Use Flask current_app.logger inside threading

Question

I am using current_app.logger and when I tried to log inside thread it says "working outside of application context". How do I log a message from a method running in a thread?

def background():
    current_app.logger.debug('logged from thread')

@app.route('/')
def index():
    Thread(target=background).start()
    return 'Hello, World!'

Exception in thread Thread-16:
Traceback (most recent call last):
  File "/usr/lib64/python3.5/threading.py", line 914, in _bootstrap_inner
    self.run()
  File "/usr/lib64/python3.5/threading.py", line 862, in run
    self._target(*self._args, **self._kwargs)
  File "/home/sapam/demo.py", line 57, in background
    current_app.logger.critical('test')
  File "/home/sapam/.virtualenvs/demo/lib/python3.5/site-packages/werkzeug/local.py", line 343, in __getattr__
    return getattr(self._get_current_object(), name)
  File "/home/sapam/.virtualenvs/demo/lib/python3.5/site-packages/werkzeug/local.py", line 302, in _get_current_object
    return self.__local()
  File "/home/sapam/.virtualenvs/demo/lib/python3.5/site-packages/flask/globals.py", line 51, in _find_app
    raise RuntimeError(_app_ctx_err_msg)
RuntimeError: Working outside of application context.

This typically means that you attempted to use functionality that needed
to interface with the current application object in a way.  To solve
this set up an application context with app.app_context().  See the
documentation for more information.

127.0.0.1 - - [13/Sep/2016 12:28:24] "GET / HTTP/1.1" 200 -

Answer 1

You use the standard logging module in the standard way: get the logger for the current module and log a message with it.

def background():
    logging.getLogger(__name__).debug('logged from thread')

---

app.logger is mostly meant for internal Flask logging, or at least logging within an app context. If you're in a thread, you're no longer in the same app context.

You can pass current_app._get_current_object() to the thread and use that instead of current_app. Or you can subclass Thread to do something similar.

def background(app):
    app.logger.debug('logged from thread')

@app.route('/')
def index():
    Thread(target=background, kwargs={'app': current_app._get_current_object()}).start()
    return 'Hello, World!'

class FlaskThread(Thread):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.app = current_app._get_current_object()

    def run(self):
        with self.app.app_context():
            super().run()

def background():
    current_app.logger.debug('logged from thread')

@app.route('/')
def index():
    FlaskThread(target=background).start()
    return 'Hello, World!'

The same works for multiprocessing.