forward() should pick answer consistent with forward_solutions_table() and default decision function

RELEASE_NOTES.rst

@@ -38,6 +38,7 @@ New Features and/or Enhancements
 Fixes
 -----
 
+* ``diffract.forward()`` should pick solution consistent with ``diffract.forward_solution_table()``, if it can. Otherwise, fall back to previous iterative method.
 * ``util.restore_reflections()`` use renamed motor axes if so defined.
 
 Maintenance

hkl/diffract.py

@@ -456,9 +456,12 @@ def forward(self, pseudo):
 
         Return the default solution using the ``_decision_fcn()``.
         """
-        solutions = self.calc.forward_iter(
-            start=self.position, end=pseudo, max_iters=self.max_forward_iterations.get()
-        )
+        try:
+            solutions = self.calc.forward(list(pseudo))
+        except ValueError:
+            solutions = self.calc.forward_iter(
+                start=self.position, end=pseudo, max_iters=self.max_forward_iterations.get()
+            )
         logger.debug("pseudo to real: %s", solutions)
         return self._decision_fcn(pseudo, solutions)
 

hkl/tests/test_diffract.py

@@ -357,3 +357,33 @@ def test_get_constraint(axis, fourc):
     else:
         with pytest.raises(KeyError):
             fourc.get_axis_constraints(axis)
+
+
+def test_i303_forward_solution_original_problem(e4cv):
+    assert e4cv is not None
+
+    # Approximate the code presented as the example problem.
+    refl = (1, 1, 1)
+    e4cv.reset_constraints()
+
+    e4cv.calc.engine.mode = "constant_phi"
+    assert e4cv.calc.engine.mode == "constant_phi"
+    CONSTANT_PHI = 23.4567
+
+    e4cv.apply_constraints({"phi": Constraint(-180, 180, CONSTANT_PHI, True)})
+    assert pytest.approx(e4cv.calc["phi"].value, abs=1e-4) == CONSTANT_PHI
+
+    # Check that phi is held constant in forward_solutions_table()
+    # Returns a pyRestTable.Table object.
+    table = e4cv.forward_solutions_table([refl], full=True, digits=4)
+    phi_column = table.labels.index("phi")
+    assert len(table.rows) > 0
+    for row in table.rows:
+        assert pytest.approx(row[phi_column], abs=1e-4) == CONSTANT_PHI
+
+    # Check that phi is held constant in forward()
+    # Returns a position namedtuple.
+    position = e4cv.forward(refl)
+    assert pytest.approx(position.phi, abs=1e-4) == CONSTANT_PHI
+
+    e4cv.reset_constraints()

hkl/tests/test_tardis.py

@@ -132,8 +132,8 @@ def test_reachable(tardis, kcf_sample, constrain):
     tardis.move(ppos)
     print("tardis position is", tardis.position)
     print("tardis position is", tardis.calc.physical_positions)
-    numpy.testing.assert_almost_equal(tardis.position, ppos)
-    numpy.testing.assert_almost_equal(tardis.calc.physical_positions, rpos)
+    numpy.testing.assert_almost_equal(tardis.position, ppos, decimal=4)
+    numpy.testing.assert_almost_equal(tardis.calc.physical_positions, rpos, decimal=4)
 
 
 def test_inversion(tardis, kcf_sample, constrain):