Python File I/O Reading and Writing to Files

File I/O (Input/Output) is a fundamental part of programming, enabling data storage and retrieval for many applications. In Python, file I/O operations are straightforward to learn and powerful in functionality, allowing us to read from and write to files with minimal code.

In this article, we’ll cover:

• Opening files and file modes.
• Reading from files.
• Writing to files.
• Working with different file formats.
• Managing file resources with with statements.

Let’s dive in!

Opening Files and File Modes

To perform any operation on a file, we first need to open it. Python provides the open() function for this purpose, which returns a file object. The open() function requires at least one argument, the file name, and optionally takes a second argument, the mode.

File Modes in Python

The most common file modes include:

• "r" (Read Mode): Opens a file for reading. This is the default mode. The file must exist; otherwise, it raises an error.
• "w" (Write Mode): Opens a file for writing. If the file exists, it truncates (empties) it before writing. If the file doesn’t exist, it creates a new one.
• "a" (Append Mode): Opens a file for appending. Data is added to the end of the file without truncating it. If the file doesn’t exist, it creates a new one.
• "r+" (Read and Write Mode): Opens a file for both reading and writing. The file must exist.
• "w+" (Write and Read Mode): Opens a file for writing and reading. If the file exists, it’s truncated; otherwise, a new file is created.
• "a+" (Append and Read Mode): Opens a file for appending and reading. If the file doesn’t exist, it creates a new one.

Syntax Example

file = open("example.txt", "r")  # Open file in read mode

To close the file after operations, we use file.close(). However, using with statements is a best practice for handling files, as it ensures files are properly closed even if an error occurs (more on this later).

Reading the Entire File

To read the entire content of a file at once, use the read() method.

with open("example.txt", "r") as file:
    content = file.read()
    print(content)

Reading Line by Line

The readline() method reads one line at a time, which is useful for large files.

with open("example.txt", "r") as file:
    line = file.readline()
    while line:
        print(line, end='')  # `end=''` avoids adding extra new lines
        line = file.readline()

Reading All Lines into a List

Using readlines(), we can read all lines and store them in a list.

with open("example.txt", "r") as file:
    lines = file.readlines()
    for line in lines:
        print(line, end='')

Writing to Files

Writing data to a file can be done with the write() and writelines() methods. Writing operations require opening the file in a mode that allows writing, such as "w", "a", "w+", or "a+".

Writing a String to a File

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

Appending Data to a File

To append data without overwriting the existing content, use "a" mode.

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

Writing Multiple Lines

The writelines() method takes a list of strings and writes them to the file. Each list item should include a newline character (\n) if you want each to start on a new line.

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

with open("example.txt", "w") as file:
    file.writelines(lines)

Working with Different File Formats

Python’s file I/O capabilities aren’t limited to plain text files. The open() function can also handle various formats, such as:

Working with CSV Files

For handling CSV files, Python provides the csv module. Here’s a quick example of reading from and writing to CSV files.

Reading a CSV File

import csv

with open("data.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)

Writing to a CSV File

import csv

data = [
    ["Name", "Age", "City"],
    ["Alice", 30, "New York"],
    ["Bob", 25, "Los Angeles"]
]

with open("data.csv", "w", newline='') as file:
    writer = csv.writer(file)
    writer.writerows(data)

Working with JSON Files

For JSON files, Python provides the json module.

Reading a JSON File

import json

with open("data.json", "r") as file:
    data = json.load(file)
    print(data)

Writing to a JSON File

import json

data = {"name": "Alice", "age": 30, "city": "New York"}

with open("data.json", "w") as file:
    json.dump(data, file)

Managing File with with Statements

Using with statements for file I/O is a best practice because it automatically closes the file, even if an error occurs within the block.

with open("example.txt", "r") as file:
    content = file.read()
    print(content)
# The file is automatically closed after the `with` block

Without with, you would need to explicitly close the file using file.close(), which increases the risk of leaving files open if an error occurs before closing.

Handling Exceptions in File I/O

Errors in file I/O are common, such as attempting to open a file that doesn’t exist or trying to write to a read-only file. To handle these cases, Python offers the try-except blocks.

try:
    with open("nonexistent.txt", "r") as file:
        content = file.read()
except FileNotFoundError:
    print("The file does not exist.")
except IOError:
    print("An error occurred while accessing the file.")

Example: Complete File I/O Operation

Here’s a complete example that reads from a file, processes the content, and writes the processed data to a new file.

try:
    # Reading from a file
    with open("input.txt", "r") as infile:
        content = infile.read()
    
    # Processing the content (e.g., making it uppercase)
    processed_content = content.upper()
    
    # Writing the processed content to a new file
    with open("output.txt", "w") as outfile:
        outfile.write(processed_content)

    print("File processing complete!")
except FileNotFoundError:
    print("The input file was not found.")
except IOError:
    print("An error occurred with file processing.")

Conclusion

Python’s file I/O functions are powerful yet easy to use, enabling us to interact with files in various formats. With the basics of reading and writing files in hand, you can now manage data storage, configuration files, and more in your Python applications.