Fix bls 3tuple

CHANGELOG.md

@@ -6,6 +6,10 @@ All notable changes to this project will be documented in this file.
 ### Added
 - ATA has been added to the list of known telescopes.
 
+### Fixed
+- Bug in selecting baselines on a UVData object using `bls` keyword with 3-tuples and
+more than one polarization (introduced in 3.1.2).
+
 ## [3.1.2] - 2024-11-21
 
 ### Added

src/pyuvdata/utils/bls.py

@@ -423,6 +423,7 @@ def _extract_bls_pol(
             )
 
         bls_2 = copy.deepcopy(bls)
+        bl_pols = set()
         for bl_i, bl in enumerate(bls):
             if len(bl) != 3:
                 raise ValueError("If some bls are 3-tuples, all bls must be 3-tuples.")
@@ -432,7 +433,6 @@ def _extract_bls_pol(
                     "The third element in a bl tuple must be a polarization string"
                 )
 
-            bl_pols = set()
             wh1 = np.where(np.logical_and(ant_1_array == bl[0], ant_2_array == bl[1]))[
                 0
             ]
@@ -454,6 +454,6 @@ def _extract_bls_pol(
                         "associated with it."
                     )
 
-            polarizations = list(bl_pols)
+        polarizations = list(bl_pols)
         bls = bls_2
     return bls, polarizations

tests/uvdata/test_uvdata.py

@@ -1612,6 +1612,11 @@ def test_select_bls(casa_uvfits, sel_type):
     first_ants = [7, 3, 8, 3, 22, 28, 9]
     second_ants = [1, 21, 9, 2, 3, 4, 23]
     pols = ["RR", "RR", "RR", "RR", "RR", "RR", "RR"]
+
+    if sel_type == "antpairpol":
+        # Also test that reading different pols at the same time works.
+        pols[-1] = "LL"
+
     new_unique_ants = np.unique(first_ants + second_ants)
     ant_pairs_to_keep = list(zip(first_ants, second_ants, strict=True))
     sorted_pairs_to_keep = [sort_bl(p) for p in ant_pairs_to_keep]
@@ -1675,9 +1680,7 @@ def test_select_bls(casa_uvfits, sel_type):
         assert pair in sorted_pairs_to_keep
 
     if sel_type == "antpairpol":
-        assert uv_object2.Npols == 1
-    if sel_type == "2_3_tuple":
-        assert uv_object2.Npols == 1
+        assert uv_object2.Npols == 2
 
     assert utils.history._check_histories(
         old_history + f"  Downselected to specific {sel_str} using pyuvdata.",