Python open() Function

The open() function in Python is used to open files and return a file object. This is the starting point for reading from or writing to a file.

Syntax

open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)

Common Parameters:

• file: Path to the file (required)
• mode: Mode to open the file in (default is 'r')
• encoding: For text files, like 'utf-8'

File Modes

Mode	Description
'r'	Read (default) – file must exist
'w'	Write – creates or overwrites file
'a'	Append – adds to file, creates if needed
'x'	Create – fails if file exists
'b'	Binary mode (e.g., 'rb', 'wb')
't'	Text mode (default)
'+'	Update mode (read & write)

Example 1: Reading a File

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

The contents of example.txt

Example 2: Writing to a File

file = open("output.txt", "w")
file.write("Hello, world!")
file.close()

Creates a file with "Hello, world!"

Example 3: Using with Statement (Best Practice)

with open("data.txt", "r") as f:
    print(f.read())

Why use with? It automatically closes the file after use, even if an error occurs.

Example 4: Reading Line by Line

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

Common Errors

• FileNotFoundError – when opening a non-existent file in read mode
• PermissionError – when writing to a file you don’t have access to

Use Cases

• Reading configuration files
• Logging messages to a log file
• Processing CSV or text data

Interview Tip

Always use the with open(...) pattern to handle files safely in coding interviews.

Summary

• open() is used to read/write/create files
• Different modes like 'r', 'w', 'a' change behavior
• Use with to manage files safely

Practice Problem

Write a Python script to copy the contents of one file into another file:

with open("source.txt", "r") as source:
    with open("destination.txt", "w") as dest:
        dest.write(source.read())