Add len extention to the json parser

This extends the jsonpath_rw parser with customisation
for gabbi.

This first one is `len`

Author: Mehdi Abaakouk

gabbi/case.py

@@ -29,11 +29,11 @@
 import unittest
 from unittest import case
 
-import jsonpath_rw
 import six
 from six.moves.urllib import parse as urlparse
 import wsgi_intercept
 
+from gabbi import json_parser
 from gabbi import utils
 
 REPLACERS = [
@@ -193,7 +193,7 @@ def extract_json_path_value(data, path):
 
         The input data is a Python datastructre, not a JSON string.
         """
-        path_expr = jsonpath_rw.parse(path)
+        path_expr = json_parser.parse(path)
         matches = [match.value for match in path_expr.find(data)]
         try:
             return matches[0]

gabbi/gabbits_intercept/data.yaml

@@ -25,6 +25,7 @@ tests:
       response_json_paths:
           $[0]: 1
           $[1]: 2
+          $.`len`: 2
 
     - name: load json file
       url: /

gabbi/json_parser.py

@@ -0,0 +1,56 @@
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+import jsonpath_rw
+
+
+class Len(jsonpath_rw.JSONPath):
+    """The JSONPath referring to the len of the current object.
+
+    Concrete syntax is '`len`'.
+    """
+
+    def find(self, datum):
+        datum = jsonpath_rw.DatumInContext.wrap(datum)
+        try:
+            value = len(datum.value)
+        except TypeError:
+            return []
+        else:
+            return [jsonpath_rw.DatumInContext(value,
+                                               context=None,
+                                               path=Len())]
+
+    def __eq__(self, other):
+        return isinstance(other, Len)
+
+    def __str__(self):
+        return '`len`'
+
+    def __repr__(self):
+        return 'Len()'
+
+
+class GabbiJsonPathParser(jsonpath_rw.parser.JsonPathParser):
+    """Custom gabbi LALR-parser for JsonPath"""
+
+    def p_jsonpath_named_operator(self, p):
+        "jsonpath : NAMED_OPERATOR"
+        if p[1] == 'len':
+            p[0] = Len()
+        else:
+            super(GabbiJsonPathParser, self).p_jsonpath_named_operator(p)
+
+
+def parse(path):
+    return GabbiJsonPathParser().parse(path)