diff --git a/src/python/pants/base/specs.py b/src/python/pants/base/specs.py
index c4247cc4b276..14445fcaf035 100644
--- a/src/python/pants/base/specs.py
+++ b/src/python/pants/base/specs.py
@@ -8,6 +8,7 @@
 from abc import abstractmethod
 
 from pants.util.meta import AbstractClass
+from pants.util.objects import DatatypeFieldDecl as F
 from pants.util.objects import datatype
 
 
@@ -60,11 +61,11 @@ def to_spec_string(self):
     return '{}^'.format(self.directory)
 
 
-class Specs(datatype(['dependencies', 'tags', ('exclude_patterns', tuple)])):
+class Specs(datatype([
+    'dependencies',
+    F('tags', tuple, default_value=()),
+    F('exclude_patterns', tuple, default_value=())])):
   """A collection of Specs representing Spec subclasses, tags and regex filters."""
 
-  def __new__(cls, dependencies, tags=tuple(), exclude_patterns=tuple()):
-    return super(Specs, cls).__new__(cls, dependencies, tags, exclude_patterns)
-
   def exclude_patterns_memo(self):
     return [re.compile(pattern) for pattern in set(self.exclude_patterns or [])]
diff --git a/src/python/pants/engine/selectors.py b/src/python/pants/engine/selectors.py
index d46e4b88a1a5..9de04bea44c1 100644
--- a/src/python/pants/engine/selectors.py
+++ b/src/python/pants/engine/selectors.py
@@ -11,7 +11,8 @@
 import six
 
 from pants.util.meta import AbstractClass
-from pants.util.objects import Exactly, datatype
+from pants.util.objects import DatatypeFieldDecl as F
+from pants.util.objects import Exactly, SubclassesOf, datatype
 
 
 def type_or_constraint_repr(constraint):
@@ -96,23 +97,25 @@ def product(self):
     """The product that this selector produces."""
 
 
-class Select(datatype(['product', 'optional']), Selector):
+class Select(datatype([
+    'product',
+    F('optional', bool, default_value=False),
+]), Selector):
   """Selects the given Product for the Subject provided to the constructor.
 
   If optional=True and no matching product can be produced, will return None.
   """
 
-  def __new__(cls, product, optional=False):
-    obj = super(Select, cls).__new__(cls, product, optional)
-    return obj
-
   def __repr__(self):
     return '{}({}{})'.format(type(self).__name__,
                              type_or_constraint_repr(self.product),
                              ', optional=True' if self.optional else '')
 
 
-class SelectVariant(datatype(['product', 'variant_key']), Selector):
+class SelectVariant(datatype([
+    'product',
+    ('variant_key', SubclassesOf(six.string_types)),
+]), Selector):
   """Selects the matching Product and variant name for the Subject provided to the constructor.
 
   For example: a SelectVariant with a variant_key of "thrift" and a product of type ApacheThrift
@@ -121,11 +124,6 @@ class SelectVariant(datatype(['product', 'variant_key']), Selector):
   """
   optional = False
 
-  def __new__(cls, product, variant_key):
-    if not isinstance(variant_key, six.string_types):
-      raise ValueError('Expected variant_key to be a string, but was {!r}'.format(variant_key))
-    return super(SelectVariant, cls).__new__(cls, product, variant_key)
-
   def __repr__(self):
     return '{}({}, {})'.format(type(self).__name__,
                                type_or_constraint_repr(self.product),