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# 18.5. `asyncio` – Asynchronous I/O, event loop, coroutines and tasks

New in version 3.4.

Source code: Lib/asyncio/

Note

The asyncio package has been included in the standard library on a provisional basis. Backwards incompatible changes (up to and including removal of the module) may occur if deemed necessary by the core developers.

This module provides infrastructure for writing single-threaded concurrent code using coroutines, multiplexing I/O access over sockets and other resources, running network clients and servers, and other related primitives. Here is a more detailed list of the package contents:

- a pluggable event loop with various system-specific implementations;
- transport and protocol abstractions (similar to those in Twisted);
- concrete support for TCP, UDP, SSL, subprocess pipes, delayed calls, and others (some may be system-dependent);
- a `Future` class that mimics the one in the `concurrent.futures` module, but adapted for use with the event loop;
- coroutines and tasks based on `yield from` (PEP 380), to help write concurrent code in a sequential fashion;
- cancellation support for `Future`s and coroutines;
- synchronization primitives for use between coroutines in a single thread, mimicking those in the `threading` module;
- an interface for passing work off to a threadpool, for times when you absolutely, positively have to use a library that makes blocking I/O calls.

Asynchronous programming is more complex than classical “sequential” programming: see the Develop with asyncio page which lists common traps and explains how to avoid them. Enable the debug mode during development to detect common issues.

Table of contents:

- 18.5.1. Base Event Loop
  - 18.5.1.1. Run an event loop
  - 18.5.1.2. Calls
  - 18.5.1.3. Delayed calls
  - 18.5.1.4. Futures
  - 18.5.1.5. Tasks
  - 18.5.1.6. Creating connections
  - 18.5.1.7. Creating listening connections
  - 18.5.1.8. Watch file descriptors
  - 18.5.1.9. Low-level socket operations
  - 18.5.1.10. Resolve host name
  - 18.5.1.11. Connect pipes
  - 18.5.1.12. UNIX signals
  - 18.5.1.13. Executor
  - 18.5.1.14. Error Handling API
  - 18.5.1.15. Debug mode
  - 18.5.1.16. Server
  - 18.5.1.17. Handle
  - 18.5.1.18. Event loop examples
    - 18.5.1.18.1. Hello World with call_soon()
    - 18.5.1.18.2. Display the current date with call_later()
    - 18.5.1.18.3. Watch a file descriptor for read events
    - 18.5.1.18.4. Set signal handlers for SIGINT and SIGTERM
- 18.5.2. Event loops
  - 18.5.2.1. Event loop functions
  - 18.5.2.2. Available event loops
  - 18.5.2.3. Platform support
    - 18.5.2.3.1. Windows
    - 18.5.2.3.2. Mac OS X
  - 18.5.2.4. Event loop policies and the default policy
  - 18.5.2.5. Event loop policy interface
  - 18.5.2.6. Access to the global loop policy
- 18.5.3. Tasks and coroutines
  - 18.5.3.1. Coroutines
    - 18.5.3.1.1. Example: Hello World coroutine
    - 18.5.3.1.2. Example: Coroutine displaying the current date
    - 18.5.3.1.3. Example: Chain coroutines
  - 18.5.3.2. InvalidStateError
  - 18.5.3.3. TimeoutError
  - 18.5.3.4. Future
    - 18.5.3.4.1. Example: Future with run_until_complete()
    - 18.5.3.4.2. Example: Future with run_forever()
  - 18.5.3.5. Task
    - 18.5.3.5.1. Example: Parallel execution of tasks
  - 18.5.3.6. Task functions
- 18.5.4. Transports and protocols (callback based API)
  - 18.5.4.1. Transports
    - 18.5.4.1.1. BaseTransport
    - 18.5.4.1.2. ReadTransport
    - 18.5.4.1.3. WriteTransport
    - 18.5.4.1.4. DatagramTransport
    - 18.5.4.1.5. BaseSubprocessTransport
  - 18.5.4.2. Protocols
    - 18.5.4.2.1. Protocol classes
    - 18.5.4.2.2. Connection callbacks
    - 18.5.4.2.3. Streaming protocols
    - 18.5.4.2.4. Datagram protocols
    - 18.5.4.2.5. Flow control callbacks
    - 18.5.4.2.6. Coroutines and protocols
  - 18.5.4.3. Protocol examples
    - 18.5.4.3.1. TCP echo client protocol
    - 18.5.4.3.2. TCP echo server protocol
    - 18.5.4.3.3. UDP echo client protocol
    - 18.5.4.3.4. UDP echo server protocol
    - 18.5.4.3.5. Register an open socket to wait for data using a protocol
- 18.5.5. Streams (coroutine based API)
  - 18.5.5.1. Stream functions
  - 18.5.5.2. StreamReader
  - 18.5.5.3. StreamWriter
  - 18.5.5.4. StreamReaderProtocol
  - 18.5.5.5. IncompleteReadError
  - 18.5.5.6. LimitOverrunError
  - 18.5.5.7. Stream examples
    - 18.5.5.7.1. TCP echo client using streams
    - 18.5.5.7.2. TCP echo server using streams
    - 18.5.5.7.3. Get HTTP headers
    - 18.5.5.7.4. Register an open socket to wait for data using streams
- 18.5.6. Subprocess
  - 18.5.6.1. Windows event loop
  - 18.5.6.2. Create a subprocess: high-level API using Process
  - 18.5.6.3. Create a subprocess: low-level API using subprocess.Popen
  - 18.5.6.4. Constants
  - 18.5.6.5. Process
  - 18.5.6.6. Subprocess and threads
  - 18.5.6.7. Subprocess examples
    - 18.5.6.7.1. Subprocess using transport and protocol
    - 18.5.6.7.2. Subprocess using streams
- 18.5.7. Synchronization primitives
  - 18.5.7.1. Locks
    - 18.5.7.1.1. Lock
    - 18.5.7.1.2. Event
    - 18.5.7.1.3. Condition
  - 18.5.7.2. Semaphores
    - 18.5.7.2.1. Semaphore
    - 18.5.7.2.2. BoundedSemaphore
- 18.5.8. Queues
  - 18.5.8.1. Queue
  - 18.5.8.2. PriorityQueue
  - 18.5.8.3. LifoQueue
    - 18.5.8.3.1. Exceptions
- 18.5.9. Develop with asyncio
  - 18.5.9.1. Debug mode of asyncio
  - 18.5.9.2. Cancellation
  - 18.5.9.3. Concurrency and multithreading
  - 18.5.9.4. Handle blocking functions correctly
  - 18.5.9.5. Logging
  - 18.5.9.6. Detect coroutine objects never scheduled
  - 18.5.9.7. Detect exceptions never consumed
  - 18.5.9.8. Chain coroutines correctly
  - 18.5.9.9. Pending task destroyed
  - 18.5.9.10. Close transports and event loops

See also

The `asyncio` module was designed in PEP 3156. For a motivational primer on transports and protocols, see PEP 3153.

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