HttpRequest.headers doesn't get updated when request.META is updated.

[lieryan]

Currently, Django docs recommended using request.headers instead of META to access headers, however the behaviour of request.headers currently makes it hard to use correctly during tests.

Currently, the following code fails:

request.META["HTTP_USER_AGENT"] = "foobar"
assert request.headers["User-Agent"] == "foobar" # works
request.META["HTTP_USER_AGENT"] = "django"
assert request.headers["User-Agent"] == "django" # fails

This is because request.headers is a @cached_property that is initialised when request.headers is first accessed, so underlying changes to META don't get reflected to request.headers.

In regular Django request, this isn't that big of a deal, because arguable request.META should be immutable anyway, and you probably should be cursed if you modify request.META in production code.

However, this is rather annoying when writing tests, because often you want to reuse the same request object to pass to different methods, or you want to test the same method with slightly different header value. Due to this caching, you have the option of either recreating the request from scratch, which can be complicated if parts setting up META is spread between setUp() and the test method, or you'd have to delete the request.headers to force it to reinitialise, which is rather non obvious and error prone, since you might forget to delete it and cause some tests to pass/fail when they shouldn't.

Also, request.headers is read only, which means that you still have to use the WSGI environment names when setting headers in tests rather than the standard HTTP name.

I'd propose HttpHeaders should be reimplemented so that it isn't a real collection, but just an accessor for request.META, also that HttpHeaders should implement __getitem__() which should also update request.META.

[Mariusz Felisiak]

Thanks for this ticket, however IMO it's not a desired change. HttpRequest.headers are read-only and immutable by design (see ​comment), moreover META contains more than only HTTP headers so modifying META via headers would be misleading. If you need a low-level modifications in your tests I would recommend to access/modify META directly.

You can start a discussion on DevelopersMailingList if you don't agree.

[Carlton Gibson]

I'd guess you'd be able to del request.headers to reset the property in your test.

[Chris Wesseling]

Replying to Carlton Gibson:

I'd guess you'd be able to del request.headers to reset the property in your test.

Modification of META is documented behaviour:
​https://docs.djangoproject.com/en/dev/ref/request-response/#django.http.HttpRequest.get_host

Shouldn't request.META.__setitem__ and request.META.__delitem__ del request.headers to synchronize if the key starts with "HTTP_"? In the current code, if any middleware has peeked into request.headers, the data gets out of sync on mutation.