flask does not see change in .js file

If you are serving your static assets with Flask (this is typically the case in a development environment), then you might need to set the SEND_FILE_MAX_AGE_DEFAULT configuration value:

Default cache control max age to use with send_static_file() (the default static file handler) and send_file(), as datetime.timedelta or as seconds. Override this value on a per-file basis using the get_send_file_max_age() hook on Flask or Blueprint, respectively. Defaults to 43200 (12 hours).

Solving this can be as simple as updating the app.config dictionary, like so:

app = Flask(__name__)
...
app.config['SEND_FILE_MAX_AGE_DEFAULT'] = 0

If you do that, your browser will not cache static assets that are served by Flask.

Caching is normally good, so it's not advisable to eliminate it entirely. And using control + F5 or whatever to do a hard refresh is obviously not a scalable solution since you have to do it in every browser on every computer.

A better idea is to let browsers cache the files most of the time, but not right after they've been updated. You can achieve this by appending the time that the source file was last updated as an argument to its path URL. For simplicity, you can use the most recent modification time for any file in the static folder (or any subfolders), as opposed to looking at each file individually.

Python

def dir_last_updated(folder):
    return str(max(os.path.getmtime(os.path.join(root_path, f))
                   for root_path, dirs, files in os.walk(folder)
                   for f in files))

@app.route('/my-site')
def my_site():
    return render_template('my-site.html',
                           last_updated=dir_last_updated('mydir/static'))

Jinja Template

<script type="text/javascript" src="/static/my-script.js?u={{ last_updated }}"></script>

HTML Result

<script type="text/javascript" src="/static/my-script.js?u=1547330602.31"></script>

Ultimately this is a frustrating browser cache issue, which can be solved by forcing the browser to do a "hard refresh", which is going to be a browser/OS dependent keystroke, but generally this works:

• Windows: Ctrl+F5
• Mac: Cmd+Shift+R
• Linux: Ctrl+Shift+R

There are other filename tricks one can use to avoid this issue (mentioned in comments of the OP). These are especially important in production where you have no control over browser behavior.

For non-Static Flask responses you can set the cache_control.max_age property, which should tell the browser when to expire the response if it is cached. For instance if you have a Flask XHR endpoint that returns JSON data you could do this:

@app.route('/_get_ajax_data/')
def get_ajax_data():
    data = {"hello": "world"}
    response = jsonify(data)
    response.cache_control.max_age = 60 * 60 * 24  # 1 day (in seconds)
    return response

You typically can also set default values in your production web server configuration for specific resource types (e.g. CSS/JS/HTML/JSON/etc)

Edit 4/1/2019 (unrelated to April Fools day)

• Mac / Safari keystroke now appears to be: Cmd+Opt+R (via comments, thanks!).
• See the new answer from @MarredCheese for a very elegant "filename trick" to force the browser to ignore cached copies for updated files.