From cbe3190f93a3e1adb9adf13d9a9af37dec96f004 Mon Sep 17 00:00:00 2001
From: Andrew Hearin <ahearin@anl.gov>
Date: Tue, 7 Jan 2020 12:22:50 -0600
Subject: [PATCH] Include support for empty user-supplied halo catalogs.
 Resolves #960

---
 .../tests/test_user_supplied_halo_catalog.py  | 19 +++++++++++++++++++
 .../sim_manager/user_supplied_halo_catalog.py |  4 +---
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/halotools/sim_manager/tests/test_user_supplied_halo_catalog.py b/halotools/sim_manager/tests/test_user_supplied_halo_catalog.py
index 95bba46cf..256256a0d 100644
--- a/halotools/sim_manager/tests/test_user_supplied_halo_catalog.py
+++ b/halotools/sim_manager/tests/test_user_supplied_halo_catalog.py
@@ -463,3 +463,22 @@ def tearDown(self):
             shutil.rmtree(self.dummy_cache_baseloc)
         except:
             pass
+
+
+def test_support_for_empty_halo_catalogs():
+    """Regression test for #960."""
+    Nhalos = 0
+    Lbox = 100
+    redshift = 0.0
+    halo_x = np.linspace(0, Lbox, Nhalos)
+    halo_y = np.linspace(0, Lbox, Nhalos)
+    halo_z = np.linspace(0, Lbox, Nhalos)
+    halo_mass = np.logspace(10, 15, Nhalos)
+    halo_id = np.arange(0, Nhalos, dtype=np.int)
+    good_halocat_args = (
+        {'halo_x': halo_x, 'halo_y': halo_y,
+        'halo_z': halo_z, 'halo_id': halo_id, 'halo_mass': halo_mass}
+        )
+    halocat = UserSuppliedHaloCatalog(Lbox=Lbox, particle_mass=100, redshift=0.,
+                    **good_halocat_args)
+    assert halocat.halo_table['halo_x'].shape == (0, )
diff --git a/halotools/sim_manager/user_supplied_halo_catalog.py b/halotools/sim_manager/user_supplied_halo_catalog.py
index 1e5a6ef1d..1a0cadbd9 100644
--- a/halotools/sim_manager/user_supplied_halo_catalog.py
+++ b/halotools/sim_manager/user_supplied_halo_catalog.py
@@ -177,10 +177,8 @@ def _parse_constructor_kwargs(self, **kwargs):
             halo_id = np.array(kwargs['halo_id'])
             assert type(halo_id) is np.ndarray
             Nhalos = custom_len(halo_id)
-            assert Nhalos > 1
         except KeyError:
-            msg = ("\nThe UserSuppliedHaloCatalog requires a ``halo_id`` keyword argument "
-                "storing an ndarray of length Nhalos > 1.\n")
+            msg = ("\nThe UserSuppliedHaloCatalog requires a ``halo_id`` keyword argument.")
             raise HalotoolsError(msg)
 
         halo_table_dict = (