prevent warning repeats

lib/iris/exceptions.py

@@ -3,6 +3,9 @@
 # This file is part of Iris and is released under the BSD license.
 # See LICENSE in the root of the repository for full licensing details.
 """Exceptions specific to the Iris package."""
+from functools import lru_cache
+import traceback
+import warnings
 
 
 class IrisError(Exception):
@@ -344,3 +347,19 @@ class IrisSaverFillValueWarning(IrisMaskValueMatchWarning, IrisSaveWarning):
  """
 
  pass
+
+
+@lru_cache(None)
+def warn_once(msg, type, stacklevel, frame):
+ """Raise a warning only if a similar one has not been raised before."""
+ warnings.warn(msg, type, stacklevel=stacklevel)
+
+
+def warn_once_at_level(msg, type, stacklevel):
+ """
+ Raise a warning only if a similar one hasn't been raised from the same line
+ for a given stack level.
+ """
+ stacklevel += 1
+ frame = traceback.format_stack()[-stacklevel]
+ warn_once(msg, type, stacklevel + 1, frame)

lib/iris/fileformats/_nc_load_rules/helpers.py

@@ -520,7 +520,7 @@ def _get_ellipsoid(cf_grid_var):
  "applied. To apply the datum when loading, use the "
  "iris.FUTURE.datum_support flag."
  )
- warnings.warn(wmsg, category=FutureWarning, stacklevel=14)
+ iris.exceptions.warn_once_at_level(wmsg, FutureWarning, category=FutureWarning, stacklevel=14)
  datum = None
 
  if datum is not None:

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

@@ -505,7 +505,7 @@ def test_datum_once(self):
  fnames = [
  "false_east_north_merc.nc",
  "non_unit_scale_factor_merc.nc",
- # toa_brightness_temperature.nc,
+ "toa_brightness_temperature.nc",
  ]
  fpaths = [
  tests.get_data_path(("NetCDF", "mercator", fname)) for fname in fnames
@@ -518,6 +518,13 @@ def test_datum_once(self):
  warnings.warn("Dummy warning", category=iris.exceptions.IrisUserWarning)
  assert len(record) == 2
 
+ with warnings.catch_warnings(record=True) as record:
+ warnings.simplefilter("default")
+ iris.load_cube(fpaths[0])
+ iris.load_cube(fpaths[1])
+ iris.load_cube(fpaths[2])
+ assert len(record) == 3
+
 
 if __name__ == "__main__":
  tests.main()