Python File Handling - Developer's Notes

If you're looking to learn how to work with files in Python, you've come to the right place. Handling file input and output operations is an integral skill for any Python developer. This site provides a comprehensive set of notes and code examples for manipulating files in Python.

Whether you need to open, read, write, append, or close files, these notes have you covered.

You'll learn how to:

  • Properly open and close files using the built-in open() function and file objects
  • Read and write files with methods like read(), readline(), write(), and more
  • Append new data to existing files without overwriting
  • Iterate through files line-by-line or in chunks
  • Use file modes like 'r', 'w', 'a', 'r+' for different file manipulations
  • Employ best practices like context managers to ensure proper file handling
  • Serialize Python objects like dicts and lists to files using json
  • Deserialize JSON data from files back into Python objects
  • Process large files without loading everything into memory

Whether you're a beginner looking to learn file I/O basics or an experienced programmer needing a Python file handling reference, this site has what you need. The organized, example-driven notes provide clear explanations and actionable code snippets.

So dive in and let these notes equip you with the knowledge to handle file operations in Python like a pro!

Opening Files

To open a file in Python, use the built-in open() function. This will return a file object that can be used to read and write to the file.

file = open('data.txt', 'r')

The first argument is the file path and name. The second argument is the mode in which to open the file. Some common modes are:

  • 'r' - Read only
  • 'w' - Write only (overwrites existing file)
  • 'a' - Append to existing file
  • 'r+' - Read and write

Reading Files

Once you have a file object, you can read from it using various methods:

print(file.read()) # Reads entire file
print(file.read(10)) # Reads 10 characters
print(file.readline()) # Reads next line

for line in file:
print(line) # Iterates over lines
  • file.read() reads the entire contents of the file as a string.
  • file.read(n) reads n characters from the current position.
  • file.readline() reads a single line.
  • You can loop over the file object to iterate over each line.

Writing Files

To write to a file, open it in write 'w' or append 'a' mode.

file = open('data.txt', 'w')

file.write('This will overwrite the file')

file = open('data.txt', 'a')
file.write('This will append to the file')

Closing Files

It's important to close files when you are finished:


This releases any system resources taken up by the open file. It's best practice to use try/finally to ensure files are closed:

file = open('data.txt')
# manipulate file

This Python file I/O guide covers the basics of working with files. Open files, read and write data, and make sure to close your files. Following these practices will help ensure your programs interact with files robustly.


How do I append to a file without overwriting it?

Use 'a' mode when opening the file to append instead of overwrite:

file = open('data.txt', 'a')
How do I write a list or dictionary to a file?

Use json.dump() to serialize the object to a string and write to file:

import json

data = ['foo', {'bar': 'baz'}]

with open('data.json', 'w') as f:
json.dump(data, f)
How can I read a large file without loading the entire thing into memory?

Loop line by line to process a large file iteratively:

with open('large_file.txt') as f:
for line in f:
# process line

Or read chunks at a time:

with open('large_file.txt') as f:
chunk = f.read(1024)
while chunk:
# process chunk
chunk = f.read(1024)