Posted on Dec 12, 2023

What does the "yield" keyword do in Python and Javascript?

#javascript #python #programming #beginners

The "yield" keyword in both Python and Javascript plays a key role in working with generators, but with some subtle differences:

Python:

Function creation: yield is used to create generator functions, which are functions that can pause and resume their execution multiple times, yielding values each time they resume. This allows for memory-efficient iteration over large datasets or lazy evaluation of expressions.
Pausing and resuming: Within a generator function, yield pauses execution and returns a value to the caller. The function can then resume execution when the caller requests the next value using an iterator interface.

def fibonacci(n): a, b = 0, 1 for _ in range(n): yield a a, b = b, a + b # Use the generator with a for loop for num in fibonacci(10): print(num)

This code generates the first 10 Fibonacci numbers without storing them all in memory at once.

Javascript:

Async iteration: yield is used with the async and await keywords to pause and resume execution in asynchronous code. It allows for iterating over asynchronous operations one by one, similar to a regular loop.
Returning values: Similar to Python, yield within an async generator function returns a value to the caller. However, it doesn't pause execution; instead, it signals that the next value is ready when the caller resumes.

async function fetchUserRepos(username) { const response = await fetch(`https://api.github.com/users/${username}/repos`); const repos = await response.json(); for (const repo of repos) { yield repo.name; } } (async () => { const repoNames = []; for await (const name of fetchUserRepos('<insert_userName_here>')) { repoNames.push(name); } console.log(repoNames); // Prints all repository names })();

This code fetches a user's repositories from Github and iterates over them one by one, waiting for each repository to be fetched before proceeding.

Key differences:

Feature	Python	Javascript
Execution	Synchronous	Asynchronous
Value consumption	`for` loop or iterators	`next()` method
Generator completion	`return` statement or end of function	`return` statement or end of function

Purpose:

In Python, yield is primarily used for memory-efficient iteration and lazy evaluation. In Javascript, it's used for asynchronous iteration and managing control flow in async functions.