Python BlockingIOError — What It Means & How to Fix It

What Does This Error Mean?

BlockingIOError means your code tried to do an I/O operation — like reading from a socket or pipe — but the resource was not ready yet and your code was set up to not wait. Instead of blocking (pausing until data arrives), Python raises this error immediately. This is most common in network programming and async code that uses non-blocking sockets.

Affected Models

Python 3.x
Network and socket programming
Async I/O code

Common Causes

A socket was set to non-blocking mode using socket.setblocking(False), and a read or write was attempted before data was available
Using os.read() or os.write() on a file descriptor that has been put into non-blocking mode
Calling socket.recv() on a non-blocking socket when the send buffer is full
Using asyncio or other async frameworks incorrectly — mixing blocking calls with non-blocking resources
A pipe or FIFO that has no data available and was opened in non-blocking mode

How to Fix It

Understand whether you actually need non-blocking I/O. If you are writing a simple script, switch the socket back to blocking mode: socket.setblocking(True). This removes the error entirely.

Non-blocking mode is only needed for high-performance servers handling thousands of connections simultaneously. Most scripts do not need it.
If you need non-blocking mode, catch BlockingIOError (or its alias EAGAIN/EWOULDBLOCK) and retry the operation when the resource becomes ready.

Example: try: data = sock.recv(1024) except BlockingIOError: pass — then use select() or poll() to wait until data is available.
Use the select module to wait until a socket is ready before reading or writing. select.select([sock], [], []) blocks until the socket has data to read.

This is the traditional approach for non-blocking I/O without an async framework.
If you are using asyncio, replace synchronous socket calls with await-based equivalents. Use asyncio.open_connection() instead of raw sockets.

Mixing raw non-blocking sockets with asyncio is a common source of confusion. Let asyncio manage the I/O loop entirely.
Check the errno attribute on the exception. errno.EAGAIN and errno.EWOULDBLOCK both mean 'not ready yet, try again'. Handle them the same way.

On Windows, the equivalent error code is errno.WSAEWOULDBLOCK. Cross-platform code should check for all three.

When to Call a Professional

BlockingIOError is always a code design issue — you are mixing non-blocking I/O with synchronous-style calls. You can fix it yourself by either switching to blocking mode, or properly handling the would-block condition in your async loop.

Frequently Asked Questions

What is the difference between BlockingIOError and TimeoutError?

BlockingIOError means the operation could not start at all because the resource was not ready right now. TimeoutError means the operation started but took too long to complete. BlockingIOError is immediate — it does not wait at all. TimeoutError means you did wait, but not long enough.

Is BlockingIOError the same as EAGAIN?

Yes. On Unix systems, when a non-blocking I/O call would have blocked, the OS returns the EAGAIN error code. Python maps this to BlockingIOError. You may also see EWOULDBLOCK, which Python also maps to BlockingIOError — they are functionally the same.

Should I just set all my sockets to blocking mode to avoid this?

For most scripts and applications, yes — blocking mode is simpler and perfectly fine. Non-blocking mode only matters when you need to handle many connections at once in a single thread. If you are using asyncio or a framework like aiohttp, the framework handles non-blocking I/O for you — you do not need to manage it manually.