Add an error if parameters are not properly passed to flag_init method

src/pyuvdata/uvdata/mwa_corr_fits.py

@@ -536,6 +536,7 @@ def correct_cable_length(self, cable_lens, ant_1_inds, ant_2_inds):
     def flag_init(
         self,
         num_fine_chan,
+        *,
         edge_width=80e3,
         start_flag=2.0,
         end_flag=0.0,
@@ -551,6 +552,9 @@ def flag_init(
 
         Parameters
         ----------
+        num_fine_chans : int
+            Number of fine channels in each data file (number of fine channels
+            per coarse channel).
         edge_width: float
             The width to flag on the edge of each coarse channel, in hz. Set to
             0 for no edge flagging.
@@ -565,7 +569,8 @@ def flag_init(
         freq_inds : array_like of int, optional
             Frequency indices that were kept if frequency selection was done.
         n_orig_freq : int, optional
-            Number of original frequencies if frequency selection was done.
+            Number of original frequencies if frequency selection was done. Must
+            be passed if freq_inds is not None.
 
         Raises
         ------
@@ -576,6 +581,8 @@ def flag_init(
             (0 also acceptable).
             If end_flag is not an integer multiple of the integration time
             (0 also acceptable).
+        AssertionError
+            If freq_inds is not None and n_orig_freq is None.
 
         """
         if (edge_width % self.channel_width[0]) > 0:
@@ -594,6 +601,11 @@ def flag_init(
                 "integration_time of the data or zero."
             )
 
+        if freq_inds is not None and n_orig_freq is None:
+            raise AssertionError(
+                "If freq_inds is not None, n_orig_freq must be passed."
+            )
+
         num_ch_flag = int(edge_width / self.channel_width[0])
         num_start_flag = int(start_flag / self.integration_time[0])
         num_end_flag = int(end_flag / self.integration_time[0])
@@ -680,7 +692,8 @@ def _read_fits_file(
         file_nums : array
             List of included file numbers ordered by coarse channel
         num_fine_chans : int
-            Number of fine channels in each data file
+            Number of fine channels in each data file (number of fine channels
+            per coarse channel).
         int_time : float
             The integration time of each observation.
         map_inds : array

tests/uvdata/test_mwa_corr_fits.py

@@ -652,6 +652,22 @@ def test_flag_init_errors(flag_file_init, err_type, read_kwargs, err_msg):
         uv.read(flag_file_init, **read_kwargs)
 
 
+@pytest.mark.filterwarnings(
+    "ignore:coarse channels are not contiguous for this observation"
+)
+@pytest.mark.filterwarnings("ignore:some coarse channel files were not submitted")
+def test_flag_init_error_freq_sel(flag_file_init):
+    from pyuvdata.uvdata.mwa_corr_fits import MWACorrFITS
+
+    mwa_obj = MWACorrFITS()
+    mwa_obj.read_mwa_corr_fits(flag_file_init, flag_init=False)
+
+    with pytest.raises(
+        AssertionError, match="If freq_inds is not None, n_orig_freq must be passed."
+    ):
+        mwa_obj.flag_init(32, freq_inds=np.arange(10), n_orig_freq=None)
+
+
 def test_read_metadata_only(tmp_path):
     """Test reading an MWA corr fits file as metadata only."""
     uvd = UVData()