fix: stitch_pars bug in stitching values other than fixed values (#991)

* Fix bug in stitch_pars to use stitch_with argument
* Refactor stitch_pars to inside _make_stitch_pars function 
* Add test for stitch_pars

src/pyhf/optimize/common.py

@@ -3,6 +3,31 @@
 from ..tensor.common import _TensorViewer
 
 
+def _make_stitch_pars(tv=None, fixed_values=None):
+    """
+    Construct a callable to stitch fixed paramter values into the unfixed parameters. See :func:`shim`.
+
+    This is extracted out to be unit-tested for proper behavior.
+
+    If ``tv`` or ``fixed_values`` are not provided, this returns the identity callable.
+
+    Args:
+        tv (~pyhf.tensor.common._TensorViewer): tensor viewer instance
+        fixed_values (`list`): default set of values to stitch parameters with
+
+    Returns:
+        callable (`func`): a callable that takes nuisance parameter values as input
+    """
+    if tv is None or fixed_values is None:
+        return lambda pars, stitch_with=None: pars
+
+    def stitch_pars(pars, stitch_with=fixed_values):
+        tb, _ = get_backend()
+        return tv.stitch([tb.astensor(stitch_with, dtype='float'), pars])
+
+    return stitch_pars
+
+
 def _get_tensor_shim():
     """
     A shim-retriever to lazy-retrieve the necessary shims as needed.
@@ -96,15 +121,13 @@ def shim(
 
         tv = _TensorViewer([fixed_idx, variable_idx])
         # NB: this is a closure, tensorlib needs to be accessed at a different point in time
-        def stitch_pars(pars, stitch_with=fixed_values):
-            tb, _ = get_backend()
-            return tv.stitch([tb.astensor(fixed_values, dtype='float'), pars])
+        stitch_pars = _make_stitch_pars(tv, fixed_values)
 
     else:
         variable_init = init_pars
         variable_bounds = par_bounds
         minimizer_fixed_vals = fixed_vals
-        stitch_pars = lambda pars, stitch_with=None: pars
+        stitch_pars = _make_stitch_pars()
 
     objective_and_grad = _get_tensor_shim()(
         objective,

tests/test_optim.py

@@ -1,6 +1,7 @@
 import pyhf
 from pyhf.optimize.mixins import OptimizerMixin
-from pyhf.optimize.common import _get_tensor_shim
+from pyhf.optimize.common import _get_tensor_shim, _make_stitch_pars
+from pyhf.tensor.common import _TensorViewer
 import pytest
 from scipy.optimize import minimize, OptimizeResult
 import iminuit
@@ -409,3 +410,29 @@ def test_get_tensor_shim(monkeypatch):
         _get_tensor_shim()
 
     assert 'No optimizer shim for fake_backend.' == str(excinfo.value)
+
+
+def test_stitch_pars(backend):
+    tb, _ = backend
+
+    passthrough = _make_stitch_pars()
+    pars = ['a', 'b', 1.0, 2.0, object()]
+    assert passthrough(pars) == pars
+
+    fixed_idx = [0, 3, 4]
+    variable_idx = [1, 2, 5]
+    fixed_vals = [10, 40, 50]
+    variable_vals = [20, 30, 60]
+    tv = _TensorViewer([fixed_idx, variable_idx])
+    stitch_pars = _make_stitch_pars(tv, fixed_vals)
+
+    pars = tb.astensor(variable_vals)
+    assert tb.tolist(stitch_pars(pars)) == [10, 20, 30, 40, 50, 60]
+    assert tb.tolist(stitch_pars(pars, stitch_with=tb.zeros(3))) == [
+        0,
+        20,
+        30,
+        0,
+        0,
+        60,
+    ]