SRJahirTech

Python Full Tutorial -- Part 8: File Handling

By Suraj Ahir 2025-12-29 11 min read

Almost every real Python application reads or writes files. Configuration files, log files, CSV exports, JSON API responses, uploaded images -- file I/O is unavoidable. Python makes it clean with context managers and has excellent built-in support for the most common file formats.

Reading and Writing Text Files

# Read entire file
with open("data.txt", "r", encoding="utf-8") as f:
    content = f.read()

# Read line by line (memory efficient for large files)
with open("data.txt", "r") as f:
    for line in f:
        print(line.strip())  # strip() removes trailing newline

# Read all lines into a list
with open("data.txt", "r") as f:
    lines = f.readlines()

# Write file (creates or overwrites)
with open("output.txt", "w") as f:
    f.write("First line\n")
    f.write("Second line\n")

# Append to file
with open("log.txt", "a") as f:
    f.write("New entry\n")

CSV Files

import csv

users = [
    {"name": "Suraj", "age": 25, "city": "Mumbai"},
    {"name": "Raj",   "age": 22, "city": "Delhi"}
]

# Write CSV
with open("users.csv", "w", newline="") as f:
    writer = csv.DictWriter(f, fieldnames=["name", "age", "city"])
    writer.writeheader()
    writer.writerows(users)

# Read CSV
with open("users.csv", "r") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(f"{row['name']} lives in {row['city']}")

JSON Files

import json

config = {
    "database": {"host": "localhost", "port": 5432},
    "debug": False,
    "allowed_hosts": ["localhost", "example.com"]
}

# Write JSON
with open("config.json", "w") as f:
    json.dump(config, f, indent=2)

# Read JSON
with open("config.json", "r") as f:
    loaded = json.load(f)
print(loaded["database"]["host"])  # localhost

# String operations
json_str = json.dumps(config, indent=2)  # Python -> JSON string
parsed   = json.loads(json_str)           # JSON string -> Python

Modern Paths with pathlib

from pathlib import Path

project = Path("/home/suraj/myapp")
config  = project / "config" / "settings.json"

print(config.exists())    # True/False
print(config.suffix)      # .json
print(config.stem)        # settings
print(config.parent)      # /home/suraj/myapp/config

# Read/write directly
text = config.read_text(encoding="utf-8")
config.write_text("{\"debug\": true}")

# Create directories safely
(project / "logs").mkdir(parents=True, exist_ok=True)

# Find all Python files recursively
for py_file in project.rglob("*.py"):
    print(py_file)

Frequently Asked Questions

In Part 9, we cover error handling -- writing code that recovers gracefully from failures.

Written by

Suraj Ahir

Cloud & DevOps engineer running four live production services on my own AWS infrastructure. I write everything on this site myself — no ghostwriters, no AI filler.

More about me → GitHub LinkedIn

Working with Binary Files

import struct

# Read a binary file
with open("image.png", "rb") as f:      # rb = read binary
    header = f.read(8)                   # Read first 8 bytes
    print(header)                        # b"\x89PNG\r\n\x1a\n"

# Write binary data
with open("output.bin", "wb") as f:     # wb = write binary
    f.write(b"\x00\x01\x02\x03")

# struct: pack/unpack binary data
data = struct.pack(">IH", 1234567890, 65535)  # Big-endian uint32, uint16
print(len(data))  # 6 bytes

value, flag = struct.unpack(">IH", data)
print(value, flag)  # 1234567890 65535

Configuration Management with configparser

import configparser

# config.ini
# [database]
# host = localhost
# port = 5432
# name = myapp

config = configparser.ConfigParser()
config.read("config.ini")

host = config["database"]["host"]      # localhost
port = config["database"].getint("port")  # 5432 as int
debug = config["app"].getboolean("debug")  # True/False from yes/no/true/false

# Write config
config["new_section"] = {"key": "value", "number": "42"}
with open("output.ini", "w") as f:
    config.write(f)

Async File Operations with aiofiles

import asyncio
import aiofiles   # pip install aiofiles

async def read_multiple_files(filenames: list) -> dict:
    """Read multiple files concurrently."""
    results = {}
    
    async def read_one(fn):
        async with aiofiles.open(fn, mode="r") as f:
            content = await f.read()
        results[fn] = content
    
    await asyncio.gather(*[read_one(fn) for fn in filenames])
    return results

async def main():
    files = ["config.json", "settings.yaml", "data.csv"]
    contents = await read_multiple_files(files)
    for filename, content in contents.items():
        print(f"{filename}: {len(content)} chars")

asyncio.run(main())

Excel Files with openpyxl

from openpyxl import Workbook, load_workbook
from openpyxl.styles import Font, PatternFill

# Create Excel file
wb = Workbook()
ws = wb.active
ws.title = "Sales Report"

# Headers with formatting
headers = ["Date", "Product", "Units", "Revenue"]
for col, header in enumerate(headers, start=1):
    cell = ws.cell(row=1, column=col, value=header)
    cell.font = Font(bold=True)
    cell.fill = PatternFill("solid", fgColor="4472C4")

# Data
data = [
    ("2026-01-01", "Python Course", 45, 44955),
    ("2026-01-02", "DevOps Course", 23, 22977),
]
for row_num, row_data in enumerate(data, start=2):
    for col_num, value in enumerate(row_data, start=1):
        ws.cell(row=row_num, column=col_num, value=value)

wb.save("sales_report.xlsx")

# Read Excel file
wb = load_workbook("sales_report.xlsx")
ws = wb.active
for row in ws.iter_rows(min_row=2, values_only=True):
    date, product, units, revenue = row
    print(f"{product}: {units} units = Rs.{revenue}")

Working with ZIP Archives

import zipfile
from pathlib import Path

# Create ZIP archive
with zipfile.ZipFile("backup.zip", "w", zipfile.ZIP_DEFLATED) as zf:
    for path in Path("./myproject").rglob("*.py"):
        zf.write(path, path.relative_to("."))

# Extract ZIP
with zipfile.ZipFile("backup.zip", "r") as zf:
    zf.extractall("/tmp/restored/")
    print(zf.namelist())  # List contents

# Check ZIP without extracting
with zipfile.ZipFile("backup.zip") as zf:
    total = sum(info.file_size for info in zf.infolist())
    print(f"Total uncompressed: {total:,} bytes")