Academy — Sanjib Sikder

Data That Survives

Every program you've written so far forgets everything when it ends. Variables live in memory — the moment the program stops, they're gone.

Files change this. Writing data to a file means it persists. Reading a file means your program can work with data that was prepared elsewhere — by another program, another person, or a previous run of your own code.

This lesson uses Python's standard file API, running in the browser's in-memory filesystem. The syntax is identical to what you'd write on a real machine.

Writing a File

Python

Ctrl+Enter to run

with open(#22c55e">"notes.txt", "w") as f:
    f.write(#22c55e">"First line of notes.\n")
    f.write(#22c55e">"Second line of notes.\n")
    f.write(#22c55e">"Third line.\n")

print(#22c55e">"File written.")

Three things to understand here:

open("notes.txt", "w") — Opens a file called notes.txt in write mode. "w" creates the file if it doesn't exist, or overwrites it if it does.

with ... as f: — The with statement is a context manager. It opens the file, gives you access through f, and automatically closes the file when the block ends — whether or not an error occurred. Always use with for files. Forgetting to close a file is a classic source of subtle bugs.

f.write() — Writes text to the file. Notice \n — write() doesn't add newlines automatically the way print() does. You must include them explicitly.

Reading a File

Python

Ctrl+Enter to run

with open(#22c55e">"notes.txt", "w") as f:
    f.write(#22c55e">"First line of notes.\n")
    f.write(#22c55e">"Second line of notes.\n")
    f.write(#22c55e">"Third line.\n")

with open(#22c55e">"notes.txt", "r") as f:
    contents = f.read()

print(contents)

"r" opens in read mode. f.read() reads the entire file as one string. The newlines are preserved in the string.

Reading Line by Line

For large files, reading the entire content at once is wasteful. Reading line by line is more efficient and often more natural:

Python

Ctrl+Enter to run

with open(#22c55e">"notes.txt", "w") as f:
    f.write(#22c55e">"Apples\nBananas\nCherry\nMango\n")

with open(#22c55e">"notes.txt", "r") as f:
for line in f:
print(line.strip())

Looping directly over the file object reads one line at a time. strip() removes the trailing newline from each line — almost always what you want.

Alternatively, readlines() reads all lines at once into a list:

Python

Ctrl+Enter to run

with open(#22c55e">"notes.txt", "w") as f:
    f.write(#22c55e">"Apples\nBananas\nCherry\nMango\n")

with open(#22c55e">"notes.txt", "r") as f:
    lines = f.readlines()

print(lines)
print(f#22c55e">"There are {len(lines)} items.")

Appending to a File

"w" overwrites. "a" (append mode) adds to the end without touching existing content:

Python

Ctrl+Enter to run

with open(#22c55e">"log.txt", "w") as f:
    f.write(#22c55e">"Entry 1\n")

with open(#22c55e">"log.txt", "a") as f:
    f.write(#22c55e">"Entry 2\n")
f.write(#22c55e">"Entry 3\n")

with open(#22c55e">"log.txt", "r") as f:
    print(f.read())

The first open creates the file with Entry 1. The second open in append mode adds to it — Entry 1 is untouched. This is how log files, journals, and audit trails are built.

Handling Missing Files

Trying to read a file that doesn't exist raises FileNotFoundError:

Python

Ctrl+Enter to run

try:
    with open(#22c55e">"missing.txt", "r") as f:
        contents = f.read()
except FileNotFoundError:
    print(#22c55e">"File not found. Starting fresh.")

This is the standard pattern: try to read the file, handle the case where it doesn't yet exist. You'll use this whenever a program needs to load saved state at startup.

Working With CSV Data

CSV (Comma-Separated Values) is one of the most common data formats. Python's csv module handles it cleanly:

Python

Ctrl+Enter to run

import csv

# Write a CSV file

rows = [
[#22c55e">"name", "age", "city"],
[#22c55e">"Priya", 28, "Mumbai"],
[#22c55e">"Arjun", 22, "Delhi"],
[#22c55e">"Meera", 31, "Kolkata"],
]

with open(#22c55e">"people.csv", "w", newline="") as f:
    writer = csv.writer(f)
writer.writerows(rows)

# Read it back

with open(#22c55e">"people.csv", "r") as f:
    reader = csv.reader(f)
for row in reader:
    print(row)

csv.writer handles quoting and escaping automatically — you give it plain Python lists and it produces correct CSV. csv.reader parses the file and gives you each row as a list of strings.

For named column access, DictReader is even more convenient:

Python

Ctrl+Enter to run

import csv

with open(#22c55e">"people.csv", "r") as f:
    reader = csv.DictReader(f)
for person in reader:
    print(f#22c55e">"{person['name']} lives in {person['city']}")

DictReader uses the first row as keys and gives you each subsequent row as a dictionary. Accessing person["name"] is far clearer than row[0].

File Modes at a Glance

Mode	Meaning
`"r"`	Read (default). File must exist.
`"w"`	Write. Creates or overwrites.
`"a"`	Append. Creates if absent, adds to end if present.
`"r+"`	Read and write. File must exist.

What You Have Learned

Files are how programs persist data and communicate with the world outside memory.

The key ideas:

open(filename, mode) opens a file; always use it inside a with block
"r" reads, "w" writes (overwrites), "a" appends
f.read() reads the whole file; f.readlines() reads into a list; looping over f reads line by line
strip() removes trailing newlines when reading lines
FileNotFoundError is the exception to catch when a file may not exist
Python's csv module handles reading and writing CSV cleanly
csv.DictReader gives you each row as a dictionary — the most readable way to work with structured CSV data

In the next lesson, you'll explore Python's standard library — the large collection of ready-made modules that come with every Python installation.

File Handling

Data That Survives

Writing a File

Reading a File

Reading Line by Line

Appending to a File

Handling Missing Files

Working With CSV Data

File Modes at a Glance

What You Have Learned