Question: I made a change on one of the `.js` files that I use and no matter what I do, flask insists on picking up, from memory cache, the last version of the file, without the change.

To clarify, I have the following structure. It all starts with `foo.html`

```
return render_template foo.html

```

`foo.html` has a form inside that calls flask with some data and then returns a second template `bar.html`:

```
return render_template bar.html

```

This second template calls some `.js` file, placed in the `static` folder, but it doesn't update when the code changes.

I mention the structure above because if the `.js` file was placed on `foo.html` instead of `bar.html` then Flask **would** pick up the new changes on the file. But in `bar.html` Flask completely ignores them.

What is happening? 

The only thing that worked was to click on "disable cache" on the browser and reload again.

Answer: 
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.
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>

```