ValueError

ValueError

ValueError: generator already executing

Traceback

terminal

Traceback (most recent call last):
  File "main.py", line 5, in <module>
    next(g)
ValueError: generator already executing

What causes this error

Code attempted to advance (next/send/throw) a generator that is currently in the middle of executing. This happens in reentrant scenarios where the generator tries to advance itself.

How to fix it

Restructure code to avoid re-entering the generator. Use a queue to decouple production and consumption. Ensure generators are not shared between threads without synchronization.

Code that causes this error

Broken

def gen():
    yield next(g)  # trying to advance itself

g = gen()
next(g)

Fixed code

Fixed

def gen():
    yield 1
    yield 2

g = gen()
print(next(g))  # 1
print(next(g))  # 2

About ValueError

This ValueError is raised when you try to send a value to or call next() on a generator that is already running. This happens in reentrant scenarios where code inside the generator tries to advance the same generator — for example, a generator that references itself through a callback or a signal handler. Generators are designed for cooperative, sequential execution: you advance them one step at a time, and they yield back control.

Trying to advance a generator from within itself creates an impossible state. This error is rare in normal code but can appear in complex callback-driven architectures, signal handlers, and thread-unsafe generator usage. The fix is to restructure the code to avoid re-entering the generator — use a queue or buffer to decouple the producer and consumer.

Common scenarios

Infinite recursion from missing or incorrect base cases

Modifying dictionaries or sets during iteration

Calling generators or coroutines in unsupported ways

Using asyncio event loops incorrectly or attempting to nest them