bug(URI template): allow complex vars coexist with simple var

Allow situation like this:

```python
api.add_route('/files/{file_id}', resource_1)
api.add_route('/files/{file_id}.{ext}', resource_2)
```

Which ValueError was raised due to only one var node was allowed whether
other nodes are complex or not. This fix changes the behaviour so now it
only applys to simple vars.

See #564

falcon/routing/compiled.py

@@ -112,16 +112,20 @@ def line(text, indent_offset=0):
         level_indent = indent
         found_simple = False
 
-        # NOTE(kgriffs): Sort static nodes before var nodes so that
-        # none of them get masked. False sorts before True.
-        nodes = sorted(nodes, key=lambda node: node.is_var)
+        # NOTE(kgriffs & philiptzou): Sort nodes in this sequence:
+        # static nodes(0), complex var nodes(1) and simple var nodes(2).
+        # so that none of them get masked.
+        nodes = sorted(
+            nodes, key=lambda node: node.is_var + (node.is_var and
+                                                   not node.is_complex))
 
         for node in nodes:
             if node.is_var:
                 if node.is_complex:
-                    # NOTE(richardolsson): Complex nodes are nodes which contain
-                    # anything more than a single literal or variable, and they
-                    # need to be checked using a pre-compiled regular expression.
+                    # NOTE(richardolsson): Complex nodes are nodes which
+                    # contain anything more than a single literal or variable,
+                    # and they need to be checked using a pre-compiled regular
+                    # expression.
                     expression_idx = len(self._expressions)
                     self._expressions.append(node.var_regex)
 
@@ -143,7 +147,8 @@ def line(text, indent_offset=0):
                     #   /foo/{id}/bar
                     #   /foo/{name}/bar
                     #
-                    assert len([node for node in nodes if node.is_var]) == 1
+                    assert len([_node for _node in nodes
+                                if _node.is_var and not _node.is_complex]) == 1
                     found_simple = True
 
             else:
@@ -289,7 +294,8 @@ def conflicts_with(self, segment):
 
                 return False
             else:
-                # NOTE(kgriffs): Falcon does not support the following:
+                # NOTE(kgriffs & philiptzou): Falcon does not accept multiple
+                # simple var nodes exist at the same level as following:
                 #
                 #   /foo/{thing1}
                 #   /foo/{thing2}
@@ -298,8 +304,9 @@ def conflicts_with(self, segment):
                 #
                 #   /foo/{thing1}
                 #   /foo/all
+                #   /foo/{thing1}.{ext}
                 #
-                return other.is_var
+                return other.is_var and not other.is_complex
 
         # NOTE(kgriffs): If self is a static string match, then all the cases
         # for other are False, so no need to check.

tests/test_uri_templates.py

@@ -39,6 +39,28 @@ def on_get(self, req, resp, id, name51):
         self.called = True
 
 
+class FileResource(object):
+    def __init__(self):
+        self.file_id = None
+        self.called = False
+
+    def on_get(self, req, resp, file_id):
+        self.file_id = file_id
+        self.called = True
+
+
+class FileDetailsResource(object):
+    def __init__(self):
+        self.file_id = None
+        self.ext = None
+        self.called = False
+
+    def on_get(self, req, resp, file_id, ext):
+        self.file_id = file_id
+        self.ext = ext
+        self.called = True
+
+
 class TestUriTemplates(testing.TestBase):
 
     def before(self):
@@ -219,3 +241,25 @@ def test_relative_path(self):
 
         self.assertRaises(ValueError, self.api.add_route,
                           'no/leading_slash', self.resource)
+
+    def test_same_level_complex_var(self):
+        resource = FileResource()
+        details_resource = FileDetailsResource()
+        self.api.add_route('/files/{file_id}', resource)
+        self.api.add_route('/files/{file_id}.{ext}', details_resource)
+
+        # dots cause ambiguous in filenames
+        file_id_1 = self.getUniqueString().replace('.', '-')
+        file_id_2 = self.getUniqueString().replace('.', '-')
+        ext = self.getUniqueString().replace('.', '-')
+        path_1 = '/files/' + file_id_1
+        path_2 = '/files/' + file_id_2 + '.' + ext
+
+        self.simulate_request(path_1)
+        self.assertTrue(resource.called)
+        self.assertEqual(resource.file_id, file_id_1)
+
+        self.simulate_request(path_2)
+        self.assertTrue(details_resource.called)
+        self.assertEqual(details_resource.file_id, file_id_2)
+        self.assertEqual(details_resource.ext, ext)