fix(backport): Guard Minuit optimizer against provided strategy of None

* Backport:
   - PR https://github.com/scikit-hep/pyhf/pull/ 2277
   - PR https://github.com/scikit-hep/pyhf/pull/ 2278

docs/contributors.rst

@@ -31,3 +31,4 @@ Contributors include:
 - Jerry Ling
 - Nathan Simpson
 - Beojan Stanislaus
+- Daniel Werner

src/pyhf/optimize/opt_minuit.py

@@ -28,7 +28,10 @@ def __init__(self, *args, **kwargs):
         Args:
             errordef (:obj:`float`): See minuit docs. Default is ``1.0``.
             steps (:obj:`int`): Number of steps for the bounds. Default is ``1000``.
-            strategy (:obj:`int`): See :attr:`iminuit.Minuit.strategy`. Default is ``None``.
+            strategy (:obj:`int`): See :attr:`iminuit.Minuit.strategy`.
+              Default is ``None``, which results in either
+              :attr:`iminuit.Minuit.strategy` ``0`` or ``1`` from the evaluation of
+              ``int(not pyhf.tensorlib.default_do_grad)``.
             tolerance (:obj:`float`): Tolerance for termination.
               See specific optimizer for detailed meaning.
               Default is ``0.1``.
@@ -99,11 +102,14 @@ def _minimize(
             fitresult (scipy.optimize.OptimizeResult): the fit result
         """
         maxiter = options.pop('maxiter', self.maxiter)
-        # 0: Fast, user-provided gradient
-        # 1: Default, no user-provided gradient
-        strategy = options.pop(
-            'strategy', self.strategy if self.strategy else not do_grad
-        )
+        # do_grad value results in iminuit.Minuit.strategy of either:
+        #   0: Fast. Does not check a user-provided gradient.
+        #   1: Default. Checks user-provided gradient against numerical gradient.
+        strategy = options.pop("strategy", self.strategy)
+        # Guard against None from either self.strategy defaulting to None or
+        # passing strategy=None as options kwarg
+        if strategy is None:
+            strategy = 0 if do_grad else 1
         tolerance = options.pop('tolerance', self.tolerance)
         if options:
             raise exceptions.Unsupported(

tests/test_optim.py

@@ -173,28 +173,35 @@ def test_minuit_strategy_do_grad(mocker, backend):
     assert spy.spy_return.minuit.strategy == 1
 
 
-@pytest.mark.parametrize('strategy', [0, 1])
+@pytest.mark.parametrize('strategy', [0, 1, 2])
 def test_minuit_strategy_global(mocker, backend, strategy):
     pyhf.set_backend(
         pyhf.tensorlib, pyhf.optimize.minuit_optimizer(strategy=strategy, tolerance=0.2)
     )
     spy = mocker.spy(pyhf.optimize.minuit_optimizer, '_minimize')
-    m = pyhf.simplemodels.uncorrelated_background([50.0], [100.0], [10.0])
-    data = pyhf.tensorlib.astensor([125.0] + m.config.auxdata)
+    model = pyhf.simplemodels.uncorrelated_background([50.0], [100.0], [10.0])
+    data = pyhf.tensorlib.astensor([125.0] + model.config.auxdata)
 
-    do_grad = pyhf.tensorlib.default_do_grad
-    pyhf.infer.mle.fit(data, m)
+    pyhf.infer.mle.fit(data, model)
     assert spy.call_count == 1
-    assert spy.spy_return.minuit.strategy == strategy if do_grad else 1
+    assert spy.spy_return.minuit.strategy == strategy
 
-    pyhf.infer.mle.fit(data, m, strategy=0)
+    pyhf.infer.mle.fit(data, model, strategy=None)
     assert spy.call_count == 2
-    assert spy.spy_return.minuit.strategy == 0
+    assert spy.spy_return.minuit.strategy == int(not pyhf.tensorlib.default_do_grad)
 
-    pyhf.infer.mle.fit(data, m, strategy=1)
+    pyhf.infer.mle.fit(data, model, strategy=0)
     assert spy.call_count == 3
+    assert spy.spy_return.minuit.strategy == 0
+
+    pyhf.infer.mle.fit(data, model, strategy=1)
+    assert spy.call_count == 4
     assert spy.spy_return.minuit.strategy == 1
 
+    pyhf.infer.mle.fit(data, model, strategy=2)
+    assert spy.call_count == 5
+    assert spy.spy_return.minuit.strategy == 2
+
 
 def test_set_tolerance(backend):
     m = pyhf.simplemodels.uncorrelated_background([50.0], [100.0], [10.0])