Ultimate Last-Minute Revision Notes for Unit I: File Handling (Computer Science Class 12)

 Here are the Ultimate Last-Minute Revision Notes for Unit I: File Handling (Computer Science Class 12). This unit typically holds 10-15 Marks and is critical for the practical exam as well.

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1. Concept: Types of Files

Feature	Text File (.txt, .py, .csv)	Binary File (.dat, .jpg, .mp3)
Storage	Stores information in ASCII or Unicode characters.	Stores information in raw bytes (same as memory).
Readability	Human-readable.	Not human-readable (looks like garbage text).
Translation	EOL Translation: Converts \n to OS-specific newline.	No Translation: Faster and more efficient.

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2. File Modes (The open() function)

Syntax: file_obj = open("filename", "mode")

Cheat Sheet for Modes:

• r: Read only. Default mode. File must exist (else Error). Pointer at Beginning.

• w: Write only. Creates file if not exists. Overwrites (truncates) if exists. Pointer at Beginning.

• a: Append. Creates file if not exists. Adds to end. Pointer at End.

• r+: Read & Write. File must exist. Pointer at Beginning.

• w+: Write & Read. Overwrites file. Pointer at Beginning.

• a+: Append & Read. Pointer at End.

Note: For Binary files, add b to the mode (e.g., rb, wb, ab+).

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3. Text File Methods

A. Reading

1. read(n): Reads n bytes. If n is empty, reads the whole file. Returns a String.

2. readline(): Reads a single line (up to \n). Returns a String.

3. readlines(): Reads all lines. Returns a List of Strings.

B. Writing

1. write(str): Writes a string to the file.

2. writelines(list): Writes a list of strings. Note: Does not add \n automatically.

The with Statement (Best Practice)

Automatically closes the file, even if an error occurs.

with open("data.txt", "r") as f:
    print(f.read())
# No f.close() needed here

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4. Binary Files (pickle Module)

Used for Serialization (converting Python objects like Lists/Dictionaries to byte stream).

Import: import pickle

A. Writing (dump)

Python

import pickle
data = [10, "Ali", 95.5]
with open("student.dat", "wb") as f:
    pickle.dump(data, f)  # Syntax: dump(object, file_handle)

B. Reading (load)

Crucial: You must use a try-except block to catch EOFError (End of File Error) because binary files don't have a specific EOF marker like text files.

Python

import pickle
with open("student.dat", "rb") as f:
    try:
        while True:
            rec = pickle.load(f)  # Syntax: load(file_handle)
            print(rec)
    except EOFError:
        pass 

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5. CSV Files (csv Module)

Import: import csv

A. Writing

Crucial: Use newline='' in open() to prevent empty lines between rows in Windows.

Python

import csv
with open("data.csv", "w", newline='') as f:
    wo = csv.writer(f)
    wo.writerow(["Roll", "Name"])       # Write Header
    wo.writerow([1, "Amit"])            # Write Single Row
    wo.writerows([[2, "Bob"], [3, "C"]]) # Write Multiple Rows

B. Reading

Python

import csv
with open("data.csv", "r") as f:
    ro = csv.reader(f)
    for row in ro:
        print(row)  # 'row' is a List of Strings e.g., ['1', 'Amit']

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6. File Pointers (Random Access)

1. tell(): Returns current cursor position (Integer).

2. seek(offset, whence): Moves cursor.

   • Offset: Number of bytes to move.

   • Whence (Reference point):

     • 0: Beginning (Default).

     • 1: Current Position (Binary rb/wb only).

     • 2: End of File (Binary rb/wb only).

Example: f.seek(-10, 2) moves 10 bytes backwards from the end of a binary file.

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7. "Golden Code" (Memorize these Patterns)

Scenario A: Count words starting with 'T' in a Text File.

Python

def count_T():
    c = 0
    f = open("story.txt", "r")
    data = f.read()      # Read whole file
    words = data.split() # Split into list of words
    for w in words:
        if w[0].upper() == 'T':
            c += 1
    print(c)
    f.close()

Scenario B: Search for a record in Binary File.

Python

import pickle
def search(roll_no):
    f = open("student.dat", "rb")
    found = False
    try:
        while True:
            s = pickle.load(f) # s is like {'roll':1, 'name':'A'}
            if s['roll'] == roll_no:
                print("Found:", s)
                found = True
    except EOFError:
        if not found: print("Not Found")
    f.close()

Scenario C: Update a record in Binary File. Logic: Read all records into a list → Modify the specific record in the list → Truncate file (mode 'wb') and dump the whole list back.

Python

import pickle
def update_marks(roll, new_marks):
    # 1. Read all data
    f = open("student.dat", "rb")
    all_data = []
    try:
        while True:
            all_data.append(pickle.load(f))
    except EOFError:
        f.close()

    # 2. Modify data in memory
    for s in all_data:
        if s['roll'] == roll:
            s['marks'] = new_marks
    
    # 3. Write back to file
    f = open("student.dat", "wb")
    for s in all_data:
        pickle.dump(s, f)
    f.close()