7.3 File Modes (r, w, a, b)

When working with files in Python, the file mode determines how a file is opened and what operations are allowed on the file. You use the open() function to specify the file mode, which controls whether you can read from or write to the file, and whether you're working with a text file or a binary file. Python provides several file modes, each serving different purposes, such as reading, writing, or appending data.

In this section, we'll explore the most common file modes in Python and their combinations, including how to use them with text and binary files.

7.3.1 Overview of File Modes

Here’s a quick summary of the most commonly used file modes:

• r: Read mode (default). Opens a file for reading. The file must already exist, or an error will be raised.
• w: Write mode. Opens a file for writing. If the file exists, it is overwritten. If it doesn’t exist, a new file is created.
• a: Append mode. Opens a file for appending data at the end. If the file doesn’t exist, it is created. The existing content is preserved.
• b: Binary mode. Used in combination with other modes (r, w, or a) to indicate binary file operations instead of text mode.

You can combine these modes, such as "rb" for reading a binary file or "w+" for writing and reading a file.

7.3.2 r: Read Mode

The r mode is used to open a file for reading only. The file must already exist, and if it does not, Python raises a FileNotFoundError.

Usage:

file = open("example.txt", "r")  # Open for reading (text mode by default)

Example:

try:
    with open("example.txt", "r") as file:
        content = file.read()
        print(content)
except FileNotFoundError:
    print("The file does not exist.")

In this example:

• The file is opened in read mode (r), which allows the program to read its contents.
• If the file does not exist, a FileNotFoundError is raised.

7.3.3 w: Write Mode

The w mode is used to open a file for writing. If the file already exists, it is overwritten (the old content is lost). If the file does not exist, it is created.

Usage:

file = open("example.txt", "w")  # Open for writing (text mode by default)

Example:

with open("output.txt", "w") as file:
    file.write("This is a new file.\n")
    file.write("All previous content is overwritten.")

In this example:

• The file is opened in write mode (w). If output.txt already exists, it will be overwritten. If it doesn’t exist, it will be created.
• The write() method writes text to the file.

7.3.4 a: Append Mode

The a mode opens a file for appending. Data is added at the end of the file without erasing the existing content. If the file doesn’t exist, it is created.

Usage:

file = open("example.txt", "a")  # Open for appending (text mode by default)

Example:

with open("output.txt", "a") as file:
    file.write("\nThis line will be appended to the file.")

In this example:

• The file is opened in append mode (a). New content is added at the end of the file without modifying or removing existing content.
• If output.txt doesn’t exist, it will be created.

7.3.5 b: Binary Mode

The b mode is used to handle binary files like images, audio, or video files. It is combined with other modes (r, w, or a) to indicate binary file operations instead of text.

Usage:

• rb: Read a binary file.
• wb: Write to a binary file.
• ab: Append to a binary file.

Example: Reading a Binary File:

with open("example_image.png", "rb") as file:
    binary_data = file.read()
    print(binary_data[:10])  # Display the first 10 bytes

In this example:

• The file is opened in binary read mode (rb) to read binary data from the file. The data is returned as a sequence of bytes.
• The first 10 bytes are printed.

Example: Writing to a Binary File:

binary_data = b'\x00\x01\x02\x03'

with open("output_binary_file.bin", "wb") as file:
    file.write(binary_data)

In this example:

• The file is opened in binary write mode (wb).
• Binary data is written to the file using the write() method. If the file already exists, it will be overwritten.

7.3.6 Combined Modes

You can combine different file modes to achieve more specific behavior, such as reading and writing in the same file or working with both text and binary files.

1. r+: Read and Write

The r+ mode opens a file for both reading and writing. The file must already exist, and no data is overwritten until you explicitly write to the file.

Usage:

file = open("example.txt", "r+")

Example:

with open("example.txt", "r+") as file:
    content = file.read()  # Read the existing content
    print(content)
    file.write("\nAppending new content.")

In this example:

• The file is opened in read and write mode (r+).
• First, the existing content is read using read(). Then, new data is written to the file, starting at the current file pointer position.

2. w+: Write and Read

The w+ mode opens a file for writing and reading. If the file already exists, it is overwritten, and if it doesn’t exist, a new file is created.

Usage:

file = open("example.txt", "w+")

Example:

with open("output.txt", "w+") as file:
    file.write("New content for reading and writing.\n")
    file.seek(0)  # Move the file pointer to the beginning
    print(file.read())  # Read the written content

In this example:

• The file is opened in write and read mode (w+).
• After writing data to the file, the seek(0) method moves the file pointer to the beginning of the file, allowing the program to read the newly written content.

3. a+: Append and Read

The a+ mode opens a file for appending and reading. Data is appended to the file, and you can also read the file’s existing content. If the file doesn’t exist, it is created.

Usage:

file = open("example.txt", "a+")

Example:

with open("output.txt", "a+") as file:
    file.write("\nAppending new content with a+ mode.")
    file.seek(0)  # Move the file pointer to the beginning to read
    print(file.read())  # Read the entire file content

In this example:

• The file is opened in append and read mode (a+).
• New data is appended to the file, and then the file pointer is moved to the beginning to read the complete content of the file.

7.3.7 Common Errors and Exceptions

When working with file modes, certain errors may arise if the mode is not used correctly:

• FileNotFoundError: Raised when attempting to open a file for reading (r or r+) that doesn’t exist.
• PermissionError: Raised if you don’t have the required permissions to read or write to the file.

Example:

try:
    with open("non_existent_file.txt", "r") as file:
        content = file.read()
except FileNotFoundError:
    print("The file was not found.")
except PermissionError:
    print("You don't have permission to access this file.")

In this example:

• The FileNotFoundError exception is caught if the file doesn’t exist.
• The PermissionError is caught if there’s a permission issue.

7.3.8 Summary of File Modes

|
| w | Opens a file for writing. Overwrites the file if it exists. Creates a new file if it doesn’t exist. |
| a | Opens a file for appending. Adds new data at the end without overwriting. Creates a new file if it doesn’t exist. |
| b | Opens a file in binary mode. Combined with other modes (r, w, a). |
| r+ | Opens a file for reading and writing. The file must exist. |
| w+ | Opens a file for writing and reading. Overwrites the file if it exists or creates a new one. |
| a+ | Opens a file for appending and reading. Creates a new file if it doesn’t exist. |

Understanding file modes is crucial for managing files properly in Python. Each mode serves a specific purpose and allows you to control how your program interacts with files, whether you're reading, writing, or appending text or binary data.