fix: jax backend tolist for tracers in logging (#2580)

* JAX can not convert a tracer during .tolist (because it has no data to put in a list).
  This fix checks if JAX is currently in tracing time and instead returns the tracer
  (or rather its abstract value for a little nicer representation). This preserves the
  same error message as for the NumPy backend in case there's a shape mismatch in data.
* Add Peter Fackeldey to contributors list.

Author: pfackeldey

docs/contributors.rst

@@ -36,3 +36,4 @@ Contributors include:
 - Lorenz Gaertner
 - Melissa Weber Mendonça
 - Matthias Bussonnier
+- Peter Fackeldey

src/pyhf/tensor/jax_backend.py

@@ -2,6 +2,7 @@
 
 config.update('jax_enable_x64', True)
 
+from jax.core import Tracer
 from jax import Array
 import jax.numpy as jnp
 from jax.scipy.special import gammaln, xlogy
@@ -14,6 +15,13 @@
 log = logging.getLogger(__name__)
 
 
+def _currently_jitting():
+    """
+    JAX turns arrays into Tracers during jit-compilation, so check for that.
+    """
+    return isinstance(jnp.array(1), Tracer)
+
+
 class _BasicPoisson:
     def __init__(self, rate):
         self.rate = rate
@@ -184,6 +192,9 @@ def conditional(self, predicate, true_callable, false_callable):
         return true_callable() if predicate else false_callable()
 
     def tolist(self, tensor_in):
+        if _currently_jitting():
+            # .aval is the abstract value and has a little nicer representation
+            return tensor_in.aval
         try:
             return jnp.asarray(tensor_in).tolist()
         except (TypeError, ValueError):

tests/test_backends.py

@@ -84,3 +84,21 @@ def test_backend_array_type(backend):
 def test_tensor_array_types():
     # can't really assert the content of them so easily
     assert pyhf.tensor.array_types
+
+
+@pytest.mark.only_jax
+def test_jax_data_shape_mismatch_during_jitting(backend):
+    """
+    Validate that during JAX tracing time pyhf doesn't try
+    to convert the data to a list, which is not possible with tracers,
+    for a shape mismatch.
+    Instead, return the tracer itself for a proper error message.
+    Issue: https://github.com/scikit-hep/pyhf/issues/1422
+    PR: https://github.com/scikit-hep/pyhf/pull/2580
+    """
+    model = pyhf.simplemodels.uncorrelated_background([10], [15], [5])
+    with pytest.raises(
+        pyhf.exceptions.InvalidPdfData,
+        match="eval failed as data has len 1 but 2 was expected",
+    ):
+        pyhf.infer.mle.fit([12.5], model)