fix _neuron_population for sholl_frequency over a neuron populati…

…on (#944)

neurom/features/neuronfunc.py

@@ -46,6 +46,7 @@
 import numpy as np
 
 from neurom import morphmath
+from neurom.core import Population
 from neurom.core.neuron import iter_neurites, iter_segments
 from neurom.core.dataformat import COLS
 from neurom.core.types import NeuriteType
@@ -57,7 +58,7 @@
 
 def _neuron_population(nrns):
     """Makes sure `nrns` behaves like a neuron population."""
-    return nrns.neurons if hasattr(nrns, 'neurons') else (nrns,)
+    return nrns if isinstance(nrns, Population) else (nrns,)
 
 
 @feature(shape=())

tests/features/test_get_features.py

@@ -876,6 +876,8 @@ def test_sholl_frequency():
     assert_allclose(get_feature('sholl_frequency', NEURON, neurite_type=NeuriteType.axon),
                     [1, 2, 2, 6, 2, 2, 2, 2, 2])
 
+    assert len(get_feature('sholl_frequency', POP)) == 108
+
 
 @pytest.mark.skip('test_get_segment_lengths is disabled in test_get_features')
 def test_section_path_distances_endpoint():