What is a generator's lifecycle? | Interview Question

Q: What is a generator's lifecycle?

**Target Audience:** Python developers who use generators and want to know exactly what state one is in, and what happens to its clean-up code **Difficulty:** Medium **Python version:** Checked on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); the version-specific example on CPython 3.13, with 3.12 compared > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact word an interviewer expects. Every output block in the **Verified** section was copied from a real run of the examples on this page. ## 1. Why This Even Matters — A Story First A video game has a save point. You buy the game: it is in the box, nothing has happened yet. You start playing: the game is running. You hit pause: everything freezes exactly where it is, every enemy and every item, and you can come back tomorrow and carry on. Eventually you either finish the game or quit; either way, that playthrough is

The problem

Question presented to candidate: "A generator object goes through several states between the moment you call the function and the moment it is gone. Walk me through them. What moves it from one state to the next, and what happens to code in a finally block if the consumer stops early?"

What a strong answer should cover:

The four states reported by inspect.getgeneratorstate: GEN_CREATED (body not started), GEN_RUNNING (executing right now), GEN_SUSPENDED (paused at a yield), GEN_CLOSED (finished); a generator never comes back from closed
What moves it: next() / send(value) resume it (and send must be None on a just-created generator); yield suspends it; return or an uncaught exception closes it, with the return value carried on StopIteration.value
throw(exc) raises an exception at the paused yield; close() raises GeneratorExit there, so finally blocks and with exits run; a generator that yields again after GeneratorExit gets RuntimeError
Dropping the last reference to a suspended generator closes it as well (in CPython this happens immediately through reference counting); break out of a for loop leaves it suspended, not closed
PEP 479 (default since 3.7): a StopIteration escaping the body is turned into RuntimeError; version details such as the three-argument throw() being deprecated in 3.12, gi_suspended (3.11) and close() returning a value (3.13)
Introspection: gi_frame (None once closed), gi_running, gi_frame.f_locals

Code / implementation expected: Optional — a generator with a try/finally, printing inspect.getgeneratorstate before, during and after, and calling close().

generators
lifecycle

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Python developers who use generators and want to know exactly what state one is in, and what happens to its clean-up code Difficulty: Medium Python version: Checked on CPython 3.11 (and in this page on Pyodide 0.25.1, which is CPython 3.11.3); the version-specific example on CPython 3.13, with 3.12 compared

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact word an interviewer expects. Every output block in the Verified section was copied from a real run of the examples on this page.

1. Why This Even Matters — A Story First

A video game has a save point. You buy the game: it is in the box, nothing has happened yet. You start playing: the game is running. You hit pause: everything freezes exactly where it is, every enemy and every item, and you can come back to

The solution is hidden so you can try first

13 min read · 10 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonThe four states, send(), return values and gi_frame

Practice

Pythonclose(), del, throw(), GeneratorExit and PEP 479

Practice

PythonDeprecated three-argument throw() and close() returning a value (Python 3.13)

import sys
import warnings

print("Python", sys.version_info[:2])

def gen():
    try:
        yield 1
    except ValueError as e:
        yield f"caught {e}"

# The one-argument form: pass an exception instance.
g = gen()
next(g)
print("one-arg throw ->", g.throw(ValueError("boom")))

# The old three-argument form still works but warns on 3.12+.
g = gen()
next(g)
with warnings.catch_warnings(record=True) as caught:
    warnings.simplefilter("always")
    result = g.throw(ValueError, ValueError("old style"), None)
print("three-arg throw ->", result)
for w in caught:
    print(w.category.__name__ + ":", w.message)

# Since 3.13, close() returns the value a generator returns
# after catching GeneratorExit (on 3.12 and earlier it returns None).
def summer():
    total = 0
    try:
        while True:
            total += yield
    except GeneratorExit:
        return total

s = summer()
next(s)
for n in (5, 10, 20):
    s.send(n)
print("close() returned:", s.close())

Discussion

Other approaches, follow-ups and how it went in real interviews.

Sign in to join the discussion.

No comments yet. Share how you would answer it.

The solution is hidden so you can try first

Keep your momentum going.