Index: twisted/web/http.py
===================================================================
--- twisted/web/http.py	(revision 32494)
+++ twisted/web/http.py	(working copy)
@@ -563,7 +563,9 @@
     sentLength = 0 # content-length of response, or total bytes sent via chunking
     etag = None
     lastModified = None
-    args = None
+    _args = None # .args is a property that will parse and store request args here
+    _orig_method = None # http method request originally called with
+    _orig_uri = None # uri request originally called with
     path = None
     content = None
     _forceSSL = 0
@@ -725,10 +727,13 @@
         @param version: The HTTP version of this request.
         """
         self.content.seek(0,0)
-        self.args = {}
         self.stack = []
 
-        self.method, self.uri = command, path
+        # since argument parsing is now deferred to a property we want to
+        # have explicit references to the original method and original uri
+        # to parse against in case they are subsequently changed by
+        # application code
+        self.method, self.uri = self._orig_method, self._orig_uri = command, path
         self.clientproto = version
         x = self.uri.split('?', 1)
 
@@ -736,20 +741,44 @@
             self.path = self.uri
         else:
             self.path, argstring = x
-            self.args = parse_qs(argstring, 1)
 
         # cache the client and server information, we'll need this later to be
         # serialized and sent with the request so CGIs will work remotely
         self.client = self.channel.transport.getPeer()
         self.host = self.channel.transport.getHost()
 
-        # Argument processing
-        args = self.args
+        self.process()
+
+    def args(self):
+        """
+        Return query or post arguments dict of the request on demand.
+
+        If the arguments have already been parsed, they will be returned
+        immediately, otherwise the first access will parse arguments
+        from the url or body and return it.
+        """
+        if self._args is not None:
+            return self._args
+
+        # Parse query string arguments
+        x = self._orig_uri.split('?', 1)
+        if len(x) != 1:
+            self._args = parse_qs(x[1], 1)
+        else:
+            self._args = {}
+
+        # Parse post body arguments
+        args = self._args
         ctype = self.requestHeaders.getRawHeaders('content-type')
         if ctype is not None:
             ctype = ctype[0]
 
-        if self.method == "POST" and ctype:
+        if self._orig_method == "POST" and ctype:
+
+            # preserve location in content stream
+            curr_loc = self.content.tell()
+            self.content.seek(0, 0)
+
             mfd = 'multipart/form-data'
             key, pdict = cgi.parse_header(ctype)
             if key == 'application/x-www-form-urlencoded':
@@ -768,11 +797,11 @@
                         self.channel.transport.loseConnection()
                         return
                     raise
-            self.content.seek(0, 0)
+            # restore previous location in content stream
+            self.content.seek(curr_loc, 0)
+        return args
+    args = property(args)
 
-        self.process()
-
-
     def __repr__(self):
         return '<%s %s %s>'% (self.method, self.uri, self.clientproto)
 
Index: twisted/web/test/test_http.py
===================================================================
--- twisted/web/test/test_http.py	(revision 32494)
+++ twisted/web/test/test_http.py	(working copy)
@@ -826,6 +826,42 @@
 
         self.runRequest(httpRequest, MyRequest)
 
+    def test_formNotParsedUntilArgsAccessed(self):
+        """
+        The post body is not parsed until request.args is accessed.
+        """
+        query = 'key=value'
+        httpRequest = '''\
+POST / HTTP/1.0
+Content-Length: %d
+Content-Type: application/x-www-form-urlencoded
+
+%s''' % (len(query), query)
+
+        testcase = self
+        self.parse_qs_calls = []
+        orig_parse_qs = http.parse_qs
+        def parse_qs(query_string, *args):
+            testcase.parse_qs_calls.append(query_string)
+            return orig_parse_qs(query_string, *args)
+        self.patch(http, 'parse_qs', parse_qs)
+        class MyRequest(http.Request):
+            def process(self):
+                testcase.assertEquals(self.method, "POST")
+                # Arguments have not been parsed:
+                testcase.assertEquals(testcase.parse_qs_calls, [])
+                # But have been after first access to self.args:
+                testcase.assertEquals(self.args["key"], ["value"])
+                testcase.assertEquals(testcase.parse_qs_calls, [query])
+
+                # Reading from the content file-like must produce the entire
+                # request body.
+                testcase.assertEquals(self.content.read(), query)
+                testcase.didRequest = 1
+                self.finish()
+
+        self.runRequest(httpRequest, MyRequest)
+
     def testMissingContentDisposition(self):
         req = '''\
 POST / HTTP/1.0
@@ -864,7 +900,7 @@
                 # The tempfile API used to create content returns an
                 # instance of a different type depending on what platform
                 # we're running on.  The point here is to verify that the
-                # request body is in a file that's on the filesystem. 
+                # request body is in a file that's on the filesystem.
                 # Having a fileno method that returns an int is a somewhat
                 # close approximation of this. -exarkun
                 testcase.assertIsInstance(self.content.fileno(), int)