2. Reading and writing files efficiently

Quiz: 0/5

When working with files in Python, you often need to read data from a file or write data to a file. This lesson will teach you how to do that efficiently.

Reading a file

Python provides several ways to read a file:

a) .read() → Read the entire file at once

This method reads the entire file as a single string.

with open("example.txt", "r") as file:  # Open the file in read mode
    content = file.read()  # Read all contents
    print(content)  # Print the file content

Best for: Small files, since it loads everything into memory.

b) .readline() → Read one line at a time

This method reads only one line at a time, useful when dealing with large files.

with open("example.txt", "r") as file:
    line1 = file.readline()  # Read the first line
    print(line1)

    line2 = file.readline()  # Read the second line
    print(line2)

Best for: Reading a file line by line when you don’t want to load everything at once.

c) .readlines() → Read all lines into a list

This method reads all lines from the file and stores them in a list, where each line is an item.

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

Best for: When you need to process each line separately but still want to keep them in memory.

Writing and appending to a file

If you want to modify a file, you need to open it in write w or append a mode.

a) Writing with .write() → Overwrites the file

The .write() method writes text into a file. If the file already exists, it deletes its content before writing new data.

with open("example.txt", "w") as file:
    file.write("Hello, World!\n")  # Write a single line to the file

Be careful! This will erase the existing file content before writing.

b) Appending with .write() → Adds text to the end of a file

If you want to add more text without deleting the existing content, use append mode a.

with open("example.txt", "a") as file:
    file.write("This is a new line.\n")

Best for: Adding new information without deleting previous content.

c) Writing multiple lines with .writelines()

If you have a list of strings and want to write them to a file, use .writelines().

lines = ["Line 1\n", "Line 2\n", "Line 3\n"]

with open("example.txt", "w") as file:
    file.writelines(lines)  # Writes all lines at once

Note: Each string in the list should already end with \n (new line character).

Using with open() to manage files safely

Using with open() is the recommended way to work with files because it automatically closes the file when you're done.

with open("example.txt", "r") as file:
    content = file.read()
    print(content)  # File closes automatically after this block

Why is this important?
If you don't close the file, it might cause errors or lock issues. Using with open() ensures that files are closed properly.

Summary

Method	Purpose
.read()	Read the whole file at once
.readline()	Read one line at a time
.readlines()	Read all lines into a list
.write()	Write text (overwrites the file)
.writelines()	Write multiple lines from a list
"a" mode	Append text without deleting content
with open()	Automatically closes the file