fix: Step sizes for fixed parameters in minuit should be 0.0 (#1054)

* Step sizes are set to 0.0 for fixed parameters in a fit for minuit
   - The step size acts as the uncertainty for fixed parameters
* Add test for step sizes for fixed parameters

Author: kratsg

src/pyhf/optimize/opt_minuit.py

@@ -44,6 +44,7 @@ def _get_minimizer(
         for index, val in fixed_vals:
             fixed_bools[index] = True
             init_pars[index] = val
+            step_sizes[index] = 0.0
 
         # Minuit requires jac=callable
         if do_grad:

tests/test_optim.py

@@ -454,3 +454,15 @@ def test_init_pars_sync_fixed_values_minuit(mocker):
     opt._get_minimizer(None, [9, 9, 9], [(0, 10)] * 3, fixed_vals=[(0, 1)])
     assert minimizer.Minuit.from_array_func.call_args[1]['start'] == [1, 9, 9]
     assert minimizer.Minuit.from_array_func.call_args[1]['fix'] == [True, False, False]
+
+
+def test_step_sizes_fixed_parameters_minuit(mocker):
+    opt = pyhf.optimize.minuit_optimizer()
+
+    # patch all we need
+    from pyhf.optimize import opt_minuit
+
+    minimizer = mocker.patch.object(opt_minuit, 'iminuit')
+    opt._get_minimizer(None, [9, 9, 9], [(0, 10)] * 3, fixed_vals=[(0, 1)])
+    assert minimizer.Minuit.from_array_func.call_args[1]['fix'] == [True, False, False]
+    assert minimizer.Minuit.from_array_func.call_args[1]['error'] == [0.0, 0.01, 0.01]