Added graphcat.passthrough(). Closes #6.

features/graph.feature

@@ -341,3 +341,12 @@ Feature: Graph
         And testing if the graph contains task "A" should return True
         And testing if the graph contains task "C" should return False
 
+
+    Scenario: Passthrough
+        Given an empty graph
+        When adding tasks ["A", "B", "C"] with functions [graphcat.constant(42), graphcat.constant(10), graphcat.passthrough("lhs")]
+        And adding links [("A", ("C", "lhs")), ("B", ("C", "rhs"))]
+        And updating tasks ["C"]
+        Then the task ["A", "B", "C"] outputs should be [42, 10, 42]
+
+

graphcat/__init__.py

@@ -787,6 +787,28 @@ def null(name, inputs):
     pass
 
 
+def passthrough(input, index=0):
+    """Factory for task functions that pass-through incoming data.
+
+    Callers can use this function to temporarily bypass tasks in the graph.
+
+    Parameters
+    ----------
+    input: hashable object, required
+        The named input that will pass-through to the task output.
+    index: integer, required
+        Index of the named inputs that will pass-through to the task output.
+
+    Returns
+    -------
+    fn: function
+        Task function that will pass input `input` index `index` to its output.
+    """
+    def implementation(name, inputs):
+        return inputs[input][index]
+    return implementation
+
+
 def raise_exception(exception):
     """Factory for task functions that raise an exception when executed.