File Handling — Insertion, Deletion, Updating Data

Python has no direct method to insert/delete/modify a line in the middle of a text file — because files are stored as a continuous byte stream. The standard approach is to read all content, modify it in memory, and rewrite the file (usually via a temporary file).

Updating (Modifying) a Line

# Suppose marks.txt contains:
# Riya,85
# Amit,70
# Zoya,90

with open("marks.txt", "r") as f:
    lines = f.readlines()

# Update Amit's marks to 75
for i, line in enumerate(lines):
    name, marks = line.strip().split(",")
    if name == "Amit":
        lines[i] = f"{name},75\n"

with open("marks.txt", "w") as f:
    f.writelines(lines)

Inserting a Line at a Specific Position

with open("marks.txt", "r") as f:
    lines = f.readlines()

lines.insert(1, "Karan,88\n")   # insert a new line at index 1

with open("marks.txt", "w") as f:
    f.writelines(lines)

Deleting a Line

with open("marks.txt", "r") as f:
    lines = f.readlines()

lines = [line for line in lines if not line.startswith("Zoya")]

with open("marks.txt", "w") as f:
    f.writelines(lines)

Using a Temporary File (safer for large files)

import os

def update_record(filename, target_name, new_line):
    temp_file = filename + ".tmp"
    with open(filename, "r") as infile, open(temp_file, "w") as outfile:
        for line in infile:
            if line.startswith(target_name):
                outfile.write(new_line + "\n")
            else:
                outfile.write(line)
    os.replace(temp_file, filename)   # atomically replace original

update_record("marks.txt", "Riya", "Riya,95")

Updating a CSV file

import csv

with open("students.csv", "r") as f:
    rows = list(csv.reader(f))

for row in rows:
    if row[0] == "Amit":
        row[1] = "23"   # update age

with open("students.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(rows)

Deleting a file entirely

import os

if os.path.exists("temp.txt"):
    os.remove("temp.txt")
else:
    print("File does not exist")

Summary — file "CRUD" via Python