diff --git a/lib/gpt/core/coordinates.py b/lib/gpt/core/coordinates.py
index abb03a758..73757b1fa 100644
--- a/lib/gpt/core/coordinates.py
+++ b/lib/gpt/core/coordinates.py
@@ -131,7 +131,7 @@ def mat_backward(dst, src):
 
 def coordinate_mask(field, mask):
     assert isinstance(mask, numpy.ndarray)
-    assert field.otype.data_otype() == gpt.ot_singlet
+    assert field.otype.is_singlet
 
     x = gpt.coordinates(field)
     field[x] = mask.astype(field.grid.precision.complex_dtype).reshape((len(mask), 1))
diff --git a/lib/gpt/core/expr.py b/lib/gpt/core/expr.py
index 267140bd4..d4fb82a1c 100644
--- a/lib/gpt/core/expr.py
+++ b/lib/gpt/core/expr.py
@@ -113,7 +113,7 @@ def __mul__(self, l):
                 if uf & gpt.factor_unary.BIT_CONJ != 0:
                     lhs = lhs.conj()
                 res = gpt.tensor(np.tensordot(lhs, l.array, axes=mt[1]), mt[0]())
-                if res.otype == gpt.ot_singlet:
+                if res.otype.is_singlet:
                     res = complex(res.array)
                 return res
             assert 0
@@ -247,7 +247,7 @@ def get_otype_from_expression(e):
     bare_otype = None
     for coef, term in e.val:
         if len(term) == 0:
-            t_otype = gpt.ot_singlet
+            t_otype = gpt.ot_singlet()
         else:
             t_otype = None
             t_adj = False
diff --git a/lib/gpt/core/lattice.py b/lib/gpt/core/lattice.py
index 10eeae6c6..93b2f26f0 100644
--- a/lib/gpt/core/lattice.py
+++ b/lib/gpt/core/lattice.py
@@ -288,8 +288,8 @@ def __itruediv__(self, expr):
         return self
 
     def __lt__(self, other):
-        assert self.otype.data_otype() == gpt.ot_singlet
-        assert other.otype.data_otype() == gpt.ot_singlet
+        assert self.otype.is_singlet
+        assert other.otype.is_singlet
         res = gpt.lattice(self)
         params = {"operator": "<"}
         cgpt.binary(res.v_obj[0], self.v_obj[0], other.v_obj[0], params)
diff --git a/lib/gpt/core/object_type/__init__.py b/lib/gpt/core/object_type/__init__.py
index f7b4c3a28..e90f5045e 100644
--- a/lib/gpt/core/object_type/__init__.py
+++ b/lib/gpt/core/object_type/__init__.py
@@ -38,7 +38,7 @@ def gpt_object(first, ot):
 ###
 # Container objects without (lie) group structure
 def singlet(grid):
-    return gpt_object(grid, ot_singlet)
+    return gpt_object(grid, ot_singlet())
 
 
 def matrix_color(grid, ndim):
diff --git a/lib/gpt/core/object_type/base.py b/lib/gpt/core/object_type/base.py
index 9f60fe8da..35bdc3ee2 100644
--- a/lib/gpt/core/object_type/base.py
+++ b/lib/gpt/core/object_type/base.py
@@ -36,6 +36,7 @@ class ot_base:
 
     # list of object types to which I can convert and converter function
     ctab = {}
+    is_singlet = False
 
     # safe cast of data_alias
     def data_otype(self):
diff --git a/lib/gpt/core/object_type/complex_additive_group.py b/lib/gpt/core/object_type/complex_additive_group.py
index d1ffaac20..f7889e91a 100644
--- a/lib/gpt/core/object_type/complex_additive_group.py
+++ b/lib/gpt/core/object_type/complex_additive_group.py
@@ -30,13 +30,13 @@ class ot_complex_additive_group(ot_singlet):
 
     def __init__(self):
         self.__name__ = "ot_complex_additive_group"
-        self.data_alias = lambda: ot_singlet
+        self.data_alias = lambda: ot_singlet()
         self.rmtab = {
-            "ot_singlet": (lambda: ot_singlet, None),
+            "ot_singlet": (lambda: ot_singlet(), None),
         }
         self.mtab = {
             self.__name__: (lambda: self, None),
-            "ot_singlet": (lambda: ot_singlet, None),
+            "ot_singlet": (lambda: ot_singlet(), None),
         }
 
     # this is always multiplicative identity, not neutral element of group
@@ -87,7 +87,7 @@ def __init__(self, n):
         }
         self.otab = {self.__name__: (lambda: ot_matrix_complex_additive_group(n), [])}
         self.itab = {
-            self.__name__: (lambda: ot_singlet, (0, 0)),
+            self.__name__: (lambda: ot_singlet(), (0, 0)),
         }
         self.cache = {}
 
@@ -119,7 +119,7 @@ def coordinates(self, l, c=None):
         assert l.otype.__name__ == self.__name__
         if c is None:
             r = [None] * self.shape[0] * 2
-            a = gpt.separate_indices(l, (0, lambda: ot_singlet), self.cache)
+            a = gpt.separate_indices(l, (0, lambda: ot_singlet()), self.cache)
             for i in a:
                 r[i[0]] = gpt.component.real(a[i])
                 r[i[0] + self.shape[0]] = gpt.component.imag(a[i])
diff --git a/lib/gpt/core/object_type/container.py b/lib/gpt/core/object_type/container.py
index 6dfca4937..f70b320cf 100644
--- a/lib/gpt/core/object_type/container.py
+++ b/lib/gpt/core/object_type/container.py
@@ -65,13 +65,14 @@ class ot_singlet(ot_base):
     colortrace = (None, None, None)
     v_otype = ["ot_singlet"]
     mtab = {
-        "ot_singlet": (lambda: ot_singlet, None),
+        "ot_singlet": (lambda: ot_singlet(), None),
     }
+    is_singlet = True
 
-    def data_otype(self=None):
-        return ot_singlet
+    def data_otype(self):
+        return ot_singlet()
 
-    def identity():
+    def identity(self):
         return 1.0
 
 
@@ -84,7 +85,7 @@ def __init__(self, ndim):
         self.shape = (ndim, ndim)
         self.transposed = (1, 0)
         self.spintrace = (None, None, None)  # do nothing
-        self.colortrace = (0, 1, lambda: ot_singlet)
+        self.colortrace = (0, 1, lambda: ot_singlet())
         self.v_otype = ["ot_mcolor%d" % ndim]  # cgpt data types
         self.mtab = {
             self.__name__: (lambda: self, (1, 0)),
@@ -115,7 +116,7 @@ def __init__(self, ndim):
         }
         self.otab = {self.__name__: (lambda: ot_matrix_color(ndim), [])}
         self.itab = {
-            self.__name__: (lambda: ot_singlet, (0, 0)),
+            self.__name__: (lambda: ot_singlet(), (0, 0)),
         }
 
     def compose(self, a, b):
@@ -133,7 +134,7 @@ def __init__(self, ndim):
         self.nfloats = 2 * ndim * ndim
         self.shape = (ndim, ndim)
         self.transposed = (1, 0)
-        self.spintrace = (0, 1, lambda: ot_singlet)
+        self.spintrace = (0, 1, lambda: ot_singlet())
         self.colortrace = (None, None, None)  # do nothing
         self.v_otype = ["ot_mspin%d" % ndim]
         self.mtab = {
@@ -197,7 +198,7 @@ def __init__(self, ndim):
             "ot_singlet": (lambda: self, None),
         }
         self.otab = {self.__name__: (lambda: ot_matrix_spin(ndim), [])}
-        self.itab = {self.__name__: (lambda: ot_singlet, (0, 0))}
+        self.itab = {self.__name__: (lambda: ot_singlet(), (0, 0))}
 
     def compose(self, a, b):
         return a + b
@@ -269,7 +270,7 @@ def __init__(self, spin_ndim, color_ndim):
             ),
         }
         self.itab = {
-            self.__name__: (lambda: ot_singlet, ([0, 1], [0, 1])),
+            self.__name__: (lambda: ot_singlet(), ([0, 1], [0, 1])),
         }
         self.mtab = {
             "ot_singlet": (lambda: self, None),
@@ -346,7 +347,7 @@ def __init__(self, n):
             "ot_singlet": (lambda: self, None),
         }
         self.itab = {
-            self.__name__: (lambda: ot_singlet, (0, 0)),
+            self.__name__: (lambda: ot_singlet(), (0, 0)),
         }
 
 
@@ -367,7 +368,7 @@ def __init__(self, n):
         self.shape = (n, n)
         self.transposed = (1, 0)
         self.spintrace = (None, None, None)
-        self.colortrace = (0, 1, lambda: ot_singlet)
+        self.colortrace = (0, 1, lambda: ot_singlet())
         self.vector_type = ot_vector_singlet(n)
         self.mtab = {
             self.__name__: (lambda: self, (1, 0)),
diff --git a/lib/gpt/core/object_type/real_additive_group.py b/lib/gpt/core/object_type/real_additive_group.py
index 7cb5dcb20..e5b29e5a7 100644
--- a/lib/gpt/core/object_type/real_additive_group.py
+++ b/lib/gpt/core/object_type/real_additive_group.py
@@ -30,13 +30,13 @@ class ot_real_additive_group(ot_singlet):
 
     def __init__(self):
         self.__name__ = "ot_real_additive_group"
-        self.data_alias = lambda: ot_singlet
+        self.data_alias = lambda: ot_singlet()
         self.rmtab = {
-            "ot_singlet": (lambda: ot_singlet, None),
+            "ot_singlet": (lambda: ot_singlet(), None),
         }
         self.mtab = {
             self.__name__: (lambda: self, None),
-            "ot_singlet": (lambda: ot_singlet, None),
+            "ot_singlet": (lambda: ot_singlet(), None),
         }
 
     # this is always multiplicative identity, not neutral element of group
@@ -87,7 +87,7 @@ def __init__(self, n):
         }
         self.otab = {self.__name__: (lambda: ot_matrix_real_additive_group(n), [])}
         self.itab = {
-            self.__name__: (lambda: ot_singlet, (0, 0)),
+            self.__name__: (lambda: ot_singlet(), (0, 0)),
         }
         self.cache = {}
 
@@ -114,7 +114,7 @@ def coordinates(self, l, c=None):
         assert l.otype.__name__ == self.__name__
         if c is None:
             r = [None] * self.shape[0]
-            a = gpt.separate_indices(l, (0, lambda: ot_singlet), self.cache)
+            a = gpt.separate_indices(l, (0, lambda: ot_singlet()), self.cache)
             for i in a:
                 r[i[0]] = a[i]
             return r
diff --git a/lib/gpt/core/object_type/u_1.py b/lib/gpt/core/object_type/u_1.py
index fb0724f96..936b845c3 100644
--- a/lib/gpt/core/object_type/u_1.py
+++ b/lib/gpt/core/object_type/u_1.py
@@ -34,13 +34,13 @@ def identity(self):
 
     def __init__(self, name):
         self.__name__ = name
-        self.data_alias = lambda: ot_singlet
+        self.data_alias = lambda: ot_singlet()
         self.rmtab = {
-            "ot_singlet": (lambda: ot_singlet, None),
+            "ot_singlet": (lambda: ot_singlet(), None),
         }
         self.mtab = {
             self.__name__: (lambda: self, None),
-            "ot_singlet": (lambda: ot_singlet, None),
+            "ot_singlet": (lambda: ot_singlet(), None),
         }
 
 
diff --git a/lib/gpt/core/tensor.py b/lib/gpt/core/tensor.py
index 9ef9a5ee8..27c87265b 100644
--- a/lib/gpt/core/tensor.py
+++ b/lib/gpt/core/tensor.py
@@ -75,7 +75,7 @@ def adj(self):
         return tensor(np.transpose(self.array.conj(), self.otype.transposed), self.otype)
 
     def reduced(self):
-        if self.otype.data_otype() == gpt.ot_singlet:
+        if self.otype.is_singlet:
             return complex(self.array)
         return self
 
@@ -92,7 +92,7 @@ def trace(self, t):
             if ct[0] is not None:
                 res = tensor(np.trace(res.array, offset=0, axis1=ct[0], axis2=ct[1]), ct[2]())
 
-        if res.otype == gpt.ot_singlet:
+        if res.otype.is_singlet:
             res = complex(res.array)
         return res
 
diff --git a/lib/gpt/core/util.py b/lib/gpt/core/util.py
index d4b03c41e..e04c47d19 100644
--- a/lib/gpt/core/util.py
+++ b/lib/gpt/core/util.py
@@ -52,7 +52,7 @@ def to_num(x):
 
 # tensor
 def value_to_tensor(val, otype):
-    if otype.data_otype() == gpt.ot_singlet:
+    if otype.is_singlet:
         # this is not ideal, can we do a subclass of complex that preserves otype info?
         return complex(val)
     return gpt.tensor(val, otype)
diff --git a/lib/gpt/create/sparse_grid.py b/lib/gpt/create/sparse_grid.py
index d24d7d281..99e28328f 100644
--- a/lib/gpt/create/sparse_grid.py
+++ b/lib/gpt/create/sparse_grid.py
@@ -26,7 +26,7 @@ def coordinates(src, position, spacing):
 
 
 def zn(src, position, spacing, rng, n):
-    singlet = gpt.lattice(src.grid, gpt.ot_singlet)
+    singlet = gpt.lattice(src.grid, gpt.ot_singlet())
     singlet.checkerboard(src.checkerboard())
     pos = coordinates(src, position, spacing)
     singlet_full = gpt.lattice(singlet)
diff --git a/lib/gpt/ml/layer/nearest_neighbor.py b/lib/gpt/ml/layer/nearest_neighbor.py
index e4c67ae45..24145490c 100644
--- a/lib/gpt/ml/layer/nearest_neighbor.py
+++ b/lib/gpt/ml/layer/nearest_neighbor.py
@@ -22,7 +22,7 @@
 
 
 class nearest_neighbor(cshift):
-    def __init__(self, grid, ot_input=g.ot_singlet, ot_weights=g.ot_singlet, activation=sigmoid):
+    def __init__(self, grid, ot_input=g.ot_singlet(), ot_weights=g.ot_singlet(), activation=sigmoid):
         nd = grid.nd
         super().__init__(
             grid,