bpo-1346874: Add 'Expect: 100-Continue' support to httplib

[tbartlett0]

Previously, http.client would always send content body immediately and ignore any 100 responses. This change makes HTTPClient.request() wait for a Continue response if the Expect: 100-Continue header is set, and adds a parameter to HTTPClient.getresponse() that will cause it to return 100 responses instead of eating them.

https://bugs.python.org/issue1346874

[tbartlett0]

This PR is intended to be backwards-compatible, but I have identified two potential edge cases:

• clients that currently set Expect: 100-Continue with a broken server that ignores the header: This will introduce a delay of expect_timeout seconds to each request. However, this can be fixed by a) not sending the header or b) setting expect_timeout to 0.
• anyone who redefines HTTPClient.response_class: this is likely to be a breaking change, as a new kwarg is used on HTTPResponse.begin(). (This seems like a fairly rare situation, as it's not exposed as an __init__ param, but chances are that someone, somewhere has done it...)

[quadoss]

Is there an update on this one, its been in open state for couple of years and I too have been hitting this and wanting to see when this would be merged

[emnoor-reef]

Just bumping. Is this PR on consideration for getting merged? Or, are there any other plans for #42550 ? I am encountering this problem at work.

[python-cla-bot]

All commit authors signed the Contributor License Agreement.

[IngridMorstrad]

Hi, @tbartlett0
Is this PR still being worked on? Or can someone else (me) take it over?

[AA-Turner]

Tim would need to (re-)sign the CLA.

[tbartlett0]

Hi, I haven't touched this since it was submitted, since there seemed to be limited interest and I ended up working around it in other ways. If someone else wants to take it on I have no problem with that (either using my code or rewriting their own, there might be a better way to achieve it)

[tbartlett0]

@IngridMorstrad
I (re-)signed the CLA, hopefully that clears any issues if you wish to take this on now.

[IngridMorstrad]

I don't have permissions here, so created a new PR: #133276

[github-actions]

This PR is stale because it has been open for 30 days with no activity.