Comment 3 for bug 1011769

Exactly, it's a limitation of httplib where it doesn't know how to use "Expect: 100-continue" headers. That's the only way for HTTP requests to fail early. Curl I think does it by default when sending a PUT.

Of course expect headers add an extra RTT to requests, so if latency is important, you should probably just make sure your content is valid as best you can and forego that step.