Enclose field names containing literals in quotes

When creating the text representation of a path, field names that contain lexer literals (like a period ".') should be enclosed in quotes (or brackets).

The proposed fix only adds quotes if needed (as opposed to always). This avoids changing current paths/behavior drastically since it seems no one has run into this so far.

jsonpath_ng/jsonpath.py

@@ -1,6 +1,9 @@
-from __future__ import unicode_literals, print_function, absolute_import, division, generators, nested_scopes
+from __future__ import (absolute_import, division, generators, nested_scopes,
+                        print_function, unicode_literals)
+
 import logging
 from itertools import *  # noqa
+from jsonpath_ng.lexer import JsonPathLexer
 
 # Get logger name
 logger = logging.getLogger(__name__)
@@ -615,7 +618,14 @@ def filter(self, fn, data):
         return data
 
     def __str__(self):
-        return ','.join(map(str, self.fields))
+        # If any JsonPathLexer.literals are included in field name need quotes
+        # This avoids unnecessary quotes to keep strings short.
+        # Test each field whether it contains a literal and only then add quotes
+        # The test loops over all literals, could possibly optimize to short circuit if one found
+        fields_as_str = ("'" + str(f) + "'" if any([l in f for l in JsonPathLexer.literals]) else
+                         str(f) for f in self.fields)
+        return ','.join(fields_as_str)
+
 
     def __repr__(self):
         return '%s(%s)' % (self.__class__.__name__, ','.join(map(repr, self.fields)))

tests/test_jsonpath.py

@@ -233,6 +233,8 @@ def test_child_paths(self):
     def test_descendants_paths(self):
         self.check_paths([('foo..baz', {'foo': {'baz': 1, 'bing': {'baz': 2}}}, ['foo.baz', 'foo.bing.baz'] )])
 
+    def test_literals_in_field_names(self):
+        self.check_paths([("A.'a.c'", {'A' : {'a.c': 'd'}}, ["A.'a.c'"])])
 
     #
     # Check the "auto_id_field" feature