Demos of how global variables play out when you're running python on multiple threads.

global_vars_threading.py

"""Demo behavior of global variables across threads.
"""
from threading import Thread
import time

shared_var = "outside"  # global variable


def thread1(threadname: str) -> None:
    global shared_var

    for _ in range(5):
        time.sleep(0.25)
        print(f"From {threadname}: shared_var = {shared_var} | {shared_var == threadname}")
        shared_var = threadname


def thread2(threadname: str) -> None:
    global shared_var
    for _ in range(5):
        shared_var = threadname
        time.sleep(0.25)
        print(f"From {threadname}: shared_var = {shared_var} | {shared_var == threadname}")


def main() -> None:
    thread_fn1 = Thread(target=thread1, args=("A",))
    thread_fn2 = Thread(target=thread2, args=("B",))
    thread_fn1.start()
    thread_fn2.start()
    thread_fn1.join()
    thread_fn2.join()


if __name__ == "__main__":
    main()

leaked_context_app.py

"""Demo of different methods for maintaining a global context in a flask app.

---

Run the app:

```
gunicorn --reload -k gevent -b '0.0.0.0:3005' scratch.leaked_context.leaked_context_app:app
```

Now make requests:

```
import requests

url = "http://0.0.0.0:3005/"

requests.get(url, json={"name": "A", "seconds": 5}) # In one console
requests.get(url, json={"name": "B", "seconds": 1}) # In another
```
"""

import json
import logging
import time
from flask import Flask, g, jsonify, request
from werkzeug.local import get_ident, Local  # type: ignore

logging.basicConfig(level=logging.INFO)
logger = logging.getLogger(__name__)

app = Flask(__name__)

SHARED_GLOBAL = "[unset]"
SHARED_LOCAL = Local()

get_thread_name = lambda: hex(id(get_ident()))


@app.route("/")
def my_route():
    global SHARED_GLOBAL

    body = json.loads(request.get_data(as_text=True))  # request.get_json() or json.loads
    seconds = body.get("seconds", 1)
    name = body.get("name", "[name]")

    g.name = name  # This will work
    SHARED_LOCAL = name  # This will work
    SHARED_GLOBAL = name  # This will not

    poll_context = 0.5
    tot_sec = 0
    while tot_sec <= seconds:

        # Logging info is interesting to check out (-vvs)
        msg = f"""
        {name} @ {tot_sec} out of {seconds}. Thread = {get_thread_name()}"""
        for nm, v in [("g", g.name), ("Local", SHARED_LOCAL), ("global", SHARED_GLOBAL)]:
            _is_same = v == name
            msg += f"""
            ({name}) {nm:8s} {v} | name correct? {_is_same}"""
        logger.info(msg)

        time.sleep(poll_context)
        tot_sec += poll_context

    return jsonify({})