Python – Catching a 500 server error in Flask

flaskpython

I love Flask's error catching. It's beautifully simple:

@app.errorhandler(404)
def pageNotFound(error):
    return "page not found"

works like charm. But it doesn't work for the 500 error code. I want to catch Python errors when something goes wrong an exception is raised in the code. Is that possible?

I should note that if I explicitly call return abort(500) in a view then the 500 errorhandler does work. So this is explicitly for when the Python code fails.

Is this possible?

Best Answer

What you have described is, by default, how Flask works. My assumption is that you are running in debug mode, and therefore exceptions are being shown to you in the debug screen. Make sure debug mode is off, then try again. Here is a comment directly from the code itself:

Default exception handling that kicks in when an exception occurs that is not caught. In debug mode the exception will be re-raised immediately, otherwise it is logged and the handler for a 500 internal server error is used. If no such handler exists, a default 500 internal server error message is displayed.

Related Solutions

Python – How to write a `try`/`except` block that catches all exceptions

Apart from a bare except: clause (which as others have said you shouldn't use), you can simply catch Exception:

import traceback
import logging

try:
    whatever()
except Exception as e:
    logging.error(traceback.format_exc())
    # Logs the error appropriately.

You would normally only ever consider doing this at the outermost level of your code if for example you wanted to handle any otherwise uncaught exceptions before terminating.

The advantage of except Exception over the bare except is that there are a few exceptions that it wont catch, most obviously KeyboardInterrupt and SystemExit: if you caught and swallowed those then you could make it hard for anyone to exit your script.

Python – Configure Flask dev server to be visible across the network

While this is possible, you should not use the Flask dev server in production. The Flask dev server is not designed to be particularly secure, stable, or efficient. See the docs on deploying for correct solutions.

The --host option to flask run, or the host parameter to app.run(), controls what address the development server listens to. By default it runs on localhost, change it to flask run --host=0.0.0.0 (or app.run(host="0.0.0.0")) to run on all your machine's IP addresses.

0.0.0.0 is a special value that you can't use in the browser directly, you'll need to navigate to the actual IP address of the machine on the network. You may also need to adjust your firewall to allow external access to the port.

The Flask quickstart docs explain this in the "Externally Visible Server" section:

If you run the server you will notice that the server is only accessible from your own computer, not from any other in the network. This is the default because in debugging mode a user of the application can execute arbitrary Python code on your computer.

If you have the debugger disabled or trust the users on your network, you can make the server publicly available simply by adding --host=0.0.0.0 to the command line:
$ flask run --host=0.0.0.0
This tells your operating system to listen on all public IPs.

Best Answer

Related Solutions

Python – How to write a `try`/`except` block that catches all exceptions

Python – Configure Flask dev server to be visible across the network

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