How to Replace a Specific Line in a File Using Python

To replace a specific line in a file using Python, read every line into a list with readlines(), assign new text to that line’s index, then write the list back with writelines(). Python can’t overwrite one line in place, so the file is always rewritten:

with open("stores.txt") as f:
    lines = f.readlines()
lines[2] = "Status: closed\n"   # line 3
with open("stores.txt", "w") as f:
    f.writelines(lines)

Every output below comes from a real run on Python 3.12.5 under Windows 11, using a small store record from Austin, Texas.

I’ll also cover matching a line by its text, huge files that shouldn’t sit in memory, the fileinput module, regex edits, and the newline mistakes that quietly break files.

Replace a specific line by number with readlines()

Here’s the sample file, stores.txt, with five lines. Line 3 holds the status, and since lists start at zero, that’s lines[2]:

Store: Austin, TX
Manager: Emily Carter
Status: open
Last audit: 2026-03-14
Monthly rent: $4,250.00

with open("stores.txt", encoding="utf-8") as f:
    lines = f.readlines()

print("Line 3 before:", repr(lines[2]))
lines[2] = "Status: closed\n"          # index 2 is line 3

with open("stores.txt", "w", encoding="utf-8") as f:
    f.writelines(lines)

with open("stores.txt", encoding="utf-8") as f:
    print(f.read())

Output:

Line 3 before: 'Status: open\n'
Store: Austin, TX
Manager: Emily Carter
Status: closed
Last audit: 2026-03-14
Monthly rent: $4,250.00

Notice the new text ends with \n. Each item from readlines() keeps its newline, so yours needs one too. If reading files is new to you, start with reading a file line by line in Python.

Replace a line that matches text in a file

Line numbers shift when someone edits the file by hand. Matching on a prefix is sturdier. This version uses pathlib and stops at the first hit:

from pathlib import Path

path = Path("stores.txt")
lines = path.read_text(encoding="utf-8").splitlines(keepends=True)

for i, line in enumerate(lines):
    if line.startswith("Manager:"):
        print(f"Found on line {i + 1}: {line.strip()}")
        lines[i] = "Manager: James Walker\n"
        break                          # first match only
else:
    print("No Manager line found")

path.write_text("".join(lines), encoding="utf-8")
print(path.read_text(encoding="utf-8"))

Output:

Found on line 2: Manager: Emily Carter
Store: Austin, TX
Manager: James Walker
Status: open
Last audit: 2026-03-14
Monthly rent: $4,250.00

Drop the break to replace every matching line. The for/else pattern prints a warning when nothing matches. For changing words inside a line instead, see replacing multiple characters in a string.

How do you replace a line in a large file without loading it all?

A 2 GB log won’t fit comfortably in a list. Stream it instead: copy each line to a temporary file, swap in the changed one, then replace the original with os.replace():

import os
import tempfile

src = "orders.csv"
with open(src, "w", encoding="utf-8") as f:            # sample: 100,000 order rows
    f.write("order_id,state,total_usd\n")
    for n in range(1, 100_001):
        f.write(f"{n},TX,{n % 500 + 0.99:.2f}\n")

target = 50_000                                         # line number to change
fd, tmp = tempfile.mkstemp(dir=".", suffix=".tmp")
with open(src, encoding="utf-8") as fin, os.fdopen(fd, "w", encoding="utf-8") as fout:
    for number, line in enumerate(fin, start=1):
        if number == target:
            print("Old:", line.strip())
            line = "49999,CA,499.99\n"
        fout.write(line)
os.replace(tmp, src)                                    # swap in the new file

with open(src, encoding="utf-8") as f:
    for number, line in enumerate(f, start=1):
        if number == target:
            print("New:", line.strip())
            break

Output:

Old: 49999,TX,499.99
New: 49999,CA,499.99

Memory use stays flat whatever the file size. Creating the temp file in the same folder matters, because os.replace() is only atomic on one drive. If you want a backup first, copying a file with shutil takes one line.

Edit a file in place with the fileinput module

fileinput with inplace=True does the temp-file work for you. While the loop runs, print() writes into the file, and backup keeps the old copy:

import fileinput

with fileinput.input("stores.txt", inplace=True, backup=".bak", encoding="utf-8") as f:
    for line in f:
        if f.filelineno() == 5:
            line = "Monthly rent: $4,400.00\n"
        print(line, end="")            # print() writes into the file here

with open("stores.txt", encoding="utf-8") as f:
    print(f.read())
with open("stores.txt.bak", encoding="utf-8") as f:
    print("Backup still has:", f.readlines()[4].strip())

Output:

Store: Austin, TX
Manager: Emily Carter
Status: open
Last audit: 2026-03-14
Monthly rent: $4,400.00

Backup still has: Monthly rent: $4,250.00

Forget end="" and you’ll get blank lines between every row, because each line already has a newline. I skip fileinput in code that logs to the console, since stray prints land in the file, and write the list to a file myself instead.

Update part of a line with re.subn()

Sometimes you only need to change a value inside a line. re.subn() with re.MULTILINE edits matching lines across the whole text and tells you how many changed:

import re
from pathlib import Path

path = Path("stores.txt")
text = path.read_text(encoding="utf-8")

new_text, count = re.subn(r"^(Last audit: )\d{4}-\d{2}-\d{2}$",
                          r"\g<1>2026-10-10", text, flags=re.MULTILINE)
path.write_text(new_text, encoding="utf-8")
print(f"Replaced {count} line(s)")
print(new_text)

Output:

Replaced 1 line(s)
Store: Austin, TX
Manager: Emily Carter
Status: open
Last audit: 2026-10-10
Monthly rent: $4,250.00

A count of zero means the pattern missed, which is a good thing to check before saving. Writing the result back works like any string written to a file in Python.

Avoid newline and line-number mistakes when replacing lines

Two bugs cause most broken files. A replacement without \n glues two lines together, and a line number past the end raises IndexError:

lines = ["Store: Austin, TX\n", "Status: open\n", "Manager: Emily Carter\n"]

bad = lines.copy()
bad[1] = "Status: closed"              # forgot the newline
good = lines.copy()
good[1] = "Status: closed\n"
print("Missing newline:", repr("".join(bad)))
print("With newline:   ", repr("".join(good)))

line_number = 7
if not 1 <= line_number <= len(lines):
    print(f"Line {line_number} doesn't exist; the file has {len(lines)} lines")

Output:

Missing newline: 'Store: Austin, TX\nStatus: closedManager: Emily Carter\n'
With newline:    'Store: Austin, TX\nStatus: closed\nManager: Emily Carter\n'
Line 7 doesn't exist; the file has 3 lines

Remember that people count lines from 1 and lists count from 0. If stray newlines are already in your data, removing newlines from a string cleans them up.

Line replacement methods compared

Method	Memory	Best for
readlines() + index	Whole file	Small files, known line number
Loop + startswith()	Whole file	Lines found by their text
Temp file + os.replace()	One line	Large files, safe writes
fileinput inplace	One line	Quick scripts with a backup
re.subn()	Whole file	Changing a value inside lines

If you’re working with files a lot, these pick up where this one stops:

• Read a text file with pandas
• Read a file into an array in Python
• Read a file as a string in Python
• Write a variable to a file in Python

Frequently asked questions