# Python Select Module

The `select` module provides access to platform-specific I/O monitoring functions. It allows you to wait for I/O completion on multiple file descriptors (like sockets or files) simultaneously. This is known as I/O multiplexing.

## Importing the Module

```python
import select
```

## `select.select()`

The most common function is `select()`, which is available on most operating systems. It waits until one or more file descriptors are ready for some kind of I/O.

### Syntax

```python
readable, writable, exceptional = select.select(rlist, wlist, xlist[, timeout])
```

*   `rlist`: Wait until ready for reading.
*   `wlist`: Wait until ready for writing.
*   `xlist`: Wait for an "exceptional condition".
*   `timeout`: Time in seconds to wait (float). If omitted, it blocks until at least one file descriptor is ready.

### Example: Monitoring Stdin and Sockets

```python
import select
import socket
import sys

# Create a TCP/IP socket
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.setblocking(0)
server.bind(('localhost', 10000))
server.listen(5)

inputs = [server, sys.stdin]
outputs = []

while inputs:
    readable, writable, exceptional = select.select(inputs, outputs, inputs)

    for s in readable:
        if s is server:
            # A "readable" server socket is ready to accept a connection
            connection, client_address = s.accept()
            connection.setblocking(0)
            inputs.append(connection)
        elif s is sys.stdin:
            # Handle standard input
            print(sys.stdin.readline())
        else:
            data = s.recv(1024)
            if data:
                print(f"Received: {data}")
            else:
                inputs.remove(s)
                s.close()
```

## `poll()` and `epoll()`

On Linux and Unix systems, `poll()` and `epoll()` are available and are generally more efficient than `select()` for large numbers of file descriptors.

### `select.poll()`

```python
import select

# Only available on Unix/Linux
if hasattr(select, 'poll'):
    p = select.poll()
    # p.register(fd, eventmask)
    # events = p.poll(timeout)
```

### `select.epoll()`

Available on Linux 2.5.44 and newer.

```python
import select

# Only available on Linux
if hasattr(select, 'epoll'):
    ep = select.epoll()
    # ep.register(fd, eventmask)
    # events = ep.poll(timeout)
```

[[programming/python/python]]