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

Question

I tried using this

@app.after_request
def add_header(response):
    response.headers['Cache-Control'] = 'max-age=300'
    return response

But this causes a duplicate Cache-Control header to appear. I only want max-age=300, NOT the max-age=1209600 line!

$ curl -I http://my.url.here/
HTTP/1.1 200 OK
Date: Wed, 16 Apr 2014 14:24:22 GMT
Server: Apache
Cache-Control: max-age=300
Content-Length: 107993
Cache-Control: max-age=1209600
Expires: Wed, 30 Apr 2014 14:24:22 GMT
Content-Type: text/html; charset=utf-8

Solution 1

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_request
def add_header(response):
    response.cache_control.max_age = 300
    return response

Solution 2

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_request
def 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.

Comments

[turdus-merula]

Also response.cache_control.public = True.

[kevlarr]

Kudos for the if 'Cache-Control' not in ... bit, very smart to encourage people to check that!

[TheRealFakeNews]

@aldel What file would contain app = Flask(__name__) ?

[aldel]

@AlanH It has to exist somewhere in a Flask app. It would usually be one of the top-level files, i.e., the one that you run as __main__, or one that it imports.

[Mohamed Diaby]

max-age=0 can also be added to force the cache to revalidate because no-store will only prevent a new resource from being cached, but not prevent the cache from responding with a resource from a previous request response.headers["Cache-Control"] = "no-store, max-age=0"