Test that a warning is properly raised

yt/frontends/ramses/fields.py

@@ -6,6 +6,7 @@
 
 from yt import units
 from yt._typing import KnownFieldsT
+from yt.fields.field_detector import FieldDetector
 from yt.fields.field_info_container import FieldInfoContainer
 from yt.frontends.ramses.io import convert_ramses_conformal_time_to_physical_age
 from yt.utilities.cython_fortran_utils import FortranFile
@@ -218,12 +219,16 @@ def _temperature(field, data):
         else:
 
             def _temperature(field, data):
-                warnings.warn(
-                    "Trying to calculate temperature but the cooling tables couldn't be found or read. "
-                    "yt will return T/µ instead of T — this is equivalent to assuming µ=1.0",
-                    category=RuntimeWarning,
-                    stacklevel=1,
-                )
+                if not isinstance(data, FieldDetector):
+                    warnings.warn(
+                        "Trying to calculate temperature but the cooling tables "
+                        "couldn't be found or read. yt will return T/µ instead of "
+                        "T — this is equivalent to assuming µ=1.0. To suppress this, "
+                        "derive the temperature from temperature_over_mu with "
+                        "some values for mu.",
+                        category=RuntimeWarning,
+                        stacklevel=1,
+                    )
                 return data["gas", "temperature_over_mu"]
 
         self.add_field(

yt/frontends/ramses/tests/test_outputs_pytest.py

@@ -55,3 +55,28 @@ def test_field_config_2(custom_ramses_fields_conf):
         assert ("ramses", f) in ds.field_list
     for f in custom_grav:
         assert ("gravity", f) in ds.field_list
+
+
+@requires_file(output_00080)
+@requires_file(ramses_new_format)
+def test_warning_T2():
+    ds1 = yt.load(output_00080)
+    ds2 = yt.load(ramses_new_format)
+
+    # Should not raise warnings
+    ds1.r["gas", "temperature_over_mu"]
+    ds2.r["gas", "temperature_over_mu"]
+
+    # We cannot read the cooling tables of output_00080
+    # so this should raise a warning
+    with pytest.warns(
+        RuntimeWarning,
+        match=(
+            "Trying to calculate temperature but the cooling tables couldn't be "
+            r"found or read\. yt will return T/µ instead of T.*"
+        ),
+    ):
+        ds1.r["gas", "temperature"]
+
+    # But this one should not
+    ds2.r["gas", "temperature"]