fix logic

Author: neutrinoceros

yt/fields/field_functions.py

@@ -1,3 +1,4 @@
+import warnings
 from collections.abc import Callable
 from inspect import Parameter, signature
 
@@ -113,3 +114,16 @@ def validate_field_function(function: Callable) -> None:
             f"Received field function {function} with invalid signature. "
             f"Expected {nokeyword} to be allowed as a keyword argument."
         )
+
+    defaults = tuple(
+        name
+        for name in ("data", "field")
+        if name in fparams and fparams[name].default is not Parameter.empty
+    )
+    if defaults:
+        warnings.warn(
+            f"Received field function {function} with default values for parameters {defaults!r}. "
+            "These default values will never be used. Drop them to avoid this warning.",
+            UserWarning,
+            stacklevel=2,
+        )