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ch-1-5
UNIT 1 ▪ CHAPTER 5
05
Text Files
write · writelines · read · readline · readlines · seek · tell · Worked Programs
A text file stores ordinary characters — letters, digits, punctuation, whitespace — that any editor (Notepad, VS Code) can display. In Python, every read from a text file gives you a string (
str); every write sends a string back to disk.
Real-life analogy. A text file is like a printed book. You can open it at page 1 and read line by line, you can flip to any page you like, or you can turn to the last page and add a new paragraph. Python’s
read, readline, readlines, seek and write are just the tools for doing exactly those actions.
5.1 Text Files — A Quick Recap
From Chapter 4 you already know:
- Text mode is the default of
open();"r"="rt","w"="wt","a"="at". - Always prefer
with open(…) as f:so the file closes automatically. - A text file ends each line with the newline character
\n.
5.1.1 Text-file modes — one-glance table
| Mode | File must exist? | Position | Read? | Write? | Truncates? |
|---|---|---|---|---|---|
"r" | Yes | Start | ✅ | ❌ | — |
"w" | No (creates) | Start | ❌ | ✅ | ⚠ Yes |
"a" | No (creates) | End | ❌ | ✅ | No |
"x" | Must not exist | Start | ❌ | ✅ | — |
"r+" | Yes | Start | ✅ | ✅ | No |
"w+" | No (creates) | Start | ✅ | ✅ | ⚠ Yes |
"a+" | No (creates) | End | ✅ | ✅ | No |
5.2 Writing to a Text File
5.2.1 write(s) — write one string
write() sends the string to the file and returns the number of characters written. It does not add a newline automatically — you must include \n yourself when you want one.
with open("marks.txt", "w") as f:
n = f.write("Asha 92\n") # writes 8 characters, returns 8
f.write("Rahul 78\n")
f.write("Riya 85\n")
print("First line wrote", n, "characters.")
Mnemonic — “write() writes exactly what you give it.” No extra newline, no extra space between arguments, no magic formatting.
5.2.2 writelines(list_of_strings) — write many strings at once
writelines() accepts any iterable of strings and writes each one in order. It, too, does not add newlines.
lines = ["Asha 92\n", "Rahul 78\n", "Riya 85\n"]
with open("marks.txt", "w") as f:
f.writelines(lines)
Despite its name,
writelines() does not write “one line per item.” If your strings don’t end with \n, everything merges onto one line.
5.2.3 Writing numbers — you must convert to str first
Text files store only characters. If you pass a plain int or float, Python raises TypeError. Use str() or an f-string.
marks = 85
with open("marks.txt", "w") as f:
# f.write(marks) # ❌ TypeError: write() requires str
f.write(str(marks) + "\n") # ✅ "85\n"
f.write(f"Total = {marks}\n") # ✅ f-string, most readable
5.2.4 "w" versus "a" — in action
with open("log.txt", "w") as f: # wipes old file
f.write("Line 1\n")
with open("log.txt", "a") as f: # keeps Line 1, adds more
f.write("Line 2\n")
f.write("Line 3\n")
with open("log.txt", "r") as f:
print(f.read())
Line 1
Line 2
Line 3
5.3 Reading from a Text File
Python offers four ways to read a text file. Each gives you the data in a different shape.
Imagine the file contains exactly:
Asha 92
Rahul 78
Riya 85
The four read methods below operate on this same file. Notice the difference in the shape of the return value.
5.3.1 read() — entire file as one string
with open("marks.txt", "r") as f:
content = f.read()
print(type(content), len(content))
print(content)
<class 'str'> 24
Asha 92
Rahul 78
Riya 85
Internally the newline characters are still there — they just don’t look special when printed.
5.3.2 read(n) — read at most n characters
Handy when the file is huge and you don’t want to load it all at once.
with open("marks.txt", "r") as f:
first8 = f.read(8) # first 8 characters
next10 = f.read(10) # next 10 characters
print(repr(first8))
print(repr(next10))
'Asha 92\nR'
'ahul 78\nRi'
5.3.3 readline() — one line at a time
Returns the next line including the trailing \n. Returns an empty string "" when there is nothing more to read.
with open("marks.txt", "r") as f:
print(repr(f.readline())) # 'Asha 92\n'
print(repr(f.readline())) # 'Rahul 78\n'
print(repr(f.readline())) # 'Riya 85\n'
print(repr(f.readline())) # '' (EOF)
Use
line.rstrip("\n") or line.strip() when you want the line without the trailing newline.
5.3.4 readlines() — every line as a list
with open("marks.txt", "r") as f:
rows = f.readlines()
print(rows)
['Asha 92\n', 'Rahul 78\n', 'Riya 85\n']
Each string keeps its trailing \n. readlines() loads the whole file into memory — avoid it on very large files.
5.3.5 Iterating over the file object directly (most Pythonic)
A file object is its own iterator — looping over it gives one line at a time without loading the whole file.
with open("marks.txt", "r") as f:
for line in f: # one line per iteration
print(line.rstrip()) # strip the trailing newline
Asha 92
Rahul 78
Riya 85
This is the preferred way to process a file line by line: short, memory-efficient, works on files of any size.
5.3.6 Side-by-side comparison
| Method | Returns | Memory | Best for |
|---|---|---|---|
read() | str — whole file | O(file size) | Small files |
read(n) | str — up to n chars | O(n) | Chunked reading |
readline() | str — one line | O(line) | Manual line-by-line |
readlines() | list[str] | O(file size) | When you need random access to lines |
for line in f | Each line in turn | O(line) | Processing any file — recommended |
5.4 The File Pointer — tell() and seek()
Every open file has an invisible file pointer — a marker that remembers where the next read or write will take place. It starts at byte 0 (or at the end for append mode) and moves forward automatically as you read or write.
5.4.1 f.tell() — where is the pointer now?
with open("marks.txt", "r") as f:
print(f.tell()) # 0 — at the start
f.read(4) # read 'Asha'
print(f.tell()) # 4 — moved 4 bytes forward
f.readline() # read to end of line
print(f.tell()) # 8 — after '\n'
5.4.2 f.seek(offset, whence=0) — jump to a new position
seek moves the pointer so the next read starts from a chosen position.
whence | Meaning | Where offset is measured from |
|---|---|---|
0 (default) | os.SEEK_SET | Start of the file |
1 | os.SEEK_CUR | Current position |
2 | os.SEEK_END | End of the file |
In text mode (the default), only
seek(0) or a position previously returned by tell() is guaranteed to work. Use whence=1 / whence=2 in binary mode (covered in Chapter 6).
with open("marks.txt", "r") as f:
f.read(4) # pointer now at 4
f.seek(0) # back to the beginning
print(f.readline()) # 'Asha 92'
5.5 Line Endings — the \n Character
Python uses \n (LF) as its single “newline” character in text mode. If you’re on Windows, the operating system actually stores \r\n (CR + LF) on disk — but Python transparently translates between the two for you. You don’t have to worry about it if you keep the file open in text mode.
Problems with extra blank lines (
\r\r\n) usually mean the file was opened in binary mode when it should have been text, or vice-versa. Stick to text mode for text files and this cannot happen.
5.6 Appending to a Text File
Append mode ("a") is perfect for logs, chat histories, and any file that grows over time.
import datetime
with open("visitor_log.txt", "a") as f:
f.write(f"{datetime.datetime.now()} visit\n")
Run the program three times and open the log — three time-stamped lines, not one.
5.7 CBSE-style Worked Programs
5.7.1 Write the user’s name and marks to a file
with open("student.txt", "w") as f:
f.write(input("Name : ") + "\n")
f.write(input("Marks: ") + "\n")
print("Saved to student.txt")
5.7.2 Count the total number of lines, words and characters in a file
lines = words = chars = 0
with open("story.txt", "r") as f:
for line in f:
lines += 1
words += len(line.split())
chars += len(line)
print("Lines :", lines)
print("Words :", words)
print("Characters:", chars)
5.7.3 Count vowels in a file
vowels = "aeiouAEIOU"
count = 0
with open("story.txt", "r") as f:
for ch in f.read():
if ch in vowels:
count += 1
print("Vowels:", count)
5.7.4 Count lines starting with a particular letter
letter = "T"
n = 0
with open("story.txt", "r") as f:
for line in f:
if line.startswith(letter):
n += 1
print(f"Lines beginning with '{letter}': {n}")
5.7.5 Copy one text file into another
with open("story.txt", "r") as src, open("copy.txt", "w") as dst:
dst.write(src.read())
print("Copied.")
5.7.6 Display only the lines containing a keyword
keyword = input("Search word: ").lower()
with open("story.txt", "r") as f:
for line in f:
if keyword in line.lower():
print(line.rstrip())
5.7.7 Word-frequency counter
freq = {}
with open("story.txt", "r") as f:
for line in f:
for w in line.lower().split():
w = w.strip(".,!?;:'\"()")
if w:
freq[w] = freq.get(w, 0) + 1
# top 5 most frequent words
top = sorted(freq.items(), key=lambda x: x[1], reverse=True)[:5]
for word, count in top:
print(f"{word:<15} {count}")
5.7.8 Replace every occurrence of one word with another
old, new = "Delhi", "Mumbai"
with open("article.txt", "r") as f:
text = f.read()
text = text.replace(old, new)
with open("article.txt", "w") as f:
f.write(text)
print(f"Replaced '{old}' with '{new}'.")
Pattern — read-modify-write. Text files cannot be edited in place. Read the whole file, transform the string, then write it back in
"w" mode. For huge files, write to a new file and rename afterwards.
5.7.9 Count upper-case, lower-case, digits & spaces
u = l = d = sp = other = 0
with open("story.txt", "r") as f:
for ch in f.read():
if ch.isupper(): u += 1
elif ch.islower(): l += 1
elif ch.isdigit(): d += 1
elif ch.isspace(): sp += 1
else: other += 1
print(f"U={u} L={l} D={d} spaces={sp} other={other}")
5.7.10 Longest line in the file
longest = ""
with open("story.txt", "r") as f:
for line in f:
line = line.rstrip("\n")
if len(line) > len(longest):
longest = line
print("Longest line:", longest)
print("Length :", len(longest))
5.8 Bringing it all together — safe file handling
Real exam programs should combine everything you have learned: with, appropriate mode, try/except, and clear output.
def show_file(path):
try:
with open(path, "r", encoding="utf-8") as f:
for i, line in enumerate(f, 1):
print(f"{i:3}: {line.rstrip()}")
except FileNotFoundError:
print("File", path, "does not exist.")
except PermissionError:
print("Not allowed to read", path)
show_file("marks.txt")
5.9 Common Mistakes to Avoid
| # | Mistake | Fix |
|---|---|---|
| 1 | Forgetting \n with write() | Always end lines with "\n" |
| 2 | Passing an int or float to write() | Convert first — f.write(str(n)) |
| 3 | Expecting writelines() to add newlines | Include \n in each string yourself |
| 4 | Using "w" to add data — old content wiped | Use "a" (append) |
| 5 | Reading a file twice without seek(0) | Second read() returns "" — pointer is at the end |
| 6 | Loading a huge file with readlines() | Use for line in f: instead |
| 7 | Leaving \n on each line from readline() | Use line.rstrip() before processing / printing |
| 8 | Opening without with and forgetting close() | Use with open(…) as f: |
Quick-revision summary
- Three writing methods:
write(s),writelines(list). Neither adds newlines — include\nyourself. - Five reading strategies:
read(),read(n),readline(),readlines(), and the preferredfor line in f. - Each
read()/write()advances the file pointer. Usetell()to see it,seek()to move it. - Text mode returns
str; you must convert numbers withstr()before writing. - Append (
"a") keeps existing data; write ("w") wipes it; read ("r") only reads. - Read-modify-write pattern: text files cannot be edited in place.
- Wrap every file operation in
withand handleFileNotFoundError.