diff --git a/README.md b/README.md
index 0694664..87b1dd9 100644
--- a/README.md
+++ b/README.md
@@ -86,6 +86,11 @@ from py2vega import py2vega, Variable
 
 py2vega('3 if value > 0 else 4', whitelist=['value'])  # Returns "value > 0 ? 3 : 4"
 py2vega('3 if my_variable > 0 else 4', whitelist=['value'])  # Raises a SyntaxError, `my_variable` is not whitelisted
+
+# Allow any member access on `value`
+py2vega('3 if value.member1 > value.member2 else 4', whitelist=[Variable('value')])  # Returns "value.member1 > value.member2 ? 3 : 4"
+
+# Restrict member access explicitly
 py2vega('3 if value.member1 > value.member2 else 4', whitelist=[Variable('value', ['member1', 'member2'])])  # Returns "value.member1 > value.member2 ? 3 : 4"
 py2vega('3 if value.member3 > 0 else 4', whitelist=[Variable('value', ['member1', 'member2'])])  # Raises a SyntaxError, `value.member3` is not whitelisted`
 ```
diff --git a/py2vega/main.py b/py2vega/main.py
index ae2740e..0ec64d9 100644
--- a/py2vega/main.py
+++ b/py2vega/main.py
@@ -12,8 +12,13 @@
 class Variable():
     """Helper class for defining a variable in whitelisting."""
 
-    def __init__(self, name, members):
-        """Construct a Variable, given its name and available members."""
+    def __init__(self, name, members=None):
+        """Construct a Variable, given its name and available members.
+
+        If members is None, all direct members are allowed. This is useful for
+        dynamic objects such as Vega's `datum`, where available field names are
+        data-dependent and not known ahead of time."""
+
         self.name = name
         self.members = members
 
@@ -76,6 +81,9 @@ def validate(nodes, origin_node):
 
 def valid_attribute_impl(node, var):
     """Check the attribute access validity. Returns True if the member access is valid, False otherwise."""
+    if var.members is None:
+        return True
+
     if node.value.id == var.name and node.attr in var.members:
         return True
 
diff --git a/test/test_py2vega.py b/test/test_py2vega.py
index 4935c65..bb270f0 100644
--- a/test/test_py2vega.py
+++ b/test/test_py2vega.py
@@ -174,6 +174,9 @@ def test_attribute():
 
     assert py2vega('3 if value.member1 > value.member2 else 4', whitelist=[Variable('value', ['member1', 'member2'])]) == "((value.member1 > value.member2) ? 3 : 4)"
 
+    # Variable without explicit members# Variable without explicit members
+    assert py2vega("'red' if datum.code > 80 else 'blue'", whitelist=[Variable("datum")], ) == "((datum.code > 80) ? 'red' : 'blue')" 
+
     # Nested member access
     whitelisted_vars = [Variable('nested_var', [Variable('var', ['test']), 'x'])]