Coroutines and await
Explore how Python uses coroutines defined with async def and the await keyword to handle asynchronous tasks. Understand how these concepts improve concurrency by allowing tasks to pause and resume without blocking the main thread, making code more readable and efficient. Discover how to chain coroutines, handle exceptions, and avoid common pitfalls like forgetting to await, preparing you to write scalable asynchronous Python programs.
In the previous lesson, we examined how the event loop schedules and resumes tasks. The next step is to define the tasks managed by the event loop. In standard Python execution, functions run in a strictly linear manner. Once invoked, a function runs from beginning to end without interruption and blocks the thread until it returns. This behavior works well for straightforward computations but becomes inefficient when a function must wait for slow operations such as network requests or database queries. During this time, the thread remains idle.
To address this limitation, Python provides coroutines. A coroutine is a special type of function that can suspend its execution, yield control back to the event loop, and later resume precisely where it left off. This capability allows the program to perform other work while waiting for external events to complete.
Coroutines fundamentally change how concurrent code is structured. Instead of relying on callback chains or complex thread coordination, developers can write code that remains sequential and easier to read. The asynchronous runtime handles task scheduling, suspension, and resumption, allowing code to remain clear while supporting high levels of concurrency.
Defining coroutines with async def
A coroutine is defined using the async def syntax. Although it resembles a standard function definition, its execution semantics are fundamentally different.
When a regular function is called, Python immediately executes its body. In contrast, when an async function is invoked, its body does not run right away. Instead, Python returns a coroutine object. This coroutine object represents a suspended computation, a task that is defined but not yet scheduled for execution. It must be explicitly awaited or passed to the event loop before any of its internal code runs.
We can observe this behavior by calling an async function and inspecting the returned value. ...