Merge pull request astropy#17586 from eerovaher/deprecate-unit-str-ar…

…ithmetic

Deprecate multiplying and dividing units with `str` or `bytes`

astropy/units/core.py

@@ -18,7 +18,7 @@
 
 from astropy.utils.compat import COPY_IF_NEEDED
 from astropy.utils.decorators import deprecated, lazyproperty
-from astropy.utils.exceptions import AstropyWarning
+from astropy.utils.exceptions import AstropyDeprecationWarning, AstropyWarning
 from astropy.utils.misc import isiterable
 
 from .errors import UnitConversionError, UnitParserWarning, UnitsError, UnitsWarning
@@ -786,15 +786,26 @@ def __pow__(self, p: Real) -> CompositeUnit:
                 ) from None
             return CompositeUnit(1, [self], [sanitize_power(p)], _error_check=False)
 
-    def __truediv__(self, m):
-        if isinstance(m, (bytes, str)):
-            m = Unit(m)
+    @staticmethod
+    def _warn_about_operation_with_deprecated_type(op: str, other: bytes | str) -> None:
+        warnings.warn(
+            AstropyDeprecationWarning(
+                f"{op} involving a unit and a '{type(other).__name__}' instance are "
+                f"deprecated since v7.1. Convert {other!r} to a unit explicitly."
+            ),
+            stacklevel=3,
+        )
 
+    def __truediv__(self, m):
         if isinstance(m, UnitBase):
             if m.is_unity():
                 return self
             return CompositeUnit(1, [self, m], [1, -1], _error_check=False)
 
+        if isinstance(m, (bytes, str)):
+            self._warn_about_operation_with_deprecated_type("divisions", m)
+            return self / Unit(m)
+
         try:
             # Cannot handle this as Unit, re-try as Quantity
             from .quantity import Quantity
@@ -805,6 +816,7 @@ def __truediv__(self, m):
 
     def __rtruediv__(self, m):
         if isinstance(m, (bytes, str)):
+            self._warn_about_operation_with_deprecated_type("divisions", m)
             return Unit(m) / self
 
         try:
@@ -825,16 +837,17 @@ def __rtruediv__(self, m):
             return NotImplemented
 
     def __mul__(self, m):
-        if isinstance(m, (bytes, str)):
-            m = Unit(m)
-
         if isinstance(m, UnitBase):
             if m.is_unity():
                 return self
             elif self.is_unity():
                 return m
             return CompositeUnit(1, [self, m], [1, 1], _error_check=False)
 
+        if isinstance(m, (bytes, str)):
+            self._warn_about_operation_with_deprecated_type("products", m)
+            return self * Unit(m)
+
         # Cannot handle this as Unit, re-try as Quantity.
         try:
             from .quantity import Quantity
@@ -845,6 +858,7 @@ def __mul__(self, m):
 
     def __rmul__(self, m):
         if isinstance(m, (bytes, str)):
+            self._warn_about_operation_with_deprecated_type("products", m)
             return Unit(m) * self
 
         # Cannot handle this as Unit.  Here, m cannot be a Quantity,

astropy/units/tests/test_quantity.py

@@ -16,7 +16,7 @@
 from astropy.units.quantity import _UNIT_NOT_INITIALISED
 from astropy.utils import isiterable
 from astropy.utils.compat import COPY_IF_NEEDED
-from astropy.utils.exceptions import AstropyWarning
+from astropy.utils.exceptions import AstropyDeprecationWarning, AstropyWarning
 from astropy.utils.masked import Masked
 
 """ The Quantity class will represent a number + unit + uncertainty """
@@ -1608,16 +1608,33 @@ def test_quantity_initialized_with_quantity():
 
 
 def test_quantity_string_unit():
-    q1 = 1.0 * u.m / "s"
+    with pytest.warns(
+        AstropyDeprecationWarning,
+        match=(
+            "^divisions involving a unit and a 'str' instance are deprecated since "
+            r"v7\.1\. Convert 's' to a unit explicitly\.$"
+        ),
+    ):
+        q1 = 1.0 * u.m / "s"
     assert q1.value == 1
     assert q1.unit == (u.m / u.s)
 
-    q2 = q1 * "m"
+    with pytest.warns(
+        AstropyDeprecationWarning,
+        match=(
+            "^products involving a unit and a 'str' instance are deprecated since "
+            r"v7\.1\. Convert 'm' to a unit explicitly\.$"
+        ),
+    ):
+        q2 = q1 * "m"
     assert q2.unit == ((u.m * u.m) / u.s)
 
 
 def test_quantity_invalid_unit_string():
-    with pytest.raises(ValueError):
+    with (
+        pytest.raises(ValueError),
+        pytest.warns(AstropyDeprecationWarning, match="^products involving .* a 'str'"),
+    ):
         "foo" * u.m
 
 

astropy/units/tests/test_units.py

@@ -731,9 +731,23 @@ def test_repr_latex():
 
 
 def test_operations_with_strings():
-    assert u.m / "5s" == (u.m / (5.0 * u.s))
+    with pytest.warns(
+        AstropyDeprecationWarning,
+        match=(
+            "^divisions involving a unit and a 'str' instance are deprecated since "
+            r"v7\.1\. Convert '5s' to a unit explicitly\.$"
+        ),
+    ):
+        assert u.m / "5s" == (u.m / (5.0 * u.s))
 
-    assert u.m * "5s" == (5.0 * u.m * u.s)
+    with pytest.warns(
+        AstropyDeprecationWarning,
+        match=(
+            "^products involving a unit and a 'str' instance are deprecated since "
+            r"v7\.1\. Convert '5s' to a unit explicitly\.$"
+        ),
+    ):
+        assert u.m * "5s" == (5.0 * u.m * u.s)
 
 
 def test_comparison():
@@ -756,19 +770,29 @@ def test_compose_into_arbitrary_units():
 
 
 def test_unit_multiplication_with_string():
-    """Check that multiplication with strings produces the correct unit."""
-    u1 = u.cm
-    us = "kg"
-    assert us * u1 == u.Unit(us) * u1
-    assert u1 * us == u1 * u.Unit(us)
+    with pytest.warns(
+        AstropyDeprecationWarning,
+        match=(
+            "^products involving a unit and a 'str' instance are deprecated since "
+            r"v7\.1\. Convert 'kg' to a unit explicitly\.$"
+        ),
+    ):
+        assert "kg" * u.cm == u.kg * u.cm
+    with pytest.warns(AstropyDeprecationWarning, match="^products involving .* 'str'"):
+        assert u.cm * "kg" == u.cm * u.kg
 
 
 def test_unit_division_by_string():
-    """Check that multiplication with strings produces the correct unit."""
-    u1 = u.cm
-    us = "kg"
-    assert us / u1 == u.Unit(us) / u1
-    assert u1 / us == u1 / u.Unit(us)
+    with pytest.warns(
+        AstropyDeprecationWarning,
+        match=(
+            "^divisions involving a unit and a 'str' instance are deprecated since "
+            r"v7\.1\. Convert 'kg' to a unit explicitly\.$"
+        ),
+    ):
+        assert "kg" / u.cm == u.kg / u.cm
+    with pytest.warns(AstropyDeprecationWarning, match="^divisions involving .* 'str'"):
+        assert u.cm / "kg" == u.cm / u.kg
 
 
 def test_sorted_bases():

docs/changes/units/17586.api.rst

@@ -0,0 +1,2 @@
+Automatic conversion of a ``str`` or ``bytes`` instance to a unit when it is
+multiplied or divided with an existing unit or quantity is deprecated.