astropy · eerovaher · Aug 20, 2025 · Aug 20, 2025 · eerovaher · Aug 20, 2025
diff --git a/astropy/io/votable/tests/test_table.py b/astropy/io/votable/tests/test_table.py
@@ -356,7 +356,7 @@ def test_write_jybeam_unit(tmp_path, recwarn):
     t = Table(
         {
             "flux": [5 * (u.Jy / u.beam)],
-            "foo": [0 * u.Unit("Crab", format="ogip")],
+            "foo": [0 * u.Unit("Crab", format="ogip", parse_strict="warn")],
             "bar": [1 * u.def_unit("my_unit")],
         }
     )

diff --git a/astropy/units/core.py b/astropy/units/core.py
@@ -2033,11 +2033,13 @@ def __call__(
             if isinstance(s, bytes):
                 s = s.decode("ascii")
 
-            try:
-                return f._validate_unit(s, detailed_exception=False)  # Try a shortcut
+            try:  # Try a shortcut
+                return f._validate_unit(
+                    s, detailed_exception=False, deprecations="raise"
+                )
             except (AttributeError, ValueError):
                 # No `f._validate_unit()` (AttributeError)
-                # or `s` was a composite unit (ValueError).
+                # or `s` was a composite unit or deprecated (ValueError).
                 pass
 
             try:

diff --git a/astropy/units/format/base.py b/astropy/units/format/base.py
@@ -9,7 +9,7 @@
 import numpy as np
 
 from astropy.units.core import CompositeUnit, NamedUnit, Unit, get_current_unit_registry
-from astropy.units.errors import UnitsWarning
+from astropy.units.errors import UnitParserWarning, UnitsWarning
 from astropy.units.utils import maybe_simple_fraction
 from astropy.utils.misc import did_you_mean
 
@@ -253,14 +253,28 @@ def _did_you_mean_units(cls, unit: str) -> str:
         return did_you_mean(unit, cls._units, fix=cls._fix_deprecated)
 
     @classmethod
-    def _validate_unit(cls, unit: str, detailed_exception: bool = True) -> UnitBase:
+    def _validate_unit(
+        cls,
+        unit: str,
+        detailed_exception: bool = True,
+        deprecations: Literal["silent", "warn", "raise"] = "warn",
+    ) -> UnitBase:
+        if unit in cls._deprecated_units:
+            if deprecations == "warn":
+                warnings.warn(cls._deprecated_unit_message(unit), UnitParserWarning)
+            elif deprecations == "raise":
+                raise ValueError()
         try:
             return cls._units[unit]
         except KeyError:
             if detailed_exception:
                 raise ValueError(cls._invalid_unit_error_message(unit)) from None
             raise ValueError() from None
 
+    @classmethod
+    def _deprecated_unit_message(cls, unit: str) -> str:
+        return f"The unit '{unit}' has been deprecated in the {cls.__name__} standard."
+
     @classmethod
     def _invalid_unit_error_message(cls, unit: str) -> str:
         return (

diff --git a/astropy/units/format/generic.py b/astropy/units/format/generic.py
@@ -410,7 +410,9 @@ class Generic(Base, _GenericParserMixin):
     """
 
     @classmethod
-    def _validate_unit(cls, s: str, detailed_exception: bool = True) -> UnitBase:
+    def _validate_unit(
+        cls, s: str, detailed_exception: bool = True, deprecations: str = "warn"
+    ) -> UnitBase:
         registry = get_current_unit_registry().registry
         if s in cls._unit_symbols:
             s = cls._unit_symbols[s]

diff --git a/astropy/units/format/ogip.py b/astropy/units/format/ogip.py
@@ -362,12 +362,3 @@ def format_exponential_notation(
         cls, val: UnitScale | np.number, format_spec: str = "g"
     ) -> str:
         return format(val, format_spec)
-
-    @classmethod
-    def _validate_unit(cls, unit: str, detailed_exception: bool = True) -> UnitBase:
-        if unit in cls._deprecated_units:
-            warnings.warn(
-                f"The unit '{unit}' has been deprecated in the OGIP standard.",
-                UnitsWarning,
-            )
-        return super()._validate_unit(unit, detailed_exception)
diff --git a/astropy/units/format/vounit.py b/astropy/units/format/vounit.py
@@ -20,12 +20,7 @@
     dimensionless_unscaled,
     si_prefixes,
 )
-from astropy.units.errors import (
-    UnitParserWarning,
-    UnitScaleError,
-    UnitsError,
-    UnitsWarning,
-)
+from astropy.units.errors import UnitParserWarning, UnitScaleError, UnitsError
 from astropy.utils import classproperty
 
 from . import Base, utils
@@ -222,12 +217,8 @@ def _fix_deprecated(cls, x: str) -> list[str]:
         )
 
     @classmethod
-    def _validate_unit(cls, unit: str, detailed_exception: bool = True) -> UnitBase:
-        if unit in cls._deprecated_units:
-            warnings.warn(
-                UnitsWarning(
-                    f"The unit '{unit}' has been deprecated in the VOUnit standard."
-                    f" Suggested: {cls.to_string(cls._units[unit]._represents)}."
-                )
-            )
-        return super()._validate_unit(unit, detailed_exception)
+    def _deprecated_unit_message(cls, unit: str) -> str:
+        return (
+            super()._deprecated_unit_message(unit)
+            + f" Suggested: {cls.to_string(cls._units[unit]._represents)}."
+        )
diff --git a/astropy/units/tests/test_format.py b/astropy/units/tests/test_format.py
@@ -361,9 +361,12 @@ def check_roundtrip(self, unit, output_format=None):
             s = unit.to_string(output_format)
 
         if s in self.format_._deprecated_units:
-            with pytest.warns(UnitsWarning, match="deprecated") as w:
-                a = Unit(s, format=self.format_)
+            with pytest.raises(ValueError, match="deprecated"):
+                Unit(s, format=self.format_)
+            with pytest.warns(UnitParserWarning, match="deprecated") as w:
+                a = Unit(s, format=self.format_, parse_strict="warn")
             assert len(w) == 1
+            assert Unit(s, format=self.format_, parse_strict="silent") == a
         else:
             a = Unit(s, format=self.format_)  # No warning
 
@@ -373,7 +376,7 @@ def check_roundtrip_decompose(self, unit):
         ud = unit.decompose()
         s = ud.to_string(self.format_)
         assert "  " not in s
-        a = Unit(s, format=self.format_)
+        a = Unit(s, format=self.format_, parse_strict="warn")
         assert_allclose(a.decompose().scale, ud.scale, rtol=1e-5)
 
 
@@ -464,7 +467,7 @@ def test_roundtrip(self, unit):
             # as a deprecated unit.
             with pytest.warns(UnitsWarning):
                 s = unit.to_string(self.format_)
-                a = Unit(s, format=self.format_)
+                a = Unit(s, format=self.format_, parse_strict="warn")
             assert_allclose(a.decompose().scale, unit.decompose().scale, rtol=1e-9)
         else:
             self.check_roundtrip(unit)
@@ -735,9 +738,8 @@ def test_deprecated_did_you_mean_units():
     ):
         u.Unit("ANGSTROM", format="vounit")
 
-    with pytest.warns(UnitsWarning, match=r".* 0\.1nm\.") as w:
+    with pytest.raises(ValueError, match=r".* 0\.1nm\."):
         u.Unit("angstrom", format="vounit")
-    assert len(w) == 1
 
 
 @pytest.mark.parametrize("string", ["mag(ct/s)", "dB(mW)", "dex(cm s**-2)"])

diff --git a/docs/changes/units/18536.bugfix.rst b/docs/changes/units/18536.bugfix.rst
@@ -0,0 +1,7 @@
+Parsing deprecated units (e.g. ``u.Unit("erg", format="vounit",
+parse_strict=...)``) now obeys the ``parse_strict`` argument, which has so far
+been ignored in that context.
+Because by default ``parse_strict="raise"`` then parsing deprecated units can
+now cause an error where it previously succeeded with a warning, but the
+previous behaviour (for deprecated units) can be ensured in a
+backwards-compatible way by setting ``parse_strict="warn"`` explicitly.