Clean up the tip.json API

I've got someone asking to help with building a widget (#21, #41). I
believe this JSON API will enable that.

www/%participant_id/tip.json

@@ -1,3 +1,14 @@
+"""Get or change the authenticated user's tip to this person.
+
+Unauthenticated users will always get back {}, so retval.amount will be
+undefined in JavaScript. Authenticated users can pass "amount" in the
+querystring to change their tip. It will be ignored if you try to tip yourself,
+otherwise it must be one of: "0.00", "0.08", "0.16", "0.32", "0.64", "1.28"
+(you'll get a 400 otherwise). The return value for authenticated users is
+{"amount": "0.08"}, where the value is guaranteed to be one of those same
+strings, or None if it's yourself.
+
+"""
 import decimal
 
 from aspen import Response
@@ -8,66 +19,75 @@ from gittip.networks import github
 # ========================================================================== ^L
 
 
-# Get tipper and tippee.
-# ======================
-
-tipper = user.id
-if 'participant_id' in path:    # gittip
-    tippee = path['participant_id']
-elif 'login' in path:           # github
-    tippee = github.resolve(path['login'])
-else:
-    raise Response(400, "can't find tippee")
-if tippee == tipper:
-    amount = "0.00"
-else:
-
-    # Add a new tip record.
-    # =====================
-    # Insert instead of update. The analytics may be interesting some day.
-
-    if 'amount' in qs:
-        try:
-            amount = decimal.Decimal(qs['amount'])
-            if amount not in AMOUNTS:
-                raise ValueError 
-        except decimal.InvalidOperation, ValueError:
-            raise Response(400, "bad amount")
-
-        NEW_TIP = """\
-
-            INSERT INTO tips
-                        (ctime, tipper, tippee, amount)
-                 VALUES ( COALESCE (( SELECT ctime 
-                                        FROM tips 
-                                       WHERE (tipper=%s AND tippee=%s)
-                                       LIMIT 1
-                                      ), CURRENT_TIMESTAMP)
-                        , %s, %s, %s
-                         )
-
-        """
-        db.execute(NEW_TIP, (tipper, tippee, tipper, tippee, amount))
-
-
-    # Fetch the current tip record.
-    # =============================
+out = {}
+if not user.ANON:
+
+    # Get tipper and tippee.
+    # ======================
+
+    tipper = user.id
+    if 'participant_id' in path:    # gittip
+        tippee = path['participant_id']
+    elif 'login' in path:           # github
+        tippee = github.resolve(path['login'])
+    else:
+        raise Response(400, "can't find tippee")
+
+
+    # Get and maybe set amount.
+    # =========================
 
+    if tippee == tipper:
+        amount = None
     else:
-        FETCH = """\
-
-            SELECT amount
-              FROM tips
-             WHERE tippee=%s AND tipper=%s
-          ORDER BY mtime DESC
-             LIMIT 1
-
-        """
-        rec = db.fetchone(FETCH, (tippee, tipper))
-        if rec is None:
-            amount = "0.00"
+
+        # Add a new tip record.
+        # =====================
+        # Insert instead of update. The analytics may be interesting some day.
+
+        if 'amount' in qs:
+            try:
+                amount = decimal.Decimal(qs['amount'])
+                if amount not in AMOUNTS:
+                    raise ValueError 
+            except (decimal.InvalidOperation, ValueError):
+                raise Response(400, "bad amount")
+
+            NEW_TIP = """\
+
+                INSERT INTO tips
+                            (ctime, tipper, tippee, amount)
+                     VALUES ( COALESCE (( SELECT ctime 
+                                            FROM tips 
+                                           WHERE (tipper=%s AND tippee=%s)
+                                           LIMIT 1
+                                          ), CURRENT_TIMESTAMP)
+                            , %s, %s, %s
+                             )
+
+            """
+            db.execute(NEW_TIP, (tipper, tippee, tipper, tippee, amount))
+
+
+        # Fetch the current tip record.
+        # =============================
+
         else:
-            amount = str(rec['amount'])
+            FETCH = """\
+
+                SELECT amount
+                  FROM tips
+                 WHERE tippee=%s AND tipper=%s
+              ORDER BY mtime DESC
+                 LIMIT 1
+
+            """
+            rec = db.fetchone(FETCH, (tippee, tipper))
+            if rec is None:
+                amount = "0.00"
+            else:
+                amount = str(rec['amount'])
 
+    out = {"amount": amount}
 
-response.body = {"amount": amount}
+response.body = out