This is more of a proposal than a bug report. Feel free to close this issue, if this is not desired behavior for Tornado.

Instead of limiting the AsyncHTTPClient's methods to standard PUT, POST, GET, HEAD and DELETE, Tornado could allow any custom HTTP method to be used. This has some benefits when working with certain applications, like CouchDB, where a non-standard method COPY is used to do an atomic copy of a document.

This behavior could easily be achieved with following patch

diff --git a/tornado/httpclient.py b/tornado/httpclient.py
index e45bfd6..fa152dc 100644
--- a/tornado/httpclient.py
+++ b/tornado/httpclient.py
@@ -390,16 +390,13 @@ def _curl_setup_request(curl, request, buffer, headers):
         "PUT": pycurl.UPLOAD,
         "HEAD": pycurl.NOBODY,
     }
-    custom_methods = set(["DELETE"])
     for o in curl_options.values():
         curl.setopt(o, False)
     if request.method in curl_options:
         curl.unsetopt(pycurl.CUSTOMREQUEST)
         curl.setopt(curl_options[request.method], True)
-    elif request.method in custom_methods:
-        curl.setopt(pycurl.CUSTOMREQUEST, request.method)
     else:
-        raise KeyError('unknown method ' + request.method)
+        curl.setopt(pycurl.CUSTOMREQUEST, request.method)
 
     # Handle curl's cryptic options for every individual HTTP method
     if request.method in ("POST", "PUT"):