UX: add information about missing optional dependencies as warnings a…

…nd error messsages in yt.load

yt/loaders.py

@@ -30,7 +30,10 @@
     YTSimulationNotIdentified,
     YTUnidentifiedDataType,
 )
-from yt.utilities.hierarchy_inspection import find_lowest_subclasses
+from yt.utilities.hierarchy_inspection import (
+    find_lowest_subclasses,
+    get_classes_with_missing_requirements,
+)
 from yt.utilities.lib.misc_utilities import get_box_grids_level
 from yt.utilities.logger import ytLogger as mylog
 from yt.utilities.object_registries import (
@@ -42,6 +45,7 @@
 # --- Loaders for known data formats ---
 
 
+# FUTURE: embedded warnings need to have their stacklevel decremented when this decorator is removed
 @future_positional_only({0: "fn"}, since="4.2.0")
 def load(
     fn: Union[str, "os.PathLike[str]"], *args, hint: Optional[str] = None, **kwargs
@@ -106,7 +110,16 @@ def load(
     candidates = find_lowest_subclasses(candidates, hint=hint)
 
     if len(candidates) == 1:
-        return candidates[0](fn, *args, **kwargs)
+        cls = candidates[0]
+        if any(missing := get_classes_with_missing_requirements().get(cls, [])):
+            warnings.warn(
+                f"This dataset appears to be of type {cls.__name__}, "
+                "but the following requirements are currently missing "
+                f"from your installation: {', '.join([_ for _ in missing])}\n"
+                "Please make sure to install these and try again.",
+                stacklevel=3,
+            )
+        return cls(fn, *args, **kwargs)
 
     if len(candidates) > 1:
         raise YTAmbiguousDataType(fn, candidates)

yt/utilities/exceptions.py

@@ -21,11 +21,29 @@ def __init__(self, filename, *args, **kwargs):
         self.kwargs = kwargs
 
     def __str__(self):
+        from yt.utilities.hierarchy_inspection import (
+            get_classes_with_missing_requirements,
+        )
+
         msg = f"Could not determine input format from {self.filename!r}"
         if self.args:
             msg += ", " + (", ".join(f"{a!r}" for a in self.args))
         if self.kwargs:
             msg += ", " + (", ".join(f"{k}={v!r}" for k, v in self.kwargs.items()))
+
+        msg += "\n"
+
+        if len(unusable_classes := get_classes_with_missing_requirements()) > 0:
+            msg += (
+                "The following types could not be thorougly checked against your data because "
+                "their requirements are missing. "
+                "You may want to inspect this list and check your installation:"
+            )
+            for cls, missing in unusable_classes.items():
+                requirements_str = ", ".join(missing)
+                msg += f"\n- {cls.__name__} (requires: {requirements_str})"
+            msg += "\n\n"
+        msg += "Please make sure you are running a sufficiently recent version of yt."
         return msg