Text File Read and Write Programs in Python (Class 12 Computer Science)

Understand Text File Read and Write Programs

Text File Read and Write Programs: key facts

Text file modes

• •r: read only; file must exist; pointer at start
• •r+: read and write; file must exist; pointer at start
• •w: write; creates or erases the file
• •w+: write and read; creates or erases
• •a: append; creates if missing; pointer at end
• •a+: append and read; pointer at end

NCERT Table 2.1; 'r' is the default mode.

Reading methods

read()	rest of the file as one string
read(n)	next n characters
readline()	next line with its newline, '' at the end
readlines()	list of the remaining lines
for line in f	one line at a time

Each read starts where the last one stopped.

Writing methods

write(s)	writes s and returns its length; no newline added
writelines(list)	writes each string; no newline added
str(n)	convert numbers before writing

Add '\n' yourself to start a new line.

Pointer methods

tell()	current position
seek(n) or seek(n, 0)	move to position n from the start
seek(0)	back to the beginning

In text mode, seek relative to the current position or the end works only with offset 0.

Counting patterns

lines	len(f.readlines())
words	len(f.read().split())
characters	len(f.read())
a word	sum of w.lower() == 'the' over every split word

read() once per pattern, or seek(0) before reading again.

How to answer Text File Read and Write Programs questions

1. 1Write the function header exactly as the question names it.
2. 2Open the file in the right mode: 'r' to read, 'w' to create or overwrite, 'a' to add at the end (use with, or remember close()).
3. 3Choose the reading style: for line in f for line-based tasks, read() for character counts, read().split() for word tasks.
4. 4Inside the loop apply the condition (first letter, contains a digit, word equals ...), using strip() or lower() as needed.
5. 5Keep a counter or print the matching line; print the result after the loop.
6. 6Close the file (not needed inside with) and call the function if the question asks for it.
7. 7For output questions, write the file contents with a visible \n at each line end and track the pointer through every read.

Text File Read and Write Programs in real life

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

Tricks to remember Text File Read and Write Programs

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

• read, readline, readlines = all, one, list

  read gives all as a string, readline gives one line, readlines gives a list.

  Use it when: MCQs on return types.

• w wipes, a adds, r refuses

  w erases old content, a adds to the end, r refuses to work if the file is missing.

  Use it when: Choosing a mode.

• write never presses Enter

  write() and writelines() never add a newline; you must type '\n' yourself.

  Use it when: Output questions after writelines().

• The bookmark moves

  The file pointer is a bookmark: every read moves it forward, and seek(0) puts it back at page one.

  Use it when: Questions with two reads in a row.

Text File Read and Write Programs: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

f = open('poem.txt')
print(f.read(4))
print(f.readline(), end='')
print(f.readlines())
f.close()

f = open('t.txt', 'w')
n = f.write('ab\n')
f.writelines(['cd', 'ef\n'])
f.close()
f = open('t.txt', 'a')
f.write('gh')
f.close()
f = open('t.txt')
print(n, f.readlines())
f.close()

with open('poem.txt') as f:
    a = f.read(5)
    p = f.tell()
    f.seek(11)
    b = f.read(6)
    f.seek(0)
    c = f.read(4)
print(a, p, b, c, sep='|')

Where students lose marks in Text File Read and Write Programs

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

• ✗Mistake: Expecting writelines() to put each item on a new line

  Neither write() nor writelines() adds a newline.

  Wrong working

  f.writelines(['cd', 'ef']) → two lines

  Right working

  f.writelines(['cd\n', 'ef\n']) → two lines

  ✓Do this: Add '\n' to each string you want on its own line.

  Usually costs: 1 mark in output questions.

• ✗Mistake: Opening in 'w' when the question says 'add' or 'append'

  'w' erases the existing contents.

  Wrong working

  f = open('late.txt', 'w') # to add a name

  Right working

  f = open('late.txt', 'a')

  ✓Do this: Match the mode to the verb: read → r, add → a, create/overwrite → w.

  Usually costs: 1 mark.

• ✗Mistake: Printing lines with double spacing

  Each line already ends in '\n' and print() adds another.

  Wrong working

  print(line)

  Right working

  print(line.strip()) # or print(line, end='')

  ✓Do this: Strip the newline before printing.

  Usually costs: Half a mark.

• ✗Mistake: Reading the file twice without seek(0)

  After read() the pointer is at the end, so a second read() returns ''.

  Wrong working

  c = len(f.read())
  w = len(f.read().split()) # 0

  Right working

  t = f.read()
  c = len(t)
  w = len(t.split())

  ✓Do this: Read once into a variable, or call f.seek(0) before reading again.

  Usually costs: 1 mark.

Practice questions on Text File Read and Write Programs

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 text file to add data at the end without deleting what is already there?
   (a) 'r' (b) 'w' (c) 'a' (d) 'r+'

   Hint

   w wipes.

   Answer

   (c) 'a'

2. Q2 · 1 mark · Very short answerBoard level

   What type of value does readlines() return?

   Hint

   All, one, list.

   Answer

   A list of strings, one per line (each including its newline).

3. Q3 · 1 mark · Very short answerBoard level

   What happens to the contents of an existing file when it is opened in 'w' mode?

   Hint

   w wipes.

   Answer

   They are erased (the file is truncated to zero length) before writing.

4. Q4 · 2 marks · Short answerHarder board level

   poem.txt contains 'Rain falls softly' and 'on the old roof'. Write the output:

   f = open('poem.txt')
   x = f.readline()
   y = f.read(6)
   print(len(x), y)
   f.close()

   Hint

   The first line includes its newline.

   Answer

   18 on the

5. Q5 · 3 marks · Short answerHarder board level

   Write a function count_vowels() that reads poem.txt and returns the number of vowels in it. What does it return for the two-line poem above?

   Hint

   Loop over every character of read().

   Answer

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

   Output: 9

6. Q6 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): Opening a file that does not exist in 'r+' mode raises an error.
   Reason (R): 'r+' opens an existing file for both reading and writing and does not create a new file.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   Which modes create files?

   Answer

   (a) Both are true and R explains A (FileNotFoundError).

7. Q7 · 2 marks · Short answerHOTS / competency

   Predict the output:

   with open('n.txt', 'w') as f:
       for i in range(1, 4):
           f.write(str(i * i))
   with open('n.txt') as f:
       s = f.read()
   print(s, len(s), s[1:])

   Hint

   No newlines and no spaces are written.

   Answer

   149 3 49

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.What does readline() return at the end of the file?Show answer

   An empty string ''.

2. 2.Which mode is used if no mode is given?Show answer

   'r' (read).

3. 3.What does f.write('abc\n') return?Show answer

   4, the number of characters written.

4. 4.What does seek(0) do?Show answer

   Moves the file pointer back to the beginning.

5. 5.Which mode creates a file if it does not exist and keeps old data?Show answer

   'a' (or 'a+').

6. 6.How do you count the words in a text file?Show answer

   len(f.read().split()).

Text File Read and Write Programs: frequently asked questions