Working with Files in Python — Read, Write, Append, and Manage File Paths

Working with files is essential in almost every program — reading settings, saving user data, processing logs, or exporting results. Python makes file operations straightforward with built-in functions.

Table of Contents

• What is a File?
• Opening and Closing Files
• Reading Files
• Writing Files
• Appending Files
• File Paths
• File Modes
• The with Statement
• Example Program

What is a File?

A file is a named location on disk that stores data. Python treats files as streams — you open a connection, read or write data through it, then close it.

Three basic operations: open, read/write, close.

Opening and Closing Files

Use the open() function to get a file object.

file = open("example.txt", "r")   # open for reading
# ... work with the file ...
file.close()                       # always close when done

The first argument is the filename, the second is the mode (explained below).

Forgetting to close can cause data loss or file locks. The with statement (covered later) handles this automatically.

Reading Files

Read entire content

file = open("data.txt", "r")
content = file.read()
print(content)
file.close()

Read line by line

file = open("data.txt", "r")
for line in file:
    print(line.strip())   # strip removes the newline
file.close()

Read all lines into a list

file = open("data.txt", "r")
lines = file.readlines()   # returns a list of strings
file.close()

read() loads everything into memory at once — fine for small files. For large files, iterate line by line instead.

Writing Files

Writing replaces the entire file. If the file doesn't exist, Python creates it.

file = open("output.txt", "w")
file.write("Hello, World!\n")
file.write("Second line here.\n")
file.close()

After running this, output.txt will contain exactly those two lines. If the file already existed, its old content is gone.

Write multiple lines at once

lines = ["Line A\n", "Line B\n", "Line C\n"]
file = open("output.txt", "w")
file.writelines(lines)
file.close()

writelines() does not add newlines automatically — include them yourself.

Appending Files

Appending adds data to the end of an existing file without deleting what's already there.

file = open("log.txt", "a")
file.write("New log entry\n")
file.close()

If log.txt doesn't exist, append mode creates it (same as write mode).

Read + Append example

# Create initial file
file = open("log.txt", "w")
file.write("Session started\n")
file.close()

# Append later
file = open("log.txt", "a")
file.write("User logged in\n")
file.write("User logged out\n")
file.close()

# Read to verify
file = open("log.txt", "r")
print(file.read())
file.close()

Output:

Session started
User logged in
User logged out

File Paths

Absolute vs Relative

• Relative path: relative to where your script is running

  • "data.txt" — file in current directory
  • "subfolder/data.txt" — file inside a subfolder
  • "../other/data.txt" — go up one directory, then into other

• Absolute path: full path from the root

  • Windows: "C:\\Users\\Name\\docs\\file.txt"
  • Linux/Mac: "/home/name/docs/file.txt"

Using forward slashes

Python accepts forward slashes even on Windows, so you can write "C:/Users/Name/docs/file.txt" — this avoids escaping issues with backslashes.

The os.path module

import os

path = os.path.join("folder", "subfolder", "file.txt")
print(path)               # folder/subfolder/file.txt on Linux/Mac
                          # folder\subfolder\file.txt on Windows

print(os.path.exists("test.txt"))       # True or False
print(os.path.isfile("test.txt"))       # True if it's a file
print(os.path.isdir("mydir"))           # True if it's a directory

os.path.join is the safe way to build paths — it uses the correct separator for the current OS.

Check before opening

import os

filename = "notes.txt"

if os.path.exists(filename):
    file = open(filename, "r")
    print(file.read())
    file.close()
else:
    print(f"{filename} does not exist.")

Common File Modes

Working with Context Managers

The with statement closes the file automatically — even if an error occurs.

Instead of this:

file = open("data.txt", "r")
data = file.read()
file.close()

Do this:

with open("data.txt", "r") as file:
    data = file.read()
# file is closed automatically here

This is the recommended way to work with files in Python. No risk of forgetting to close.

Writing with with:

with open("output.txt", "w") as file:
    file.write("Automatically closed.\n")

Reading lines with with:

with open("data.txt", "r") as file:
    for line in file:
        print(line.strip())

Example Program

A contact list that saves to and loads from a file.

import os

FILE_NAME = "contacts.txt"

def add_contact(name, phone):
    with open(FILE_NAME, "a") as f:
        f.write(f"{name},{phone}\n")
    print(f"Saved {name}.")

def list_contacts():
    if not os.path.exists(FILE_NAME):
        print("No contacts yet.")
        return
    
    with open(FILE_NAME, "r") as f:
        for line in f:
            name, phone = line.strip().split(",")
            print(f"{name}: {phone}")

# --- Usage ---
add_contact("Alice", "555-0101")
add_contact("Bob",   "555-0202")
list_contacts()

File on disk:

Alice,555-0101
Bob,555-0202

Output:

Saved Alice.
Saved Bob.
Alice: 555-0101
Bob: 555-0202