Opening and Closing Files

Python uses the built-in open() function to work with files. The most important rule: always close files when you're done with them.

# Basic pattern (not recommended)
file = open("example.txt", "r")
content = file.read()
file.close()

# Better: use 'with' statement (auto-closes)
with open("example.txt", "r") as file:
    content = file.read()
# File is automatically closed here

File Modes

The second argument to open() specifies how the file should be opened:

# Read modes
"r"      # Read (default) — file must exist
"r+"     # Read and write — file must exist

# Write modes
"w"      # Write — creates file or OVERWRITES existing
"a"      # Append — creates file or adds to end
"x"      # Exclusive create — fails if file exists

# Binary modes (add 'b')
"rb"     # Read binary (images, PDFs, etc.)
"wb"     # Write binary

Reading Files

Read Entire File

with open("notes.txt", "r") as file:
    content = file.read()
    print(content)

Read Line by Line

# Read all lines into a list
with open("notes.txt", "r") as file:
    lines = file.readlines()
    for line in lines:
        print(line.strip())     # strip() removes trailing newline

# Memory-efficient: iterate directly
with open("notes.txt", "r") as file:
    for line in file:
        print(line.strip())

Read Specific Amount

with open("notes.txt", "r") as file:
    first_100 = file.read(100)     # Read first 100 characters
    next_line = file.readline()    # Read next line from current position

Writing Files

Write Text

# Create or overwrite a file
with open("output.txt", "w") as file:
    file.write("First line\n")
    file.write("Second line\n")

# Write multiple lines at once
lines = ["Line 1\n", "Line 2\n", "Line 3\n"]
with open("output.txt", "w") as file:
    file.writelines(lines)

Append to a File

# Add to existing file without erasing
with open("log.txt", "a") as file:
    file.write("New log entry\n")

Write with print()

with open("output.txt", "w") as file:
    print("Hello, file!", file=file)
    print(f"The answer is {42}", file=file)

Working with File Paths

import os
from pathlib import Path

# os.path (traditional)
print(os.path.exists("myfile.txt"))          # True/False
print(os.path.isfile("myfile.txt"))          # True if it's a file
print(os.path.isdir("myfolder"))             # True if it's a directory
print(os.path.join("folder", "file.txt"))    # folder/file.txt

# pathlib (modern, recommended)
p = Path("folder") / "subfolder" / "file.txt"
print(p)                     # folder/subfolder/file.txt
print(p.exists())            # True/False
print(p.suffix)              # .txt
print(p.stem)                # file
print(p.parent)              # folder/subfolder

Practical Examples

Log Writer

from datetime import datetime

def log(message, filename="app.log"):
    timestamp = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    with open(filename, "a") as file:
        file.write(f"[{timestamp}] {message}\n")

log("Application started")
log("User logged in")
log("Error: connection timeout")

Simple CSV Reader

# Reading CSV manually
with open("data.csv", "r") as file:
    header = file.readline().strip().split(",")
    print(f"Columns: {header}")

    for line in file:
        values = line.strip().split(",")
        row = dict(zip(header, values))
        print(row)

# Better: use the csv module
import csv

with open("data.csv", "r") as file:
    reader = csv.DictReader(file)
    for row in reader:
        print(row["name"], row["age"])

Word Counter

def count_words(filename):
    with open(filename, "r") as file:
        text = file.read()

    words = text.split()
    lines = text.count("\n") + 1
    chars = len(text)

    print(f"File: {filename}")
    print(f"  Lines: {lines}")
    print(f"  Words: {len(words)}")
    print(f"  Characters: {chars}")

count_words("example.txt")

Checking If a File Exists

from pathlib import Path

filepath = Path("config.txt")

if filepath.exists():
    content = filepath.read_text()
    print("File found!")
else:
    print("File not found, creating it...")
    filepath.write_text("default config")

Summary

• Always use with open(...) as file: to automatically close files
• File modes: "r" (read), "w" (write/overwrite), "a" (append), "x" (create new)
• read() loads entire file; iterate with for line in file: for large files
• write() and writelines() write to files; remember \n for newlines
• Use pathlib.Path for modern, cross-platform file path handling
• The csv module handles CSV files properly (handles quoting, escaping)