The Python yield keyword

What’s the Python yield keyword for? See notes / FastAPI with Chroma for a real world use case.

W3Schools yield tutorial #

The yield keyword turns a function into a function generator. The function generator returns an iterator.

The code in the function is not executed when it is first called, but is divided into steps, one step for each yield. Each step is executed when iterated upon. The function’s return values will be a list of values, one value for each yield.¹

In [1]: def my_func():
   ...:     yield "This"
   ...:     yield "is"
   ...:     yield "a"
   ...:     yield "test"
   ...:

In [2]: x = my_func()

In [4]: for i in x:
   ...:     print(i)
   ...:
This
is
a
test

Python 3 documentation — yield expressions #

yield expressions are used to define a generator function or an asynchronous generator function. They can only be used in the body of a function definition.

Yield expressions in async functions #

Using a yield expression in an async def function’s body causes that coroutine function to be an asynchronous generator function. Python docs: https://docs.python.org/3/reference/expressions.html#asynchronous-generator-functions .

References #

Python 3.14.2 Documentation. “6. Expressions — Yield expressions.” Last updated: January 10, 2026. https://docs.python.org/3/reference/expressions.html#yield-expressions .