Table of Contents
- Overview
- Prerequisites
- Setup and Software
- Async and Await in Python
- Basic Example
- Coroutines and Tasks
- Awaiting Multiple Coroutines
- Error Handling
- Common Pitfalls and Troubleshooting
- Frequently Asked Questions
- Conclusion
Overview
In Python, the async and await keywords are used to write asynchronous code that allows concurrent execution of tasks. This comprehensive guide will introduce you to the concepts and functionality of async and await, and show you how to leverage them in your Python programs. By the end of this tutorial, you will have a solid understanding of how to write efficient, non-blocking code using async and await.
Prerequisites
To follow along with this tutorial, you should have a basic understanding of Python syntax and programming concepts. Familiarity with functions and basic knowledge of asynchronous programming will be beneficial, but not essential.
Setup and Software
To get started, ensure you have Python 3.7 or newer installed on your system. You can check the version by opening a terminal and running the command:
bash
python --version
If you don’t have Python installed or need to update to a newer version, you can download it from the official Python website: https://www.python.org/downloads/
Async and Await in Python
Asynchronous programming allows the execution of multiple tasks concurrently without blocking the main program. Python’s async and await keywords, introduced in Python 3.5, provide a way to define and work with asynchronous code.
The async keyword is used to define asynchronous functions, also known as coroutines. These functions can be paused and resumed, allowing other tasks to run in the meantime. The awaite keyword is used inside an asynchronous function to wait for the completion of another coroutine.
Basic Example
Let’s start with a basic example to understand how async and await work. Create a new Python file, async_example.py, and add the following code:
```python
import asyncio
async def greet():
print("Hello")
await asyncio.sleep(1)
print("World")
asyncio.run(greet())
``` In this example, we define an asynchronous function called `greet()` that prints "Hello", waits for one second using `asyncio.sleep()`, and then prints "World". The `await asyncio.sleep(1)` line suspends the execution of the `greet()` coroutine for one second without blocking the program.
To run this example, open a terminal, navigate to the directory containing async_example.py, and execute the following command:
bash
python async_example.py
The output should be:
Hello
[waits for one second]
World
This demonstrates the basic usage of async and await keywords to create an asynchronous function.
Coroutines and Tasks
In Python, coroutines are special types of functions that can be paused and resumed. Async functions act as coroutines when defined with the async keyword. Coroutines are typically created using the async def syntax.
To execute a coroutine, you need an event loop. The asyncio module provides an event loop that manages the execution of coroutines. To run an async function, you can use asyncio.run() which creates a new event loop, runs the coroutine, and then closes the loop.
For more flexibility, you can wrap the coroutine in a Task object. A Task is a subclass of Future that represents an eventual result of the coroutine. Tasks can be used to schedule multiple asynchronous functions to run concurrently.
Here’s an example that demonstrates executing an async function as a Task:
```python
import asyncio
async def my_task():
print("Running my task")
async def main():
task = asyncio.create_task(my_task())
await task
asyncio.run(main())
``` In this example, we define an async function called `my_task()`. We then create a `Task` using `asyncio.create_task()` and pass the `my_task()` coroutine as an argument. Finally, we await the `Task` using `await` to ensure it completes before the program exits.
Awaiting Multiple Coroutines
To await multiple coroutines, you can use asyncio.gather() to combine them into a single awaitable. Here’s an example that demonstrates awaiting multiple coroutines:
```python
import asyncio
async def coroutine_1():
await asyncio.sleep(1)
print("Coroutine 1")
async def coroutine_2():
await asyncio.sleep(2)
print("Coroutine 2")
async def main():
await asyncio.gather(coroutine_1(), coroutine_2())
asyncio.run(main())
``` In this example, we define two coroutines, `coroutine_1()` and `coroutine_2()`. We use `await asyncio.gather()` to await both coroutines concurrently. The `gather()` function takes a variable number of awaitables and returns a single awaitable that completes when all input awaitables are done.
Error Handling
When working with asynchronous code, it’s important to handle and propagate errors properly. To handle exceptions within a coroutine, you can use a try/except block as you would in a regular synchronous code. Additionally, you can use asyncio.gather() to handle exceptions thrown by multiple coroutines.
Here’s an example that demonstrates error handling in asyncio: ```python import asyncio
async def my_task():
await asyncio.sleep(1)
raise ValueError("Something went wrong!")
async def main():
task = asyncio.create_task(my_task())
try:
await task
except ValueError as e:
print(f"Caught exception: {e}")
asyncio.run(main())
``` In this example, the `my_task()` coroutine intentionally raises a `ValueError`. We use a `try`/`except` block to catch the exception when awaiting the `Task`. The `except` block prints the caught exception.
Common Pitfalls and Troubleshooting
When working with async and await, there are a few common pitfalls you might encounter:
-
Not awaiting an async function: Remember to always use
awaitwhen calling an async function. Forgetting toawaitwill result in the function executing synchronously, blocking the event loop. -
Mixing synchronous and asynchronous code: Avoid mixing synchronous and asynchronous code without proper handling. Mixing them incorrectly can lead to blocking the event loop or unexpected behavior.
-
Using
asyncio.sleep()instead ofawait asyncio.sleep(): Ensure youawaittheasyncio.sleep()function when you want to pause the execution of a coroutine. Not usingawaitwill cause the function to continue synchronously.
Frequently Asked Questions
Q: Can I mix async/await with regular synchronous code?
A: Yes, you can mix async/await with regular synchronous code. However, you need to be careful when doing so and ensure you handle the interaction appropriately. Mixing them incorrectly can lead to blocking the event loop.
Q: Are async/await only used for network operations?
A: No, async/await can be used for any I/O-bound operation, including network operations, file system operations, and interacting with databases.
Q: Is Python’s asyncio module the only way to write asynchronous code?
A: No, Python provides other libraries and frameworks, such as trio and Tornado, that also support asynchronous programming.
Conclusion
In this tutorial, you learned about Python’s async and await keywords and how to write asynchronous code. You explored the basic usage of async and await, creating coroutines and tasks, awaiting multiple coroutines, error handling, and common pitfalls. With this knowledge, you can now leverage the power of asynchronous programming to write more efficient and responsive Python applications.