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[2026-03-24] [email protected] globals + child threads
Raw
README.md

Handling flask.g in a child thread

Solution

In order to spawn a child thread via _EXECUTOR.submit() or threading.Thread(target=...), the target function needs to be wrapped with flask.copy_current_request_context(). That only copies the Flask context, and does not copy flask.g. So in order to preserve flask.g as well we need to copy flask.g.__dict__ and then place it back on the flask.g in the new context. This is nuts!

def _make_task_closure(func: Callable[P, R]) -> Callable[P, R]:
    g_values = flask.g.__dict__.copy()

    @functools.wraps(func)
    def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
        # NOTE: The `wrapper()` is expected to be called in a new execution context, so `flask.g`
        #       below is within that context, not within the context that **WRAPPED** `func`.
        flask.g.__dict__.update(g_values)
        return func(*args, **kwargs)

    return flask.copy_current_request_context(wrapper)

Evidence

Running the app via uv run via_flask_g.py we can see the impact of the copy (or the absence of the copy).

When flask.g.__dict__ gets updated, we see the executor_result and thread_result reflecting that they are exactly incremented from the squishy value:

$ curl --include http://localhost:5001/
HTTP/1.0 200 OK
Content-Type: application/json
Content-Length: 75
Server: Werkzeug/2.0.3 Python/3.12.12
Date: Tue, 24 Mar 2026 19:39:17 GMT

{"copy":true,"executor_result":78655,"squishy":8655,"thread_result":18655}

and when we skip the copy, they fall back to default values:

$ curl --include http://localhost:5001/?copy=false
HTTP/1.0 200 OK
Content-Type: application/json
Content-Length: 72
Server: Werkzeug/2.0.3 Python/3.12.12
Date: Tue, 24 Mar 2026 19:39:19 GMT

{"copy":false,"executor_result":42,"squishy":6290,"thread_result":1337}

Including contextvars too

In addition to copying the Flask context, looking to the future we may also want to capture contextvars:

def _make_task_closure(func: Callable[P, R]) -> Callable[P, R]:
    outer_ctx = contextvars.copy_context()
    g_values = flask.g.__dict__.copy()

    @functools.wraps(func)
    def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
        flask.g.__dict__.update(g_values)
        return outer_ctx.run(func, *args, **kwargs)

    return flask.copy_current_request_context(wrapper)

We can confirm that pulling either from contextvars or from flask.g we get the desired outcome here:

$ curl --include 'http://localhost:5002/?source=contextvars'
HTTP/1.0 200 OK
Content-Type: application/json
Content-Length: 98
Server: Werkzeug/2.0.3 Python/3.12.12
Date: Tue, 24 Mar 2026 20:39:07 GMT

{"copy":true,"executor_result":77802,"source":"contextvars","squishy":7802,"thread_result":17802}
$
$
$ curl --include 'http://localhost:5002/?source=flask.g'
HTTP/1.0 200 OK
Content-Type: application/json
Content-Length: 94
Server: Werkzeug/2.0.3 Python/3.12.12
Date: Tue, 24 Mar 2026 20:39:09 GMT

{"copy":true,"executor_result":73418,"source":"flask.g","squishy":3418,"thread_result":13418}

and if we set ?copy=false, both of contextvars and flask.g leave the value unset

$ curl --include 'http://localhost:5002/?source=contextvars&copy=false'
HTTP/1.0 200 OK
Content-Type: application/json
Content-Length: 95
Server: Werkzeug/2.0.3 Python/3.12.12
Date: Tue, 24 Mar 2026 20:39:28 GMT

{"copy":false,"executor_result":42,"source":"contextvars","squishy":2798,"thread_result":1337}
$
$
$ curl --include 'http://localhost:5002/?source=flask.g&copy=false'
HTTP/1.0 200 OK
Content-Type: application/json
Content-Length: 91
Server: Werkzeug/2.0.3 Python/3.12.12
Date: Tue, 24 Mar 2026 20:39:33 GMT

{"copy":false,"executor_result":42,"source":"flask.g","squishy":3488,"thread_result":1337}
Raw
pyproject.toml
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requires-python = "==3.12.12"
dependencies = [
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"jinja2==3.0.3",
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Raw
uv.lock
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Raw
via_contextvars.py
import concurrent.futures
import contextvars
import functools
import random
import threading
from collections.abc import Callable
from typing import Literal, ParamSpec, TypeVar
import flask
P = ParamSpec("P")
R = TypeVar("R")
app = flask.Flask(__name__)
_EXECUTOR = concurrent.futures.ThreadPoolExecutor(max_workers=1)
_SQUISHY = contextvars.ContextVar[int | None]("squishy", default=None)
_Source = Literal["contextvars", "flask.g"]
def _determine_copy(copy_args: list[str]) -> bool:
if len(copy_args) != 1:
return True
copy_arg = copy_args[0].lower()
return copy_arg in ("1", "true", "yes")
def _determine_source(source_args: list[str]) -> _Source:
if len(source_args) != 1:
return "contextvars"
source_arg = source_args[0].lower()
if source_arg != "flask.g":
return "contextvars"
return "flask.g"
def _maybe_get_squishy(source: _Source) -> int | None:
if source == "contextvars":
value = _SQUISHY.get()
elif source == "flask.g":
value = getattr(flask.g, "_squishy", None)
else:
raise NotImplementedError("Unsupported source", source)
if value is None:
return None
if not isinstance(value, int):
raise TypeError("Squishy must be integer", type(value), value)
return value
def _get_squishy(source: _Source) -> int:
squishy = _maybe_get_squishy(source)
if squishy is None:
raise ValueError("Required squishy")
return squishy
@app.before_request
def _before_request_hook() -> None:
squishy = random.randint(1000, 9999)
flask.g._squishy = squishy
_SQUISHY.set(squishy)
class _Container:
def __init__(self) -> None:
self.value: int | None = None
def _thread_func(container: _Container, source: _Source) -> None:
squishy = _maybe_get_squishy(source)
if squishy is None:
container.value = 1337
else:
container.value = squishy + 10000
def _executor_func(source: _Source) -> int:
squishy = _maybe_get_squishy(source)
if squishy is None:
return 42
return squishy + 70000
def _make_task_closure(func: Callable[P, R], copy: bool) -> Callable[P, R]:
if not copy:
return flask.copy_current_request_context(func)
outer_ctx = contextvars.copy_context()
g_values = flask.g.__dict__.copy()
@functools.wraps(func)
def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
flask.g.__dict__.update(g_values)
return outer_ctx.run(func, *args, **kwargs)
return flask.copy_current_request_context(wrapper)
@app.route("/")
def hello_world():
copy = _determine_copy(flask.request.args.getlist("copy"))
source = _determine_source(flask.request.args.getlist("source"))
squishy = _get_squishy(source)
copied_executor_func = _make_task_closure(_executor_func, copy=copy)
future = _EXECUTOR.submit(copied_executor_func, source)
executor_result = future.result()
container = _Container()
copied_thread_func = _make_task_closure(_thread_func, copy=copy)
t = threading.Thread(target=copied_thread_func, args=(container, source))
t.start()
t.join()
thread_result = container.value
return {
"copy": copy,
"source": source,
"squishy": squishy,
"executor_result": executor_result,
"thread_result": thread_result,
}
def main() -> None:
port = 5002
host = "0.0.0.0"
app.run(host=host, port=port)
if __name__ == "__main__":
main()
# uv run via_contextvars.py
Raw
via_flask_g.py
import concurrent.futures
import contextvars
import functools
import random
import threading
from collections.abc import Callable
from typing import ParamSpec, TypeVar
import flask
P = ParamSpec("P")
R = TypeVar("R")
app = flask.Flask(__name__)
_EXECUTOR = concurrent.futures.ThreadPoolExecutor(max_workers=1)
_SQUISHY = contextvars.ContextVar[int | None]("squishy", default=None)
def _determine_copy(copy_args: list[str]) -> bool:
if len(copy_args) != 1:
return True
copy_arg = copy_args[0].lower()
return copy_arg in ("1", "true", "yes")
def _maybe_get_squishy() -> int | None:
value = _SQUISHY.get()
if value is None:
return None
if not isinstance(value, int):
raise TypeError("Squishy must be integer", type(value), value)
return value
def _get_squishy() -> int:
squishy = _maybe_get_squishy()
if squishy is None:
raise ValueError("Required squishy")
return squishy
@app.before_request
def _before_request_hook() -> None:
squishy = random.randint(1000, 9999)
flask.g._squishy = squishy
_SQUISHY.set(squishy)
class _Container:
def __init__(self) -> None:
self.value: int | None = None
def _thread_func(container: _Container) -> None:
squishy = _maybe_get_squishy()
if squishy is None:
container.value = 1337
else:
container.value = squishy + 10000
def _executor_func() -> int:
squishy = _maybe_get_squishy()
if squishy is None:
return 42
return squishy + 70000
def _make_task_closure(func: Callable[P, R], copy: bool) -> Callable[P, R]:
g_values = flask.g.__dict__.copy()
@functools.wraps(func)
def wrapper(*args: P.args, **kwargs: P.kwargs) -> R:
# NOTE: The `wrapper()` is expected to be called in a new execution context, so `flask.g`
# below is within that context, not within the context that **WRAPPED** `func`.
if copy:
flask.g.__dict__.update(g_values)
return func(*args, **kwargs)
return flask.copy_current_request_context(wrapper)
@app.route("/")
def hello_world():
copy = _determine_copy(flask.request.args.getlist("copy"))
squishy = _get_squishy()
copied_executor_func = _make_task_closure(_executor_func, copy=copy)
future = _EXECUTOR.submit(copied_executor_func)
executor_result = future.result()
container = _Container()
copied_thread_func = _make_task_closure(_thread_func, copy=copy)
t = threading.Thread(target=copied_thread_func, args=(container,))
t.start()
t.join()
thread_result = container.value
return {
"copy": copy,
"squishy": squishy,
"executor_result": executor_result,
"thread_result": thread_result,
}
def main() -> None:
port = 5001
host = "0.0.0.0"
app.run(host=host, port=port)
if __name__ == "__main__":
main()
# uv run via_flask_g.py
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