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A Comprehensive Guide to File Input and Output in Python

Updated: at 04:12 AM

File input and output (I/O) is an essential aspect of programming in any language. In Python, reading and writing files provides a simple way to persistently store data, share it between programs, and interact with external systems. This comprehensive guide will introduce you to the core concepts of file I/O in Python and equip you with the skills to proficiently work with files in your own programs.


File I/O refers to the process of reading information from files on your computer’s storage devices or writing data to them using a program. Python provides many built-in functions and methods that make it easy to perform file I/O operations like:

File I/O is an essential skill for any Python developer. You can use it for data persistence, sharing data between programs, generating reports, interacting with external systems, and many other important tasks. Mastering file I/O fundamentals will give you a strong foundation to build more complex programs in Python.

In this guide, you will learn:

Along with detailed explanations of concepts, we will also look at relevant code examples and applications of file I/O. By the end, you will have a comprehensive understanding of working with files in Python. Let’s get started!

File Paths

When performing file operations, you need a way to refer to the location of the file on the filesystem. This is done using the file path, which is essentially the address of the file.

In Python, file paths can be represented as either strings or pathlib.Path objects. For example:

# String path
file_path = '/home/user/data.txt'

# pathlib Path object
from pathlib import Path
file_path = Path('/home/user/data.txt')

File paths can be either absolute or relative:

On most systems, file paths use forward slashes / as separators, even on Windows. The root folder is represented by a leading slash.

When constructing file paths manually, you should use either raw strings like r'/home/user' or Path objects to avoid issues with special characters being interpreted as escapes.

Now let’s see how to open and close files in Python.

Opening and Closing Files in Python

To start reading or writing files, you first need to open them using Python’s built-in open() function. This will return a file object that contains methods for accessing the file.

The open() function takes a minimum of two arguments:

For example:

file = open('/home/user/data.txt', 'r')

This opens the file data.txt for reading and returns a file object file that we can use to read from it.

Some commonly used file access modes are:

For example, opening a file with 'w' clears existing contents while 'a' preserves them.

Once you are done accessing the file, you should close it to free up resources using the close() method:

file = open('data.txt', 'r')
# Read file contents


It is also a good practice to use the with statement while handling files. This automatically closes the file for you after the block ends:

with open('data.txt', 'r') as file:
   # Read file

Next, let’s explore reading and writing files in Python.

Reading and Writing Files in Python

Once you have a file object opened with the appropriate access mode, you can use several methods to read from or write to the file in Python.

Reading Files

To read content from a text file, you can use the read() method on the file object:

with open('data.txt', 'r') as file:
   data =

This reads the entire contents of the file as a string into the data variable.

For larger files, you may want to read a fixed number of bytes at a time using the read(size) method:

with open('data.txt', 'r') as file:
   chunk = # Reads 1024 bytes
   while chunk:
       chunk = # Reads next 1024 bytes

This iterates over the file in fixed-size chunks.

Another common way is to read line-by-line using the readline() method:

with open('data.txt', 'r') as file:
    line = file.readline()
    while line:
        line = file.readline()

This reads each line of the file as a string into the line variable.

Finally, you can get a list of all lines using readlines():

with open('data.txt', 'r') as file:
   lines = file.readlines()

Overall, use read() for entire file, readline() for line-by-line reading, and readlines() to get a list of lines.

Writing Files

To write to files, open them in write w, append a or update + modes.

Use the write() method to write a string to the file:

with open('data.txt', 'w') as file:
    file.write('Hello world!')

This overwrites the existing file with the string. To append instead, open in 'a' mode:

with open('data.txt', 'a') as file:
   file.write('Hello world!')

You can also write a list of strings using writelines():

lines = ['First line\n', 'Second line\n']
with open('data.txt', 'w') as file:

When writing, don’t forget to add newline \n characters to move to next lines.

While writing, you may also need to check the current position in the file using the tell() method:

with open('data.txt', 'w') as file:
   print(file.tell()) # Returns current position
   print(file.tell()) # Position after write

This helps track where in the file your writes will occur.

With both reading and writing, proper flushing and buffering is needed for optimal performance - we’ll explore this next.

Buffering and Flushing File I/O

By default, Python employs buffering when reading and writing files for efficiency. This means data is stored in temporary memory buffers and accessed from there instead of hitting the disk directly every time.

The downside is you may not be able to access the data just written until the buffer is flushed out explicitly using flush() or by closing the file.

For example:

with open('data.txt', 'w') as file:
   file.flush() # Flushes write buffer to disk

Flushing forces the buffered contents to be written to the actual file on disk.

You can disable buffering altogether when opening the file by passing buffering=0:

file = open('data.txt', 'w', buffering=0)

However, this may impact performance for large data volumes.

Alternatively, open the file in unbuffered mode using:

file = open('data.txt', 'w', buffering=1)

This flushes the output after every write operation.

For reading, you can also manually control buffering using the bufsize parameter. Setting it to low values forces more frequent disk access.

So in summary, buffering improves performance but can lead to inconsistent output. Use flushing and buffer size settings to control this tradeoff in your programs.

Next, let’s go over managing file cursors and positions.

Managing File Cursor Position

The file cursor refers to the current position in an open file where the next read or write will occur. When a file is opened, the cursor is at the beginning (index 0).

You can use the seek() method to move the file cursor to a particular byte position in the file:

with open('data.txt', 'r') as file: # Seek to 10th byte
   data = # Reads from 10th byte

seek() accepts the following values:

Some examples:

# Seek to 15th byte

# Seek back 3 bytes from current position, 1)

# Seek to end of file, 2)

You can use the tell() method to get the current cursor position:

pos = file.tell() # Get current cursor position

Seeking and telling positions allows moving the cursor to different parts of the file easily.

Error Handling for File I/O

When performing file operations, you should always add error handling in your code - like trying to open a non-existent file.

The best way is to use try-except blocks:

   f = open('missing.txt')
except FileNotFoundError:
   print('File not found')
   print('Unknown error occurred')

This catches errors cleanly and allows you to handle them gracefully instead of crashing your program.

Some common errors to handle include:

You should strive to make your file processing robust by anticipating all possible error cases and providing useful handling logic.

Processing Large Files

When working with large files in Python, you need optimized techniques to avoid Out of Memory (OOM) errors or poor performance.

Here are some best practices:

By following these tips, you can build robust programs in Python for handling large CSVs, JSON, text files, and more.


In this comprehensive guide, we covered all the key aspects of working with files in Python, including:

Python’s extensive built-in support for file I/O makes it easy to get started. With these skills, you can implement persistent storage, data sharing, generate reports, and build interfaces for your Python programs to interact with other systems.

There are many additional options available in Python’s io libraries like multi-file processing, concurrent access, in-memory buffers, and more. This foundational guide equips you with the core knowledge to handle files effectively in your own projects.

The next step is to apply these concepts to build practical programs that read, write, process, and analyze data from files robustly at scale. You could also explore advanced file formats like CSV, JSON and binary files. Mastering file I/O is a key skill on the journey to becoming a proficient Python programmer.