File Handling: Reading and Writing Files

File handling lets a Python program read from and write to files stored on disk — essential for working with logs, configuration files, datasets, and any data that needs to persist after the program finishes running.

Opening a File With open()

file = open("notes.txt", "r")   # "r" = read mode
content = file.read()
print(content)
file.close()   # always close a file when you are done with it

File Modes

The with Statement — The Recommended Way to Handle Files

with open("notes.txt", "r") as file:
    content = file.read()
    print(content)
# the file is automatically closed here, even if an error occurred above

Using with is strongly preferred over manually calling open() and close(). If an exception is raised between open() and close(), the file could be left open indefinitely — with guarantees proper cleanup no matter what happens inside the block.

Different Ways to Read a File

with open("notes.txt", "r") as file:
    full_text = file.read()        # reads the ENTIRE file as one string

with open("notes.txt", "r") as file:
    first_line = file.readline()   # reads just ONE line

with open("notes.txt", "r") as file:
    all_lines = file.readlines()   # reads every line into a LIST of strings

with open("notes.txt", "r") as file:
    for line in file:               # most memory-efficient — reads one line at a time
        print(line.strip())          # .strip() removes the trailing newline character

Writing to a File

with open("notes.txt", "w") as file:
    file.write("First line of text
")
    file.write("Second line of text
")
# IMPORTANT: "w" mode completely erases any existing content first

Appending Instead of Overwriting

with open("notes.txt", "a") as file:
    file.write("This line gets added to the end
")
# existing content is preserved; the new line is added after it

Writing Multiple Lines at Once

lines = ["First line
", "Second line
", "Third line
"]

with open("notes.txt", "w") as file:
    file.writelines(lines)

Checking If a File Exists Before Opening It

import os

if os.path.exists("notes.txt"):
    with open("notes.txt", "r") as file:
        print(file.read())
else:
    print("File does not exist")

Working With JSON Files

Since JSON is such a common data format, Python's built-in json module makes reading and writing structured data almost effortless.

import json

data = {"name": "Vishwas", "role": "Developer", "city": "Bengaluru"}

# Writing a dictionary to a JSON file
with open("profile.json", "w") as file:
    json.dump(data, file)

# Reading it back as a dictionary
with open("profile.json", "r") as file:
    loaded_data = json.load(file)
    print(loaded_data["name"])   # Vishwas

Interview tip: "What is the difference between read(), readline(), and readlines()?" is a near-guaranteed question — read() returns the whole file as one string, readline() returns just the next single line, and readlines() returns every line as a list of strings; iterating directly over the file object is the most memory-efficient option for very large files, since it never loads the entire file into memory at once.