Please elaborate why do you need to modify status code?
What's your use case?

From my perspective http proxy can write different status line if needed without changing aiohttp.
Adding status_line property doesn't make any harm but you can keep formatting code in your application. I feel the need for status_line is very rare.

By default the proxy sets the status code to 200, assuming success. It sends a modified version of the request upstream, which may or may not return a 200, at which point the status code to be sent to the client needs to be modified.

Without this patch, the only way to do that is by modifying status_line directly, which is in my opinion much more dangerous.

I still believe proxy should get response from upstream and send a copy of it to client.

No need for inplace request/response modification. All work can be done without aiohttp changing.

Even if status_line updating may be convenient for your particular code the use case is not too common for adding such hackery patch.

Then as an alternative, modifying the status attribute should be forbidden. It should be a readonly property, to make it clear that you're not supposed to modify it.

Converting status to readonly makes sense.

Fixed by #710

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