BlockingIOError is an exception that occurs when an I/O operation on a non-blocking object would normally block and it is a subclass of OSError. Commonly encountered with non-blocking sockets or file operations, this exception is raised when an I/O operation cannot be completed immediately. It plays a crucial role in asynchronous or event-driven programming by allowing developers to gracefully handle operations that need to be retried later. Let’s understand with the help of an example:

import socket

# Create a non-blocking socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setblocking(False)

try:
    s.connect(('localhost', 8080))
except BlockingIOError:
    print("Operation would block, try again later.")
finally:
    s.close()
    print("Socket closed.")

Operation would block, try again later.
Socket closed.

Explanation: If the connection cannot be established immediately then a BlockingIOError is raised and the output will be as shown above but if the connection succeeds instantly (unlikely for a non-blocking socket) only "Socket closed." will be printed.

Handling BlockingIOError

To handle a BlockingIOError, use a try-except block and catch the error using the BlockingIOError keyword. This error occurs when reading from or writing to a non-blocking resource with no available data. You can handle it by retrying the operation or logging the issue.

Syntax

try:
# Non-blocking I/O operation that may raise BlockingIOError
except BlockingIOError:
# Code to handle the situation when the operation would block
finally:
# Cleanup code that executes regardless of exceptions

Parameters: BlockingIOError is raised without user-supplied parameters however, it carries attributes like errno and strerror that provide details about the error.
Return : As an exception, it does not return a value.

Example:

When reading from a file opened in non-blocking mode, if no data is immediately available then a BlockingIOError may be raised. Handling this exception allows your program to decide whether to retry the operation later.

import os

# Open file in non-blocking mode
fd = os.open("data.txt", os.O_RDONLY | os.O_NONBLOCK)
try:
    content = os.read(fd, 1024)
    print(content.decode())
except BlockingIOError:
    print("No data available yet, try again later.")
finally:
    os.close(fd)
    print("File descriptor closed.")

The output of this code depends on whether the file data.txt has data available for reading or not.

Case 1 - Data is available in data.txt : If data.txt exists and contains some text (e.g., "Hello, world!") then the output will be:

Hello, world!
File descriptor closed.

Case 2 - No data is available yet (Non-blocking mode): If the file exists but no data is available at the moment (common in special files like pipes or sockets), a BlockingIOError is raised and the output will be:

No data available yet, try again later.
File descriptor closed.