BUG: infer_dtype with decimal/complex (#37176)

Author: jbrockmendel

pandas/_libs/lib.pyx

@@ -1414,10 +1414,12 @@ def infer_dtype(value: object, skipna: bool = True) -> str:
             return "time"
 
     elif is_decimal(val):
-        return "decimal"
+        if is_decimal_array(values):
+            return "decimal"
 
     elif is_complex(val):
-        return "complex"
+        if is_complex_array(values):
+            return "complex"
 
     elif util.is_float_object(val):
         if is_float_array(values):
@@ -1702,6 +1704,34 @@ cpdef bint is_float_array(ndarray values):
     return validator.validate(values)
 
 
+cdef class ComplexValidator(Validator):
+    cdef inline bint is_value_typed(self, object value) except -1:
+        return (
+            util.is_complex_object(value)
+            or (util.is_float_object(value) and is_nan(value))
+        )
+
+    cdef inline bint is_array_typed(self) except -1:
+        return issubclass(self.dtype.type, np.complexfloating)
+
+
+cdef bint is_complex_array(ndarray values):
+    cdef:
+        ComplexValidator validator = ComplexValidator(len(values), values.dtype)
+    return validator.validate(values)
+
+
+cdef class DecimalValidator(Validator):
+    cdef inline bint is_value_typed(self, object value) except -1:
+        return is_decimal(value)
+
+
+cdef bint is_decimal_array(ndarray values):
+    cdef:
+        DecimalValidator validator = DecimalValidator(len(values), values.dtype)
+    return validator.validate(values)
+
+
 cdef class StringValidator(Validator):
     cdef inline bint is_value_typed(self, object value) except -1:
         return isinstance(value, str)
@@ -2546,8 +2576,6 @@ def fast_multiget(dict mapping, ndarray keys, default=np.nan):
         # kludge, for Series
         return np.empty(0, dtype='f8')
 
-    keys = getattr(keys, 'values', keys)
-
     for i in range(n):
         val = keys[i]
         if val in mapping:

pandas/tests/dtypes/test_inference.py

@@ -709,6 +709,9 @@ def test_decimals(self):
         result = lib.infer_dtype(arr, skipna=True)
         assert result == "mixed"
 
+        result = lib.infer_dtype(arr[::-1], skipna=True)
+        assert result == "mixed"
+
         arr = np.array([Decimal(1), Decimal("NaN"), Decimal(3)])
         result = lib.infer_dtype(arr, skipna=True)
         assert result == "decimal"
@@ -729,6 +732,9 @@ def test_complex(self, skipna):
         result = lib.infer_dtype(arr, skipna=skipna)
         assert result == "mixed"
 
+        result = lib.infer_dtype(arr[::-1], skipna=skipna)
+        assert result == "mixed"
+
         # gets cast to complex on array construction
         arr = np.array([1, np.nan, 1 + 1j])
         result = lib.infer_dtype(arr, skipna=skipna)