Python - How select.select() works?

Python - How select.select() works?

select.select() is a function provided by the select module in Python that is used for monitoring multiple file objects (such as sockets or file descriptors) for I/O readiness. It allows you to efficiently manage multiple I/O operations without the need for creating separate threads or processes for each operation. select.select() is commonly used for building event-driven, non-blocking I/O applications.

Here's how select.select() works:

  1. Input Lists:

    select.select() takes three lists as arguments: rlist, wlist, and xlist. These lists contain file objects (sockets, pipes, or file descriptors) that you want to monitor for read, write, or exception events, respectively.

    • rlist (read list): A list of file objects that you want to check for readability.
    • wlist (write list): A list of file objects that you want to check for writability.
    • xlist (exception list): A list of file objects that you want to check for exceptional conditions (e.g., out-of-band data for sockets).

  2. Output Lists:

    select.select() returns three lists: rlist, wlist, and xlist. These lists contain the file objects that are ready for the respective I/O operation (read, write, or exception).

    • rlist: Contains the file objects from rlist that are ready for reading without blocking.
    • wlist: Contains the file objects from wlist that are ready for writing without blocking.
    • xlist: Contains the file objects from xlist that have exceptional conditions.

  3. Blocking Behavior:

    When you call select.select(), it will block (i.e., pause the execution of your program) until at least one file object in one of the input lists becomes ready for the requested I/O operation (read, write, or exception) or until a timeout period expires.

  4. Timeout:

    You can specify a timeout value as the last argument to select.select(). This value determines the maximum time select.select() will block before returning, even if no file object becomes ready during that time. A timeout value of 0 means non-blocking, and the function will return immediately.

  5. Return Values:

    select.select() returns the three output lists (rlist, wlist, xlist) that contain the file objects that are ready for the respective I/O operations. You can use these lists to perform the actual I/O operations on the ready file objects.

Here's a simple example:

import select import socket server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server_socket.bind(('localhost', 12345)) server_socket.listen(5) rlist = [server_socket] wlist = [] xlist = [] while True: readable, writable, exceptional = select.select(rlist, wlist, xlist, timeout=1) for sock in readable: if sock is server_socket: client_socket, addr = server_socket.accept() rlist.append(client_socket) else: data = sock.recv(1024) if data: # Handle data received from client pass else: # Client closed the connection sock.close() rlist.remove(sock)

In this example, select.select() is used to monitor the server_socket for incoming connections (readable list). When a client connects, it is added to the rlist, and its data can be read when ready. This allows the server to handle multiple clients without blocking on I/O operations.

Examples

  1. "Python select.select() example"

    • Description: This query likely seeks examples demonstrating the usage of select.select() in Python.
    import select import socket # Create a TCP/IP socket server = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server.setblocking(0) # Bind the socket to the address and port server_address = ('localhost', 10000) server.bind(server_address) # Listen for incoming connections server.listen(5) inputs = [server] outputs = [] while inputs: readable, writable, exceptional = select.select(inputs, outputs, inputs) for s in readable: if s is server: connection, client_address = s.accept() connection.setblocking(0) inputs.append(connection) else: data = s.recv(1024) if data: print("Received:", data.decode()) else: inputs.remove(s) s.close()

  2. "Python non-blocking socket select example"

    • Description: This query may aim to find examples illustrating non-blocking socket operations using select.select() in Python.
    import select import socket # Create a non-blocking socket server = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server.setblocking(0) # Bind the socket to the address and port server_address = ('localhost', 10000) server.bind(server_address) # Listen for incoming connections server.listen(5) inputs = [server] outputs = [] while inputs: readable, writable, exceptional = select.select(inputs, outputs, inputs) for s in readable: if s is server: connection, client_address = s.accept() connection.setblocking(0) inputs.append(connection) else: data = s.recv(1024) if data: print("Received:", data.decode()) else: inputs.remove(s) s.close()

  3. "Python asynchronous I/O with select.select()"

    • Description: This query might seek information on how to implement asynchronous I/O using select.select() in Python.
    import select import socket # Create a non-blocking socket server = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server.setblocking(0) # Bind the socket to the address and port server_address = ('localhost', 10000) server.bind(server_address) # Listen for incoming connections server.listen(5) inputs = [server] outputs = [] while inputs: readable, writable, exceptional = select.select(inputs, outputs, inputs) for s in readable: if s is server: connection, client_address = s.accept() connection.setblocking(0) inputs.append(connection) else: data = s.recv(1024) if data: print("Received:", data.decode()) else: inputs.remove(s) s.close()

  4. "Python select.select() timeout example"

    • Description: This query may be looking for examples demonstrating how to implement a timeout using select.select() in Python.
    import select import socket # Create a non-blocking socket server = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server.setblocking(0) # Bind the socket to the address and port server_address = ('localhost', 10000) server.bind(server_address) # Listen for incoming connections server.listen(5) inputs = [server] outputs = [] while inputs: readable, writable, exceptional = select.select(inputs, outputs, inputs, timeout=1) if not (readable or writable or exceptional): print("Timeout occurred.") break for s in readable: if s is server: connection, client_address = s.accept() connection.setblocking(0) inputs.append(connection) else: data = s.recv(1024) if data: print("Received:", data.decode()) else: inputs.remove(s) s.close()

  5. "Python select.select() vs. epoll()"

    • Description: This query might compare select.select() with epoll() for handling I/O operations in Python.
    # Code comparing select.select() with epoll() is more complex and requires # usage of the select and selectors modules respectively. Below is a simplified example. import select import selectors import socket # Using select.select() r, w, x = select.select([server_socket], [], []) for sock in r: data = sock.recv(1024) # Process received data # Using selectors.epoll() sel = selectors.DefaultSelector() sel.register(server_socket, selectors.EVENT_READ) for key, mask in sel.select(): sock = key.fileobj data = sock.recv(1024) # Process received data

  6. "Python select.select() multiple sockets"

    • Description: This query could be looking for examples on how to handle multiple sockets using select.select() in Python.
    import select import socket # Create multiple non-blocking sockets server1 = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server1.setblocking(0) server2 = socket.socket(socket.AF_INET, socket.SOCK_STREAM) server2.setblocking(0) # Bind the sockets to addresses and ports server_address1 = ('localhost', 10000) server1.bind(server_address1) server_address2 = ('localhost', 20000) server2.bind(server_address2) # Listen for incoming connections server1.listen(5) server2.listen(5) inputs = [server1, server2] outputs = [] while inputs: readable, writable, exceptional = select.select(inputs, outputs, inputs) for s in readable: if s in (server1, server2): connection, client_address = s.accept() connection.setblocking(0) inputs.append(connection) else: data = s.recv(1024) if data: print("Received:", data.decode()) else: inputs.remove(s) s.close()

  7. "Python select.select() for UDP sockets"

    • Description: This query may aim to find examples of using select.select() with UDP sockets in Python.
    import select import socket # Create a UDP socket udp_socket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) udp_socket.bind(('localhost', 10000)) inputs = [udp_socket] outputs = [] while inputs: readable, writable, exceptional = select.select(inputs, outputs, inputs) for s in readable: if s is udp_socket: data, address = s.recvfrom(1024) print("Received:", data.decode())

  8. "Python select.select() with stdin"

    • Description: This query might be interested in examples of using select.select() with standard input in Python.
    import select import sys while True: r, w, x = select.select([sys.stdin], [], []) if sys.stdin in r: user_input = sys.stdin.readline().strip() print("You entered:", user_input)

  9. "Python select.select() with serial ports"

    • Description: This query may seek examples demonstrating how to use select.select() with serial ports in Python.
    import select import serial # Open a serial port ser = serial.Serial('/dev/ttyUSB0', 9600) inputs = [ser] outputs = [] while inputs: readable, writable, exceptional = select.select(inputs, outputs, inputs) for s in readable: if s is ser: data = ser.read() print("Received:", data.decode())

  10. "Python select.select() for file I/O"

    • Description: This query could be interested in examples demonstrating how to use select.select() for file I/O operations in Python.
    import select import os # Open a file with open('example.txt', 'rb') as file: inputs = [file] outputs = [] while inputs: readable, writable, exceptional = select.select(inputs, outputs, inputs) for s in readable: data = s.read() if data: print("Read:", data.decode()) else: inputs.remove(s) s.close()

More Tags

angular-cdk to-date dynamics-crm-2013 documentfile npx formik icmp libx265 fileloadexception xmllint

More Python Questions

More Financial Calculators

More Statistics Calculators

More Internet Calculators

More Cat Calculators