feat(datatype): enable inference of Decimal type

ibis/expr/datatypes/cast.py

@@ -113,8 +113,18 @@ def can_cast_decimals(source: dt.Decimal, target: dt.Decimal, **kwargs) -> bool:
     target_sc = target.scale
     source_sc = source.scale
     return (
-        target_prec is None or (source_prec is not None and target_prec >= source_prec)
-    ) and (target_sc is None or (source_sc is not None and target_sc >= source_sc))
+        # If either sides precision and scale are both `None`, return `True`.
+        target_prec is None
+        and target_sc is None
+        or source_prec is None
+        and source_sc is None
+        # Otherwise, return `True` unless we are downcasting precision or scale.
+        or (
+            target_prec is None
+            or (source_prec is not None and target_prec >= source_prec)
+        )
+        and (target_sc is None or (source_sc is not None and target_sc >= source_sc))
+    )
 
 
 @castable.register(dt.Interval, dt.Interval)

ibis/expr/datatypes/tests/test_value.py

@@ -1,4 +1,5 @@
 import datetime
+import decimal
 import enum
 from collections import OrderedDict
 
@@ -46,6 +47,7 @@ class Foo(enum.Enum):
         (-32769, dt.int32),
         (-2147483649, dt.int64),
         (1.5, dt.double),
+        (decimal.Decimal(1.5), dt.decimal),
         # parametric types
         (list('abc'), dt.Array(dt.string)),
         (set('abc'), dt.Set(dt.string)),

ibis/expr/datatypes/value.py

@@ -132,6 +132,12 @@ def infer_enum(_: enum.Enum) -> dt.String:
     return dt.string
 
 
+@infer.register(decimal.Decimal)
+def infer_decimal(value: decimal.Decimal) -> dt.Decimal:
+    """Infer the [`Decimal`][ibis.expr.datatypes.Decimal] type of `value`."""
+    return dt.decimal
+
+
 @infer.register(bool)
 def infer_boolean(value: bool) -> dt.Boolean:
     return dt.boolean