	Human-readable	Not human-readable
End of file	Returns empty string ""	Raises EOFError

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Practice Programs

Program 1: Write a List to Binary File

import pickle

# Create a list and write to binary file
marks = [85, 92, 78, 95, 88]

with open("marks.dat", "wb") as f:
 pickle.dump(marks, f)

print("List written to marks.dat")
print("Data:", marks)

# Output:
# List written to marks.dat
# Data: [85, 92, 78, 95, 88]

Program 2: Read from Binary File

import pickle

# Read the list from binary file
with open("marks.dat", "rb") as f:
 marks = pickle.load(f)

print("Data read from file:", marks)
print("Total marks:", sum(marks))
print("Average:", sum(marks) / len(marks))

# Output:
# Data read from file: [85, 92, 78, 95, 88]
# Total marks: 438
# Average: 87.6

Program 3: Write Student Records (Roll, Name, Marks)

import pickle

def write_students:
 with open("students.dat", "wb") as f:
 n = int(input("How many students? "))
 for i in range(n):
 print(f"\nStudent {i+1}:")
 roll = int(input(" Roll No: "))
 name = input(" Name: ")
 marks = float(input(" Marks: "))
 record = {"roll": roll, "name": name, "marks": marks}
 pickle.dump(record, f)

 print("\nAll records written successfully!")

write_students

# Test Run:
# How many students? 3
#
# Student 1:
# Roll No: 101
# Name: Aman
# Marks: 85.5
#
# Student 2:
# Roll No: 102
# Name: Priya
# Marks: 92.0
#
# Student 3:
# Roll No: 103
# Name: Rahul
# Marks: 78.5
#
# All records written successfully!

Program 4: Search by Roll Number

import pickle

def search_student:
 roll_search = int(input("Enter roll number to search: "))
 found = False

 with open("students.dat", "rb") as f:
 try:
 while True:
 record = pickle.load(f)
 if record["roll"] == roll_search:
 print("\nStudent Found!")
 print(f" Roll No : {record['roll']}")
 print(f" Name : {record['name']}")
 print(f" Marks : {record['marks']}")
 found = True
 break
 except EOFError:
 pass

 if not found:
 print(f"Student with Roll No {roll_search} not found.")

search_student

# Test Run 1:
# Enter roll number to search: 102
# Student Found!
# Roll No : 102
# Name : Priya
# Marks : 92.0

# Test Run 2:
# Enter roll number to search: 999
# Student with Roll No 999 not found.

Program 5: Update Marks for a Roll Number

import pickle

def update_marks:
 roll_update = int(input("Enter roll number to update: "))
 new_marks = float(input("Enter new marks: "))
 found = False

 # Read all records
 records = []
 with open("students.dat", "rb") as f:
 try:
 while True:
 record = pickle.load(f)
 if record["roll"] == roll_update:
 print(f"Old marks: {record['marks']}")
 record["marks"] = new_marks
 print(f"New marks: {record['marks']}")
 found = True
 records.append(record)
 except EOFError:
 pass

 if found:
 # Write all records back
 with open("students.dat", "wb") as f:
 for record in records:
 pickle.dump(record, f)
 print("Record updated successfully!")
 else:
 print(f"Student with Roll No {roll_update} not found.")

update_marks

# Test Run:
# Enter roll number to update: 103
# Enter new marks: 88.0
# Old marks: 78.5
# New marks: 88.0
# Record updated successfully!

Program 6: Delete a Record

import pickle

def delete_student:
 roll_delete = int(input("Enter roll number to delete: "))
 found = False

 # Read all records
 records = []
 with open("students.dat", "rb") as f:
 try:
 while True:
 record = pickle.load(f)
 if record["roll"] == roll_delete:
 print(f"Deleting: {record['name']} (Roll: {record['roll']})")
 found = True
 else:
 records.append(record)
 except EOFError:
 pass

 if found:
 # Write remaining records back
 with open("students.dat", "wb") as f:
 for record in records:
 pickle.dump(record, f)
 print("Record deleted successfully!")
 else:
 print(f"Student with Roll No {roll_delete} not found.")

delete_student

# Test Run:
# Enter roll number to delete: 102
# Deleting: Priya (Roll: 102)
# Record deleted successfully!

Program 7: Display All Records

import pickle

def display_all:
 print("\n" + "=" * 40)
 print(f"{'Roll':<8}{'Name':<15}{'Marks':<10}")
 print("=" * 40)

 count = 0
 with open("students.dat", "rb") as f:
 try:
 while True:
 record = pickle.load(f)
 print(f"{record['roll']:<8}{record['name']:<15}{record['marks']:<10}")
 count += 1
 except EOFError:
 pass

 print("=" * 40)
 print(f"Total records: {count}")

display_all

# Output:
# ========================================
# Roll Name Marks
# ========================================
# 101 Aman 85.5
# 102 Priya 92.0
# 103 Rahul 78.5
# ========================================
# Total records: 3

Program 8: Employee Management System (Complete CRUD)

import pickle
import os

FILENAME = "employees.dat"

def add_employee:
 """Add a new employee record."""
 with open(FILENAME, "ab") as f:
 empid = int(input("Enter Employee ID: "))
 name = input("Enter Name: ")
 dept = input("Enter Department: ")
 salary = float(input("Enter Salary: "))
 record = {"empid": empid, "name": name, "dept": dept, "salary": salary}
 pickle.dump(record, f)
 print("Employee added successfully!\n")

def display_all:
 """Display all employee records."""
 if not os.path.exists(FILENAME):
 print("No records found.\n")
 return

 print("\n" + "=" * 55)
 print(f"{'ID':<8}{'Name':<15}{'Dept':<12}{'Salary':<12}")
 print("=" * 55)

 count = 0
 with open(FILENAME, "rb") as f:
 try:
 while True:
 rec = pickle.load(f)
 print(f"{rec['empid']:<8}{rec['name']:<15}{rec['dept']:<12}{rec['salary']:<12.2f}")
 count += 1
 except EOFError:
 pass

 print("=" * 55)
 print(f"Total employees: {count}\n")

def search_employee:
 """Search for an employee by ID."""
 empid = int(input("Enter Employee ID to search: "))
 found = False

 with open(FILENAME, "rb") as f:
 try:
 while True:
 rec = pickle.load(f)
 if rec["empid"] == empid:
 print(f"\nEmployee Found:")
 print(f" ID : {rec['empid']}")
 print(f" Name : {rec['name']}")
 print(f" Department : {rec['dept']}")
 print(f" Salary : {rec['salary']:.2f}")
 found = True
 break
 except EOFError:
 pass

 if not found:
 print(f"Employee ID {empid} not found.\n")

def update_employee:
 """Update salary of an employee."""
 empid = int(input("Enter Employee ID to update: "))
 found = False
 records = []

 with open(FILENAME, "rb") as f:
 try:
 while True:
 rec = pickle.load(f)
 if rec["empid"] == empid:
 print(f"Current salary: {rec['salary']:.2f}")
 rec["salary"] = float(input("Enter new salary: "))
 found = True
 records.append(rec)
 except EOFError:
 pass

 if found:
 with open(FILENAME, "wb") as f:
 for rec in records:
 pickle.dump(rec, f)
 print("Salary updated successfully!\n")
 else:
 print(f"Employee ID {empid} not found.\n")

def delete_employee:
 """Delete an employee record."""
 empid = int(input("Enter Employee ID to delete: "))
 found = False
 records = []

 with open(FILENAME, "rb") as f:
 try:
 while True:
 rec = pickle.load(f)
 if rec["empid"] == empid:
 print(f"Deleting: {rec['name']} (ID: {rec['empid']})")
 found = True
 else:
 records.append(rec)
 except EOFError:
 pass

 if found:
 with open(FILENAME, "wb") as f:
 for rec in records:
 pickle.dump(rec, f)
 print("Record deleted successfully!\n")
 else:
 print(f"Employee ID {empid} not found.\n")

# Main menu
def main:
 while True:
 print("=" * 35)
 print(" EMPLOYEE MANAGEMENT SYSTEM")
 print("=" * 35)
 print(" 1. Add Employee")
 print(" 2. Display All Employees")
 print(" 3. Search Employee")
 print(" 4. Update Salary")
 print(" 5. Delete Employee")
 print(" 6. Exit")
 print("=" * 35)

 choice = input("Enter your choice (1-6): ")

 if choice == "1":
 add_employee
 elif choice == "2":
 display_all
 elif choice == "3":
 search_employee
 elif choice == "4":
 update_employee
 elif choice == "5":
 delete_employee
 elif choice == "6":
 print("Thank you! Goodbye.")
 break
 else:
 print("Invalid choice. Try again.\n")

main

# Sample Interaction:
# ===================================
# EMPLOYEE MANAGEMENT SYSTEM
# ===================================
# 1. Add Employee
# 2. Display All Employees
# 3. Search Employee
# 4. Update Salary
# 5. Delete Employee
# 6. Exit
# ===================================
# Enter your choice (1-6): 1
# Enter Employee ID: 1001
# Enter Name: Anita
# Enter Department: IT
# Enter Salary: 55000
# Employee added successfully!

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Common Mistakes

Mistake	Correct Approach
Opening binary file without "b" in mode	Always use "rb", "wb", "ab" for pickle
Not handling EOFError when reading	Always wrap pickle.load in try-except EOFError
Using read / write with binary files	Use pickle.load and pickle.dump
Forgetting import pickle	Must import before using dump or load
Appending with "wb" instead of "ab"	"wb" overwrites the file; use "ab" to append
Trying to update/delete in-place	Read all records, modify in memory, write all back

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$1$2 Quick Tips

1. Always import pickle at the top, forgetting this is a common mark-losing error.
2. EOFError handling is the most tested concept. The try-while-True-except EOFError pattern appears in almost every binary file question.
3. dump writes, load reads - do not confuse with text file methods.
4. Update and Delete follow the same pattern: read all into a list, modify the list, write all back.
5. Know the difference between "wb" (overwrites) and "ab" (appends).
6. Pickling vs Unpickling definition is a 1-2 mark theory question.
7. Records are usually stored as dictionaries - practice the {"key": value} pattern.
8. Binary files cannot be opened in a text editor, this is a common True/False question.