File Handling in Python: CBSE Class 12 Computer Science Notes and Programs

What File Handling in Python is about

Python is easy to learn.
Files store data permanently.
A text file stores characters.
Every line ends with a newline.

Weightage: This chapter is part of Unit 1, Computational Thinking and Programming - 2, which carries 40 of the 70 theory marks together with Revision of Python Topics from Class XI, Functions, Exception Handling in Python and Stack.Source: CBSE Computer Science (083) syllabus 2026-27, distribution of marks, Unit 1 Computational Thinking and Programming - 2

Key concepts

Files, file types and paths

NCERT section NCERT 2.1, 2.2

Variables lose their values when a program ends; a file on secondary storage keeps data permanently. A text file stores characters (readable in Notepad), with each line ending in the newline character \n; .txt and .py files are text files. A binary file stores data as raw bytes in the form the program uses (images, .dat files made with pickle) and is not human-readable. A CSV file (comma-separated values) is a text file in which each line is a record and commas separate the fields, so spreadsheets can open it. A path locates a file: an absolute path starts from the root of the drive (C:\School\CS\notes.txt), while a relative path starts from the current working folder (notes.txt, data\notes.txt, ..\notes.txt where .. is the parent folder).

Worked example

The program runs in the folder C:\School\CS. Give the absolute path of marks.csv stored in C:\School\CS\data, and a relative path to it. Which file type suits (a) a class register opened in a spreadsheet (b) a list of Python records saved as they are?

Try it first, then show the solutionSolution

1. Absolute: from the drive root, C:\School\CS\data\marks.csv.
2. Relative: from the current folder C:\School\CS, it is data\marks.csv.
3. (a) A spreadsheet reads comma-separated text: a CSV file.
4. (b) Python objects saved as they are with pickle: a binary file.

Answer: C:\School\CS\data\marks.csv and data\marks.csv; (a) CSV file (b) binary file.

Opening, closing and file modes

NCERT section NCERT 2.3.1, 2.3.2

f = open(file_name, mode) returns a file object (file handle); the default mode is 'r'. f.close() saves any buffered data and frees the file. The modes:

Mode	Purpose	If the file does not exist	If it exists	Starts at
r	read only	FileNotFoundError	kept	beginning
r+	read and write	FileNotFoundError	kept (writing overwrites in place)	beginning
w	write only	created	emptied (truncated)	beginning
w+	write and read	created	emptied	beginning
a	append only	created	kept	end
a+	append and read	created	kept	end

Add b for binary files: rb, rb+, wb, wb+, ab, ab+. In a and a+ every write goes to the end of the file.

Worked example

notes.txt exists. Which modes (i) destroy its contents on opening, (ii) raise an error if the file is missing, (iii) let you read and also add text at the end?

Try it first, then show the solutionSolution

1. w and w+ truncate an existing file (wb and wb+ for binary files).
2. r and r+ need the file to exist; otherwise FileNotFoundError.
3. a+ opens for reading and appending (writes go to the end).

Answer: (i) w, w+ (ii) r, r+ (iii) a+

The with clause

NCERT section NCERT 2.3.3

with open('notes.txt', 'r') as f: opens the file and closes it automatically when the indented block ends, even if an exception occurs inside it. So you do not write close(). After the block, f still exists as a name but the file is closed, and reading from it raises ValueError (I/O operation on closed file).

Worked example

Rewrite with the with clause:

f = open('log.txt', 'a')
f.write('Backup done\n')
f.close()

Try it first, then show the solutionSolution

1. The with statement replaces both open-and-assign and close().

2. with open('log.txt', 'a') as f:
       f.write('Backup done\n')

Answer: The two-line with version; the file is closed automatically at the end of the block.

Writing: write() and writelines()

NCERT section NCERT 2.4

f.write(s) writes the string s and returns the number of characters written. It does not add a newline: put \n yourself. It accepts only a string, so numbers need str(). f.writelines(seq) writes every string in a list or tuple one after the other, also without adding newlines, and returns None.

Worked example

What does this print, and what does notes.txt contain afterwards?

f = open('notes.txt', 'w')
n = f.write('Python is easy to learn.\nFiles store data permanently.\n')
f.writelines(['A text file stores characters.\n', 'Every line ends with a newline.\n'])
f.close()
print(n)

Try it first, then show the solutionSolution

1. 'w' empties the file first.
2. The first string has 24 + 1 characters in line 1 and 29 + 1 in line 2 (each \n counts as one character): 25 + 30 = 55, which write() returns.
3. writelines() adds the two strings exactly as given.

Answer: It prints 55; notes.txt then holds the four lines shown in the overview.

Reading: read(), readline() and readlines()

NCERT section NCERT 2.5

f.read() returns the whole remaining file as one string; f.read(n) returns at most n characters. f.readline() returns one line including its \n (or the rest of the line if part was already read); f.readline(n) returns at most n characters of the line. f.readlines() returns a list of all remaining lines, each with its \n. At the end of the file, read() and readline() return '' and readlines() returns []. Each read moves the file pointer forward, so the next read continues where the last one stopped. for line in f: reads one line per pass and is the usual way to process a file line by line.

Worked example

Predict the output (notes.txt as in the overview):

with open('notes.txt') as f:
    print(f.read(6))
    print(f.readline(), end='')
    print(f.readline(5))
    rest = f.readlines()
    print(len(rest))

Try it first, then show the solutionSolution

1. read(6) returns 'Python'.
2. readline() returns the rest of line 1: ' is easy to learn.\n'; end='' avoids a second newline.
3. readline(5) returns the first 5 characters of line 2: 'Files'.
4. readlines() returns the remainder of line 2 plus lines 3 and 4: three items.

Answer: Python
is easy to learn.
Files
3

seek() and tell()

NCERT section NCERT 2.6

The file pointer marks where the next read or write happens. f.tell() returns its position as a number of bytes from the start. f.seek(offset, reference_point) moves it: reference point 0 = from the beginning (the default), 1 = from the current position, 2 = from the end. In text mode only seek(n) / seek(n, 0) and seek(0, 2) are allowed; non-zero offsets from 1 or 2 need binary mode. Note: tell() counts bytes, and on Windows a text file stores each newline as two bytes, so tell() values there can be larger than a character count.

Worked example

Predict the output:

with open('notes.txt') as f:
    f.seek(10)
    print(f.read(4), f.tell())
    f.seek(0)
    print(f.readline().strip())

Try it first, then show the solutionSolution

1. Line 1 is 'Python is easy to learn.': index 10 is the 'e' of 'easy' (P0 y1 t2 h3 o4 n5 space6 i7 s8 space9 e10).
2. read(4) gives 'easy' and the pointer moves to 14.
3. seek(0) goes back to the start; readline().strip() gives line 1 without its newline.

Answer: easy 14
Python is easy to learn.

Manipulating data in a text file

NCERT section NCERT 2.7

Board functions on text files follow one pattern: open in 'r' mode, read with read(), readlines() or a for loop, test each word, line or character, and count or display. Use read().split() for words, for line in f or readlines() for lines, and a loop over read() for characters. To change a file, read it, build the new text, and write it to the same or another file (there is no in-place edit of a line in a text file).

Worked example

Write a function COUNT_AE() that counts the lines of notes.txt beginning with 'A' or 'E' (either case) and returns the count.

Try it first, then show the solutionSolution

1. Read line by line and test the first character.

2. def COUNT_AE():
       c = 0
       with open('notes.txt') as f:
           for line in f:
               if line[0] in 'AaEe':
                   c += 1
       return c
   print(COUNT_AE())

3. Lines 3 ('A text...') and 4 ('Every...') qualify.

Answer: 2

Binary files with pickle: dump() and load()

NCERT section NCERT 2.8

Pickling (serialisation) turns a Python object into a byte stream; unpickling turns it back. import pickle, then pickle.dump(obj, f) writes one object to a file opened in 'wb' or 'ab', and pickle.load(f) reads the next object from a file opened in 'rb'. Each dump adds one object; load() reads them in the same order. When no object is left, load() raises EOFError, so reading all records uses a loop inside try-except EOFError. You can also store a whole list of records with a single dump and get it back with a single load.

Worked example

Write display() to print every record of student.dat.

Try it first, then show the solutionSolution

1. Open in 'rb'; load in an endless loop; stop at EOFError.

2. import pickle
   def display():
       with open('student.dat', 'rb') as f:
           try:
               while True:
                   rec = pickle.load(f)
                   print(rec)
           except EOFError:
               pass
   display()

Answer: [1, 'Asha', 88]
[2, 'Ravi', 64]
[3, 'Meena', 79]

Binary files: search, append and update

NCERT section NCERT 2.8.3

Search: load every record and compare a field, keeping a flag to print 'not found' at the end. Append: open in 'ab' and dump the new record; existing records stay. Update: the safe method reads all records into a list, changes the matching one, and rewrites the file in 'wb'. A second method opens in 'rb+', notes pos = f.tell() before each load, and on a match does f.seek(pos) and dumps the changed record over the old one; this works only if the new record pickles to the same number of bytes.

Worked example

Write update(roll, new_marks) that changes the marks of the student with the given roll number in student.dat, using the read-all-and-rewrite method. Show the file after update(2, 70).

Try it first, then show the solutionSolution

1. import pickle
   def update(roll, new_marks):
       recs = []
       with open('student.dat', 'rb') as f:
           try:
               while True:
                   recs.append(pickle.load(f))
           except EOFError:
               pass
       for r in recs:
           if r[0] == roll:
               r[2] = new_marks
       with open('student.dat', 'wb') as f:
           for r in recs:
               pickle.dump(r, f)
   update(2, 70)
   with open('student.dat', 'rb') as f:
       for i in range(3):
           print(pickle.load(f))

2. All three records are read, Ravi's marks become 70, and all three are written back.

Answer: [1, 'Asha', 88]
[2, 'Ravi', 70]
[3, 'Meena', 79]

CSV files with the csv module

NCERT section Not in NCERT; CBSE syllabus (CSV file)

import csv. Open with newline='' when writing, so that no blank lines appear between rows on Windows. Writing: w = csv.writer(f) makes a writer object; w.writerow(list) writes one row; w.writerows(list_of_lists) writes many rows. A different separator can be chosen with csv.writer(f, delimiter='|'). Reading: r = csv.reader(f) gives an object that yields each row as a list of strings, so numbers must be converted with int() or float(). Skip the header row with next(r) (or by ignoring the first row in a loop).

Worked example

Write code that reads items.csv, skips the header, prints the names of items costing more than 30 and the total of all prices.

Try it first, then show the solutionSolution

1. Each row is a list of strings, so convert the price with int().

2. import csv
   with open('items.csv') as f:
       r = csv.reader(f)
       next(r)
       total = 0
       for row in r:
           if int(row[2]) > 30:
               print(row[1], end=' ')
           total += int(row[2])
   print(total)

3. Notebook (45) and Bag (650) cost more than 30; total = 20 + 45 + 650 = 715.

Answer: Notebook Bag 715

File Handling in Python in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Text File Read and Write Programs
• Binary Files with pickle
• CSV Files with the csv Module

File Handling in Python: syntax and key facts

Open and close

f = open('name', 'mode') ... f.close(), or with open('name', 'mode') as f: (closes automatically)

Default mode is 'r'. Binary modes add b: rb, wb, ab, rb+, wb+, ab+.

Text modes in one line

r / r+	must exist, pointer at start
w / w+	create or truncate
a / a+	create or keep, writes at the end

+ adds the other operation. Opening an existing file in w mode erases it.

Write methods

f.write(str) returns the number of characters; f.writelines(list_of_str) returns None; neither adds \n

Convert numbers with str() before writing to a text file.

Read methods

read() / read(n): string; readline() / readline(n): one line with \n; readlines(): list of lines

At end of file: '' for read and readline, [] for readlines.

Pointer methods

f.tell(): current byte position; f.seek(offset, ref): ref 0 start (default), 1 current, 2 end

Text mode allows seek(n) from the start and seek(0, 2); other relative seeks need binary mode.

pickle

pickle.dump(obj, f) in wb / ab; obj = pickle.load(f) in rb; loop until EOFError

One dump writes one object; one load reads one object.

csv

csv.writer(f), .writerow(row), .writerows(rows); csv.reader(f) yields each row as a list of strings

Use newline='' with open() when writing a CSV file.

How the board asks File Handling in Python

The question patterns to expect, and the method that works for each.

Tricks to remember File Handling in Python

Short memory aids. Say each one aloud once, then use it on a question the same day.

• W wipes, A adds, R requires

  w wipes an existing file, a adds at the end, r requires the file to exist.

  Use it when: Every modes MCQ.

• Plus means both

  r+, w+ and a+ all read and write; the letter before + decides creation, truncation and starting point.

  Use it when: Choosing between r+, w+ and a+.

• readline keeps the newline

  Every line from readline() or readlines() still ends in \n, so print it with end='' or strip() it.

  Use it when: Output questions that show blank lines.

• Dump to save, Load to bring back

  dump(object, file) has the object first, like putting something into a box; load(file) takes it out.

  Use it when: Remembering the argument order of pickle.dump().

• CSV reader gives strings

  Everything that comes out of csv.reader is a string, even 650.

  Use it when: Comparisons and totals in CSV programs.

• Seek 0-1-2 = Start-Current-End

  Reference points count like a race: 0 at the start line, 1 where you are now, 2 at the finish.

  Use it when: seek() questions.

File Handling in Python: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

f = open('notes.txt')
x = f.readline()
y = f.read(5)
z = f.readlines()
print(len(x), y, len(z))
print(z[0].split()[0])
f.close()

f = open('notes.txt', 'w+')
f.write('Hello')
f.seek(0)
print(f.read())
f.close()
f = open('notes.txt', 'r+')
f.write('J')
f.seek(0)
print(f.read())
f.close()

f = open('notes.txt')
print(f.read(6), f.tell())
f.seek(15)
print(f.read(5))
f.seek(0)
print(len(f.readline()), f.tell())
f.close()

Case-study practice (Section E style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 1 + 2 marks.

Asha,12A
Ravi,11B
Meena,12C

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in File Handling in Python

The slips examiners see most often, and what to write instead.

• ✗Mistake: Comparing a CSV field with a number without converting it

  reader() gives strings, and comparing a str with an int raises TypeError (or compares text if both are strings).

  Wrong working

  if row[2] > 30:

  Right working

  if int(row[2]) > 30:

  ✓Do this: Convert every numeric field with int() or float() before comparing or adding.

  Usually costs: 1 mark in a 4-mark CSV question

• ✗Mistake: Reading a binary file without handling EOFError

  The while True loop has no other way to stop; without try-except the program crashes at the end of the file.

  Wrong working

  while True:
  rec = pickle.load(f)
  print(rec)

  Right working

  try:
  while True:
  rec = pickle.load(f)
  print(rec)
  except EOFError:
  pass

  ✓Do this: Always wrap the load loop in try-except EOFError (or store all records as one list and load once).

  Usually costs: 1 mark

• ✗Mistake: Using 'w' or 'wb' to add a record to an existing file

  w truncates the file, so every old record is lost.

  ✓Do this: Use 'a' (text, CSV) or 'ab' (binary) for adding; use 'w' only to create or rewrite the whole file.

  Usually costs: 1 mark

• ✗Mistake: Counting words with split('\n') or lines with split()

  split() with no argument splits on all whitespace (words); lines need readlines() or a for loop.

  Wrong working

  words = f.read().split('\n')

  Right working

  words = f.read().split()

  ✓Do this: Words: read().split(). Lines: readlines() or for line in f. Characters: a loop over read().

  Usually costs: 1-2 marks

• ✗Mistake: Forgetting that readline() returns the newline

  A test such as line == 'END' fails because the line is 'END\n', and printing adds an extra blank line.

  ✓Do this: Use line.strip() (or rstrip('\n')) before comparing, and print(line, end='') when printing raw lines.

  Usually costs: Half to 1 mark

• ✗Mistake: Writing a number directly with write()

  write() accepts only strings; f.write(45) raises TypeError.

  Wrong working

  f.write(marks)

  Right working

  f.write(str(marks) + '\n')

  ✓Do this: Convert with str() and add the newline yourself.

  Usually costs: Half a mark

NCERT exercise guide: File Handling in Python

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on File Handling in Python

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which mode opens a binary file for both reading and writing without erasing it? (a) wb+ (b) rb+ (c) ab (d) wb

   Hint

   Truncation comes from w.

   Answer

   (b) rb+

2. Q2 · 1 mark · Very short answerBoard level

   What does f.writelines(['a', 'b']) write, and what does it return?

   Hint

   No newlines are added.

   Answer

   It writes 'ab' and returns None.

3. Q3 · 3 marks · Short answerBoard level

   Write a function COUNT_THE() that counts how many times the word 'the' (any case) occurs as a whole word in a text file story.txt.

   Hint

   Compare each word in lower case.

   Answer

   with open('story.txt', 'w') as f:
       f.write('The cat saw the dog.\nThen the end.\n')
   def COUNT_THE():
       c = 0
       with open('story.txt') as f:
           for w in f.read().split():
               if w.lower() == 'the':
                   c += 1
       return c
   print(COUNT_THE())

   Output for this sample: 3 ('Then' and 'dog.' do not match). The first two lines only create a sample file.

4. Q4 · 2 marks · Short answerHarder board level

   Predict the output (notes.txt as in the overview):

   with open('notes.txt') as f:
       L = f.readlines()
   print(len(L), L[1][:5], L[-1].split()[-1])

   Hint

   The last word keeps its full stop.

   Answer

   4 Files newline.

5. Q5 · 3 marks · Short answerHarder board level

   Write a function DISPLAY_UPPER() that displays the lines of notes.txt that contain more than four words, in upper case.

   Hint

   len(line.split()) > 4.

   Answer

   def DISPLAY_UPPER():
       with open('notes.txt') as f:
           for line in f:
               if len(line.split()) > 4:
                   print(line.upper(), end='')
   DISPLAY_UPPER()

   Output:
   PYTHON IS EASY TO LEARN.
   A TEXT FILE STORES CHARACTERS.
   EVERY LINE ENDS WITH A NEWLINE.

6. Q6 · 3 marks · Short answerHOTS / competency

   student.dat holds records [roll, name, marks]. Write COPY_TOPPERS() that copies records with marks 80 or more into toppers.dat and returns how many were copied.

   Hint

   Open one file in rb and the other in wb.

   Answer

   import pickle
   def COPY_TOPPERS():
       c = 0
       with open('student.dat', 'rb') as src, open('toppers.dat', 'wb') as dst:
           try:
               while True:
                   rec = pickle.load(src)
                   if rec[2] >= 80:
                       pickle.dump(rec, dst)
                       c += 1
           except EOFError:
               pass
       return c
   print(COPY_TOPPERS())

   Output: 1 (only Asha, 88)

7. Q7 · 2 marks · Short answerHarder board level

   Write SHOW_CHEAP() to display the code and name of every item in items.csv priced below 50, one per line.

   Hint

   Skip the header row.

   Answer

   import csv
   def SHOW_CHEAP():
       with open('items.csv') as f:
           r = csv.reader(f)
           next(r)
           for row in r:
               if int(row[2]) < 50:
                   print(row[0], row[1])
   SHOW_CHEAP()

   Output:
   101 Pen
   102 Notebook

8. Q8 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): Opening an existing file in 'a+' mode and immediately calling read() returns an empty string.
   Reason (R): In 'a+' mode the file pointer is placed at the end of the file.

   Hint

   Where does reading start in a+?

   Answer

   (a) Both A and R are true and R is the correct explanation of A. Call seek(0) before reading.

9. Q9 · 2 marks · Short answerHarder board level

   Write a function VOWEL_COUNT() that returns the number of vowels (any case) in notes.txt, character by character.

   Hint

   Loop over read().

   Answer

   def VOWEL_COUNT():
       c = 0
       with open('notes.txt') as f:
           for ch in f.read():
               if ch.lower() in 'aeiou':
                   c += 1
       return c
   print(VOWEL_COUNT())

   Output: 35

10. Q10 · 2 marks · Short answerHOTS / competency

    What is the difference between csv.writer(f).writerow(L) and csv.writer(f).writerows(L) when L = [[1, 'A'], [2, 'B']]?

    Hint

    writerow() treats each item of L as one field.

    Answer

    writerows(L) writes two rows: 1,A and 2,B. writerow(L) writes one row whose two fields are the strings of the inner lists: "[1, 'A']","[2, 'B']".

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which mode opens a text file for reading and writing without erasing it, and fails if it does not exist?Show answer

   r+

2. 2.What does write() return?Show answer

   The number of characters written.

3. 3.What does readlines() return at the end of a file?Show answer

   An empty list [].

4. 4.What is the default reference point of seek()?Show answer

   0, the beginning of the file.

5. 5.Which exception tells a pickle.load() loop that the file has ended?Show answer

   EOFError.

6. 6.In what form does csv.reader() return each row?Show answer

   As a list of strings.

7. 7.Why is newline='' used when opening a CSV file for writing?Show answer

   To stop blank lines appearing between rows (the csv module writes its own line endings).

8. 8.Give one difference between a relative and an absolute path.Show answer

   An absolute path starts from the drive root (C:\...); a relative path starts from the current working folder.

9. 9.Which method writes several rows to a CSV file at once?Show answer

   writerows() of a csv.writer object.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Text file: characters and lines; binary file: bytes (pickle); CSV: text with comma-separated fields.
• ☐Absolute path starts from the drive root; relative path from the current folder.
• ☐r/r+ need the file; w/w+ create or erase; a/a+ create or keep and write at the end.
• ☐with open(...) as f: closes the file automatically.
• ☐write() returns characters written; writelines() writes a list of strings; neither adds \n.
• ☐read() string; readline() one line with \n; readlines() list of lines.
• ☐tell() gives the pointer position; seek(n) moves it n bytes from the start.
• ☐pickle.dump(obj, f) in wb/ab; pickle.load(f) in rb; stop at EOFError.
• ☐Update a binary file: read all, change, rewrite in wb (or rb+ with tell and seek).
• ☐csv.writer(f).writerow(row)/writerows(rows); csv.reader(f) gives lists of strings; open with newline=''.

File Handling in Python: frequently asked questions