Unable to override cookie policy

Originally opened by @KKomarov on 2023-11-15 01:09:25 in encode/httpx

I would like to be able to override the cookie policy used by a Client when making a request. However, overriding the cookie policy on the cookie jar of the Client or of the Request object has had no effect. These lines of code explains why:

https://github.com/encode/httpx/blob/497b315fc7a0ca6a54c8ec6c929568abaf6efab1/httpx/_models.py#L334
https://github.com/encode/httpx/blob/497b315fc7a0ca6a54c8ec6c929568abaf6efab1/httpx/_models.py#L1030
When initialise request from existing cookies (which is Cookies object of Client), it discards policy object from source Cookies object.

https://github.com/encode/httpx/blob/497b315fc7a0ca6a54c8ec6c929568abaf6efab1/httpx/_client.py#L400
The cookie jars of both the client and the request are merged into a newly constructed CookieJar, discarding the policies of both.

What I expected to be able to do, and what isn't working currently, is something like the following:

        cookie = MozillaCookieJar(filename='cookies.txt', policy=MyCookiePolicy())
        cookie.load()
        session = httpx.Client(headers=headers, cookies=cookie)
        session.get(url)

In my opinion Policy from CookieJar in Client should not be discarded. When CookieJar object passed in request then policy from request should override policy from Client.

Start with the linked sections of httpx/_models.py around lines 334 and 1030, then inspect httpx/_client.py around line 400. Trace how CookieJar objects are copied and merged, including the MozillaCookieJar example. Done means a policy supplied by the client is preserved, while a request-level policy overrides it.