Using Flask, how do I modify the Cache-Control header for ALL output?

Use the response.cache_control object; this is a ResponseCacheControl() instance letting you set various cache attributes directly. Moreover, it'll make sure not to add duplicate headers if there is one there already.

@app.after_requestdef add_header(response):    response.cache_control.max_age = 300    return response

You can set the default value for all static files when you create the Flask application:

app = Flask(__name__)app.config['SEND_FILE_MAX_AGE_DEFAULT'] = 300

Note that if you modify request.cache_control in after_request, as in the accepted answer, this will also modify the Cache-Control header for static files and may override the behavior you set as I showed above. I'm currently using the following code to completely disable caching for dynamically generated content but not static files:

# No cacheing at all for API endpoints.@app.after_requestdef add_header(response):    # response.cache_control.no_store = True    if 'Cache-Control' not in response.headers:        response.headers['Cache-Control'] = 'no-store'    return response

Not completely sure this is the best way, but it's working for me so far.