[formatter] implement field name alternatives (#525)

The format string '{a|b|c}' will now try to use the value from 'a' and
fall back to 'b' and 'c' if accessing a field raises an exception or
if its value is None.

test/test_util.py

@@ -259,6 +259,7 @@ class TestFormatter(unittest.TestCase):
     kwdict = {
         "a": "hElLo wOrLd",
         "b": "äöü",
+        "d": {"a": "foo", "b": 0, "c": None},
         "l": ["a", "b", "c"],
         "n": None,
         "u": "%27%3C%20/%20%3E%27",
@@ -323,6 +324,22 @@ class TestFormatter(unittest.TestCase):
         self._run_test("{missing[key]}", replacement, default)
         self._run_test("{missing:?a//}", "a" + default, default)
 
+    def test_alternative(self):
+        self._run_test("{a|z}"    , "hElLo wOrLd")
+        self._run_test("{z|a}"    , "hElLo wOrLd")
+        self._run_test("{z|y|a}"  , "hElLo wOrLd")
+        self._run_test("{z|y|x|a}", "hElLo wOrLd")
+        self._run_test("{z|n|a|y}", "hElLo wOrLd")
+
+        self._run_test("{z|a!C}"      , "Hello World")
+        self._run_test("{z|a:Rh/C/}"  , "CElLo wOrLd")
+        self._run_test("{z|a!C:RH/C/}", "Cello World")
+        self._run_test("{z|y|x:?</>/}", "")
+
+        self._run_test("{d[c]|d[b]|d[a]}", "0")
+        self._run_test("{d[a]|d[b]|d[c]}", "foo")
+        self._run_test("{d[z]|d[y]|d[x]}", "None")
+
     def test_slicing(self):
         v = self.kwdict["a"]
         self._run_test("{a[1:10]}"  , v[1:10])