add RawExpression to gremlin_python

docs/src/reference/gremlin-variants.asciidoc

@@ -157,3 +157,37 @@ g.V().both()[1]
 g.V().both().name
 ----
 
+Inserting arbitrary expressions with Gremlin-Python
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To make use of APIs provided by your graph system vendor that aren't part of
+the gremlin traversal API, you can use `RawExpression`. `RawExpression` allows
+you to construct arbitrary expressions that collect bindings. When you pass
+strings and bindings to the `RawExpression` constructor, the arguments will
+be concatenated in the script that is sent to gremlin-server.
+
+The following example uses `RawExpression` to call the `Geoshape.point` static
+constructor method on the `Geoshape` class provided by TitanDB.
+
+[source,python]
+g.V().has('location', RawExpression('Geoshape.point(', ('x', 45), ', ', ('y', -45), ')'))
+# sends the following script to gremlin-server:
+# g.V().has("location", Geoshape.point(x, y))
+#
+# with the following bindings: {'x': 45, 'y': -45}
+
+To help readability, especially if you will be using the expression more than
+once, it is recommended to use this in conjunction with a helper class as in
+the following example:
+
+[source,python]
+class Geoshape(object):
+    @staticmethod
+    def point(latitude, longitude):
+        return RawExpression('Geoshape.point(', latitude, ', ', longitude, ')')
+
+The previous traversal can now be written much more simply:
+
+[source,python]
+g.V().has('location', Geoshape.point(('x', 45), ('y', -45)))
+

gremlin-variant/src/main/groovy/org/apache/tinkerpop/gremlin/python/GremlinPythonGenerator.groovy

@@ -177,8 +177,10 @@ builtInRange = range
                         """  def ${method}(self, *args):
     self.translator.addStep(self, "${method}", *args)
     for arg in args:
-      if isinstance(arg, dict) and 1 == len(arg) and isinstance(arg.keys()[0],str):
-        self.bindings.update(arg)
+      if isinstance(arg, tuple) and 2 == len(arg) and isinstance(arg[0], str):
+        self.bindings[arg[0]] = arg[1]
+      elif isinstance(arg, RawExpression):
+        self.bindings.update(arg.bindings)
     return self
 """)
             }
@@ -230,6 +232,31 @@ builtInRange = range
             pythonClass.append("\n");
         }
         //////////////
+
+        pythonClass.append("""class RawExpression(object):
+   def __init__(self, *args):
+      self.bindings = dict()
+      self.parts = [self._process_arg(arg) for arg in args]
+
+   def _process_arg(self, arg):
+      if isinstance(arg, tuple) and 2 == len(arg) and isinstance(arg[0], str):
+         self.bindings[arg[0]] = arg[1]
+         return Raw(arg[0])
+      else:
+         return Raw(arg)
+""")
+
+        pythonClass.append("\n")
+        pythonClass.append("""class Raw(object):
+   def __init__(self, value):
+      self.value = value
+
+   def __str__(self):
+      return str(self.value)
+""")
+
+        pythonClass.append("\n")
+
         pythonClass.append("""class P(object):
    def __init__(self, operator, value, other=None):
       self.operator = operator

gremlin-variant/src/main/jython/gremlin_python/.gitignore

@@ -0,0 +1,2 @@
+__pycache__/
+*.py[cdo]