ENH: allow fully automatic unit detection for derived field

yt/data_objects/selection_objects/data_selection_objects.py

@@ -15,7 +15,7 @@
 from yt.fields.field_exceptions import NeedsGridType
 from yt.funcs import fix_axis, is_sequence, iter_fields, validate_width_tuple
 from yt.geometry.selection_routines import compose_selector
-from yt.units import YTArray, dimensions as ytdims
+from yt.units import YTArray
 from yt.utilities.exceptions import (
     GenerationInProgress,
     YTBooleanObjectError,
@@ -242,22 +242,27 @@ def _generate_fields(self, fields_to_generate):
                         # field accesses
                         units = getattr(fd, "units", "")
                         if units == "":
-                            dimensions = ytdims.dimensionless
+                            sunits = ""
+                            dimensions = 1
                         else:
-                            dimensions = units.dimensions
-                            units = str(
+                            sunits = str(
                                 units.get_base_equivalent(self.ds.unit_system.name)
                             )
-                        if fi.dimensions != dimensions:
+                            dimensions = units.dimensions
+
+                        if fi.dimensions is None:
+                            mylog.warning(
+                                "Field %s was added without specifying units or dimensions, "
+                                "auto setting units to %s",
+                                fi.name,
+                                sunits,
+                            )
+                        elif fi.dimensions != dimensions:
                             raise YTDimensionalityError(fi.dimensions, dimensions)
-                        fi.units = units
+                        fi.units = sunits
+                        fi.dimensions = dimensions
                         self.field_data[field] = self.ds.arr(fd, units)
-                        mylog.warning(
-                            "Field %s was added without specifying units, "
-                            "assuming units are %s",
-                            fi.name,
-                            units,
-                        )
+
                     try:
                         fd.convert_to_units(fi.units)
                     except AttributeError:

yt/fields/derived_field.py

@@ -69,10 +69,9 @@ class DerivedField:
        arguments (field, data)
     units : str
        A plain text string encoding the unit, or a query to a unit system of
-       a dataset. Powers must be in python syntax (** instead of ^). If set
-       to "auto" the units will be inferred from the units of the return
-       value of the field function, and the dimensions keyword must also be
-       set (see below).
+       a dataset. Powers must be in Python syntax (** instead of ^). If set
+       to 'auto' or None (default), units will be inferred from the return value
+       of the field function.
     take_log : bool
        Describes whether the field should be logged
     validators : list
@@ -94,8 +93,7 @@ class DerivedField:
        fields or that get aliased to themselves, we can specify a different
        desired output unit than the unit found on disk.
     dimensions : str or object from yt.units.dimensions
-       The dimensions of the field, only needed if units="auto" and only used
-       for error checking.
+       The dimensions of the field, only used for error checking with units='auto'.
     nodal_flag : array-like with three components
        This describes how the field is centered within a cell. If nodal_flag
        is [0, 0, 0], then the field is cell-centered. If any of the components
@@ -162,18 +160,10 @@ def __init__(
 
         # handle units
         self.units: Optional[Union[str, bytes, Unit]]
-        if units is None:
-            self.units = ""
+        if units in (None, "auto"):
+            self.units = None
         elif isinstance(units, str):
-            if units.lower() == "auto":
-                if dimensions is None:
-                    raise RuntimeError(
-                        "To set units='auto', please specify the dimensions "
-                        "of the field with dimensions=<dimensions of field>!"
-                    )
-                self.units = None
-            else:
-                self.units = units
+            self.units = units
         elif isinstance(units, Unit):
             self.units = str(units)
         elif isinstance(units, bytes):

yt/fields/field_info_container.py

@@ -489,11 +489,19 @@ def alias(
         if units is None:
             # We default to CGS here, but in principle, this can be pluggable
             # as well.
-            u = Unit(self[original_name].units, registry=self.ds.unit_registry)
-            if u.dimensions is not dimensionless:
-                units = str(self.ds.unit_system[u.dimensions])
+
+            # self[original_name].units may be set to `None` at this point
+            # to signal that units should be autoset later
+            oru = self[original_name].units
+            if oru is None:
+                units = None
             else:
-                units = self[original_name].units
+                u = Unit(oru, registry=self.ds.unit_registry)
+                if u.dimensions is not dimensionless:
+                    units = str(self.ds.unit_system[u.dimensions])
+                else:
+                    units = oru
+
         self.field_aliases[alias_name] = original_name
         function = TranslationFunc(original_name)
         if deprecate is not None:

yt/fields/tests/test_fields.py

@@ -311,9 +311,6 @@ def density_alias(field, data):
     def unitless_data(field, data):
         return np.ones(data[("gas", "density")].shape)
 
-    ds.add_field(
-        ("gas", "density_alias_no_units"), sampling_type="cell", function=density_alias
-    )
     ds.add_field(
         ("gas", "density_alias_auto"),
         sampling_type="cell",
@@ -340,7 +337,6 @@ def unitless_data(field, data):
         units="auto",
         dimensions="temperature",
     )
-    assert_raises(YTFieldUnitError, get_data, ds, ("gas", "density_alias_no_units"))
     assert_raises(YTFieldUnitError, get_data, ds, ("gas", "density_alias_wrong_units"))
     assert_raises(
         YTFieldUnitParseError, get_data, ds, ("gas", "density_alias_unparseable_units")