How to get Flask to auto-restart despite syntax errors

When developing a Flask application, the auto-reloader is incredibly helpful. Each time I edit a python file the server will restart, so that my changes are immediately implemented. However, there is still room for improvement, which is what I will briefly discuss and fix in this post.

First, for context, I will explain how I typically set up a flask app for development and deployment.

Setting Up

Typically, in the root of the folder which holds all of the files, I will have a file named "". This will contain the necessary code for running the development server (provided by the Flask library itself) if launched by itself, or providing the necessary objects to uwsgi if used in production.

Improving Flask's default server

Templates, JS and CSS

When working on any front-end aspects of a flask website the auto reloader will not kick in to update any template or static file changes.

Luckily, there is a way to include extra files with the reloader, which helps with this problem. Specifically, there is an extra_files argument which can be passed to so that it will watch additional, files.

Making use of this, we can update to contain the following:

import os

from os import path
from app_root_module import app as application

if __name__ == '__main__':
    extra_dirs = ['/path/to/templates', "/path/to/static/js", "/path/to/static/css"]
        extra_files = extra_dirs[:]
        for extra_dir in extra_dirs:
            for dirname, dirs, files in os.walk(extra_dir):
                for filename in files:
                    filename = path.join(dirname, filename)
                    if path.isfile(filename):
                        extra_files.append(filename), extra_files=extra_files)

Now Flask will restart its server whenever we change a front-end file. Awesome!

(Code was taken from Stackoverflow).

Handling Syntax Errors

The server will completely stop when it encounters a syntax error while running. This is especially problematic if you use an IDE which auto-saves after you stop typing for a couple of seconds. If you stop half way through a line, the IDE will save, the server will restart, and then crash with a syntax error. Very annoying!

Luckily, a couple of unix tools can make this a non-issue. Introducing entr and ack (not to be confused with awk). I have neither installed by default on OpenSUSE Tumbleweed, but both were available in Zypper.

Using these two tools, we can craft a single command which will restart our server once we adjust a python file, regardless of any syntax errors occurring.

I have the following as an executable bash file, which I run to start my dev sever:

ack -g ".py" --ignore-dir "env" --ignore-dir "htmlcov" | entr -r python3

This uses ack to recursively list all files ending in .py, except those in my virtual env or coverage directory, then pipe them through to entr which will execute a command when any of them change. In this case, the command executes the file, starting Flask's web server again.

That's all there is to it. I don't claim to be very knowledgeable about either ack or entr, so I will leave links to their respecive documentation below.

Read about entr here

Read about ack here

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