Removed use of catch_warnings (#5685)

* made changes, moving warnings out of for loop

* added in original functionality flip

* fixed msg not being assigned

* fixed test

* Updated tests to better document nuances

* corrected dummy warning

* review comments

* review comments

* added a msg = None

lib/iris/fileformats/_nc_load_rules/helpers.py

@@ -327,7 +327,7 @@ class UnknownCellMethodWarning(iris.exceptions.IrisUnknownCellMethodWarning):
     pass
 
 
-def parse_cell_methods(nc_cell_methods):
+def parse_cell_methods(nc_cell_methods, cf_name=None):
     """Parse a CF cell_methods attribute string into a tuple of zero or
     more CellMethod instances.
 
@@ -345,6 +345,7 @@ def parse_cell_methods(nc_cell_methods):
     results are not affected.
 
     """
+    msg = None
     cell_methods = []
     if nc_cell_methods is not None:
         for m in _split_cell_methods(nc_cell_methods):
@@ -356,10 +357,16 @@ def parse_cell_methods(nc_cell_methods):
             method_words = method.split()
             if method_words[0].lower() not in _CM_KNOWN_METHODS:
                 msg = "NetCDF variable contains unknown cell method {!r}"
-                warnings.warn(
-                    msg.format("{}".format(method_words[0])),
-                    category=UnknownCellMethodWarning,
-                )
+                msg = msg.format(method_words[0])
+                if cf_name:
+                    name = "{}".format(cf_name)
+                    msg = msg.replace("variable", "variable {!r}".format(name))
+                else:
+                    warnings.warn(
+                        msg,
+                        category=UnknownCellMethodWarning,
+                    )
+                    msg = None
             d[_CM_METHOD] = method
             name = d[_CM_NAME]
             name = name.replace(" ", "")
@@ -417,6 +424,9 @@ def parse_cell_methods(nc_cell_methods):
                 comments=d[_CM_COMMENT],
             )
             cell_methods.append(cell_method)
+        # only prints one warning, rather than each loop
+        if msg:
+            warnings.warn(msg, category=UnknownCellMethodWarning)
     return tuple(cell_methods)
 
 
@@ -447,21 +457,7 @@ def build_cube_metadata(engine):
 
     # Incorporate cell methods
     nc_att_cell_methods = getattr(cf_var, CF_ATTR_CELL_METHODS, None)
-    with warnings.catch_warnings(record=True) as warning_records:
-        cube.cell_methods = parse_cell_methods(nc_att_cell_methods)
-    # Filter to get the warning we are interested in.
-    warning_records = [
-        record
-        for record in warning_records
-        if issubclass(record.category, UnknownCellMethodWarning)
-    ]
-    if len(warning_records) > 0:
-        # Output an enhanced warning message.
-        warn_record = warning_records[0]
-        name = "{}".format(cf_var.cf_name)
-        msg = warn_record.message.args[0]
-        msg = msg.replace("variable", "variable {!r}".format(name))
-        warnings.warn(message=msg, category=UnknownCellMethodWarning)
+    cube.cell_methods = parse_cell_methods(nc_att_cell_methods, cf_var.cf_name)
 
     # Set the cube global attributes.
     for attr_name, attr_value in cf_var.cf_group.global_attributes.items():

lib/iris/tests/integration/netcdf/test_general.py

@@ -484,5 +484,40 @@ def test_path_string_save_same(self):
         self.assertCDL(tempfile_frompath)
 
 
+@tests.skip_data
+class TestWarningRepeats(tests.IrisTest):
+    def test_datum_once(self):
+        """Tests for warnings being duplicated.
+
+        Notes
+        -----
+        This test relies on `iris.load` throwing a warning. This warning might
+        be removed in the future, in which case `assert len(record) == 2 should`
+        be change to `assert len(record) == 1`.
+
+        toa_brightness_temperature.nc has an AuxCoord with lazy data, and triggers a
+        specific part of dask which contains a `catch_warnings()` call which
+        causes warnings to be repeated, and so has been removed from the
+        `fnames` list until a solution is found for such a file.
+
+        """
+        #
+        fnames = [
+            "false_east_north_merc.nc",
+            "non_unit_scale_factor_merc.nc",
+            # toa_brightness_temperature.nc,
+        ]
+        fpaths = [
+            tests.get_data_path(("NetCDF", "mercator", fname)) for fname in fnames
+        ]
+
+        with warnings.catch_warnings(record=True) as record:
+            warnings.simplefilter("default")
+            for fpath in fpaths:
+                iris.load(fpath)
+                warnings.warn("Dummy warning", category=iris.exceptions.IrisUserWarning)
+        assert len(record) == 2
+
+
 if __name__ == "__main__":
     tests.main()