📂 Learn how to read from and write to files in Python, including text files, CSV files, and how to check if a file exists. This section also includes best practices for handling files safely and efficiently.

• 📖 How to read from text files using open(), read(), readline(), and readlines()
• ✍️ How to write to text files using write() and writelines()
• ➕ How to append data to existing files
• 🆕 How to create new files
• 🔍 How to check if a file exists
• 📊 How to read and write CSV files using the built-in csv module
• 🗑️ How to rename and delete files
• 💡 Hidden tips and best practices for working with files in Python

Python provides several ways to read from a file using the built-in open() function.

The with statement ensures the file is closed automatically after use.

🔹 Example – Read entire file:

with open('readme.txt') as f:
    content = f.read()
    print(content)

🔸 Use f.readlines() to get all lines as a list:

with open('readme.txt') as f:
    lines = f.readlines()
    for line in lines:
        print(line.strip())

🔸 Use f.readline() to read one line at a time:

with open('readme.txt') as f:
    line = f.readline()
    while line:
        print(line.strip())
        line = f.readline()

Use the 'w' mode to create or overwrite a file.

🔹 Example – Write a string to a file:

with open('readme.txt', 'w') as f:
    f.write("This is a new file.")

🔹 Write multiple lines:

lines = ["Line 1\n", "Line 2\n", "Line 3\n"]
with open('readme.txt', 'w') as f:
    f.writelines(lines)

🔸 To ensure proper formatting, manually add newline characters (\n) when needed.

Use the 'a' mode to append to an existing file without overwriting it.

🔹 Example – Append text to the end of a file:

with open('readme.txt', 'a') as f:
    f.write("\nThis line was appended.")

Use the 'w' or 'x' mode to create a new file.

🔹 Using 'w':

with open('newfile.txt', 'w') as f:
    f.write("New file created!")

🔹 Using 'x' (exclusive creation):

try:
    with open('newfile.txt', 'x') as f:
        f.write("New file created!")
except FileExistsError:
    print("File already exists.")

🔸 The 'x' mode prevents accidental overwriting.

You can use either os.path.exists() or pathlib.Path().is_file().

🔹 Using os.path.exists():

import os

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

🔹 Using pathlib:

from pathlib import Path

file = Path('readme.txt')
if file.is_file():
    print("File exists.")

Use the built-in csv module to handle comma-separated values (CSV) files.

🔹 Example – Read and display CSV rows:

import csv

with open('countries.csv', newline='') as csvfile:
    reader = csv.reader(csvfile)
    for row in reader:
        print(row)

🔸 You can also use DictReader to map each row to a dictionary:

with open('countries.csv', newline='') as csvfile:
    reader = csv.DictReader(csvfile)
    for row in reader:
        print(row['name'], row['area'])

🔹 Example – Write a single row:

import csv

header = ['name', 'area', 'country_code2']
data = ['Afghanistan', 652090, 'AF']

with open('countries.csv', 'w', encoding='UTF8', newline='') as f:
    writer = csv.writer(f)
    writer.writerow(header)
    writer.writerow(data)

🔹 Write multiple rows:

rows = [
    ['Albania', 28748, 'AL'],
    ['Algeria', 2381741, 'DZ']
]
writer.writerows(rows)

🔸 Don’t forget newline='' to avoid extra blank lines in some OS environments.

Use os.rename(old_name, new_name) to rename a file.

🔹 Example:

import os
os.rename('oldfile.txt', 'newfile.txt')

Use os.remove(filename) to delete a file.

🔹 Example:

import os
os.remove('oldfile.txt')

🔸 Always check if the file exists before deleting:

if os.path.exists('oldfile.txt'):
    os.remove('oldfile.txt')

• 🔒 Always use the with statement to open files — it ensures files are closed properly.
• 📁 When writing to a subdirectory like docs/readme.txt, make sure the folder exists first.
• ⚠️ Opening too many files without closing them may lead to race conditions or resource exhaustion.
• 🧩 Use encoding='utf-8' when dealing with non-ASCII characters.
• 🧪 Use try...except blocks to handle errors gracefully (e.g., missing directories).
• 🧹 Avoid hardcoding paths — use os.path or pathlib for cross-platform compatibility.

try:
    with open('docs/report.txt', 'w', encoding='utf-8') as f:
        f.write("Sales Report\n")
        f.write("-----------\n")
        f.write("Total Revenue: $100,000\n")
except FileNotFoundError:
    print("Directory 'docs' does not exist. Please create it first.")

import csv

try:
    with open('data.csv', newline='') as csvfile:
        reader = csv.DictReader(csvfile)
        for row in reader:
            print(f"{row['Product']} - ${row['Price']}")
except FileNotFoundError:
    print("CSV file not found.")

🎉 Congratulations! You now have a solid understanding of how to work with files in Python, including reading, writing, renaming, and deleting files, and handling CSV data effectively.

Next up: 📁 Section 13: Working Directories – learn how to interact with your file system, list files, and manage directory structures.