Improved documentation with an example

Author: velochy

pymc_extras/distributions/transforms/partial_order.py

@@ -34,19 +34,66 @@ def padded_where(x, to_len, padval=-1):
 class PartialOrder(Transform):
     """Create a PartialOrder transform
 
-    This is a more flexible version of the pymc ordered transform that
+    A more flexible version of the pymc ordered transform that
     allows specifying a (strict) partial order on the elements.
 
-    It works in O(N*D) in runtime, but takes O(N^3) in initialization,
-    where N is the number of nodes in the dag and
-    D is the maximum in-degree of a node in the transitive reduction.
-
+    Examples
+    --------
+    .. code:: python
+
+        import numpy as np
+        import pymc as pm
+        import pymc_extras as pmx
+
+        # Define two partial orders on 4 elements
+        # am[i,j] = 1 means i < j
+        adj_mats = np.array([
+            # 0 < {1, 2} < 3
+            [[0, 1, 1, 0],
+            [0, 0, 0, 1],
+            [0, 0, 0, 1],
+            [0, 0, 0, 0]],
+
+            # 1 < 0 < 3 < 2
+            [[0, 0, 0, 1],
+            [1, 0, 0, 0],
+            [0, 0, 0, 0],
+            [0, 0, 1, 0]],
+        ])
+
+        # Create the partial order from the adjacency matrices
+        po = pmx.PartialOrder(adj_mats)
+
+        with pm.Model() as model:
+            # Generate 3 samples from both partial orders
+            pm.Normal("po_vals", shape=(3,2,4), transform=po,
+                        initval=po.initvals((3,2,4)))
+
+            idata = pm.sample()
+
+        # Verify that for first po, the zeroth element is always the smallest
+        assert (idata.posterior['po_vals'][:,:,:,0,0] <
+                idata.posterior['po_vals'][:,:,:,0,1:]).all()
+
+        # Verify that for second po, the second element is always the largest
+        assert (idata.posterior['po_vals'][:,:,:,1,2] >=
+                idata.posterior['po_vals'][:,:,:,1,:]).all()
+
+    Technical notes
+    ----------------
+    Partial order needs to be strict, i.e. without equalities.
+    A DAG defining the partial order is sufficient, as transitive closure is automatically computed.
+    Code works in O(N*D) in runtime, but takes O(N^3) in initialization,
+    where N is the number of nodes in the dag and D is the maximum
+    in-degree of a node in the transitive reduction.
     """
 
     name = "partial_order"
 
     def __init__(self, adj_mat):
         """
+        Initialize the PartialOrder transform
+
         Parameters
         ----------
         adj_mat: ndarray
@@ -99,10 +146,43 @@ def __init__(self, adj_mat):
         self.dag = np.swapaxes(dag_T, -2, -1)
         self.is_start = np.all(self.dag[..., :, :] == -1, axis=-1)
 
-    def initvals(self, lower=-1, upper=1):
+    def initvals(self, shape=None, lower=-1, upper=1):
+        """
+        Create a set of appropriate initial values for the variable.
+        NB! It is important that proper initial values are used,
+        as only properly ordered values are in the range of the transform.
+
+        Parameters
+        ----------
+        shape: tuple, default None
+            shape of the initial values. If None, adj_mat[:-1] is used
+        lower: float, default -1
+            lower bound for the initial values
+        upper: float, default 1
+            upper bound for the initial values
+
+        Returns
+        -------
+        vals: ndarray
+            initial values for the transformed variable
+        """
+
+        if shape is None:
+            shape = self.dag.shape[:-1]
+
+        if shape[-len(self.dag.shape[:-1]) :] != self.dag.shape[:-1]:
+            raise ValueError("Shape must match the shape of the adjacency matrix")
+
+        # Create the initial values
         vals = np.linspace(lower, upper, self.dag.shape[-2])
         inds = np.argsort(self.ts_inds, axis=-1)
-        return vals[inds]
+        ivals = vals[inds]
+
+        # Expand the initial values to the extra dimensions
+        extra_dims = shape[: -len(self.dag.shape[:-1])]
+        ivals = np.tile(ivals, extra_dims + tuple([1] * len(self.dag.shape[:-1])))
+
+        return ivals
 
     def backward(self, value, *inputs):
         minv = dtype_minval(value.dtype)

tests/distributions/test_transform.py

@@ -55,11 +55,17 @@ def test_forward_backward_dimensionality(self):
     def test_sample_model(self):
         po = PartialOrder(self.adj_mats)
         with pm.Model() as model:
-            x = pm.Normal("x", size=(2, 4), transform=po, initval=po.initvals(-1, 1))
+            x = pm.Normal(
+                "x",
+                size=(3, 2, 4),
+                transform=po,
+                initval=po.initvals(shape=(3, 2, 4), lower=-1, upper=1),
+            )
             idata = pm.sample()
 
         # Check that the order constraints are satisfied
-        xvs = idata.posterior.x.values.transpose(2, 3, 0, 1)
+        # Move chain, draw and "3" dimensions to the back
+        xvs = idata.posterior.x.values.transpose(3, 4, 0, 1, 2)
         x0 = xvs[0]  # 0 < {1, 2} < 3
         assert (
             (x0[0] < x0[1]).all()