Hello, I have added a few comments, the main concern being the use of copy instead of just dict.get, and a check for whether to run a specific function

I've addressed this concerns in each of your comments, thanks for the feedback!

Checked it out again, answered all the comments.
Looks good to me!

I don't love the idea of storing self.merged_extra and would like to see if we can cleanly avoid doing it

If I'm following the logic right (and there's a decent chance I'm not) all the stuff we store in self.merge_extra is already set as a property on the session object which we return.

If that is the case isn't session already configured and we don't need merged_extra again after this?

@ashb - here is the explanation as to why this was implemented. It boils down to the get_conn method. In run, we call get_conn. This is where the session is created via the _configure_session_from_extra. However, there are additional fields NOT included in the session that is returned that must be used later, meaning that we need to hold onto some form of extra_options.

Eventually, some form of extra_options needs to be passed to run_and_check, which is looking for the timeout, allow_redirects, and check_response fields. This mapping should be the merged mapping, which will include the timeout, allow_redirects, and check_response fields that could have been either in the Connection "extras" or the original extra_options. If we didn't create something like self.merged_extra, we'd need to change the return type of get_conn, which would be a breaking change.