REF/CLN: maybe_downcast_to_dtype (#27714)

Author: jbrockmendel

pandas/core/dtypes/cast.py

@@ -46,6 +46,7 @@
 )
 from .dtypes import DatetimeTZDtype, ExtensionDtype, PeriodDtype
 from .generic import (
+    ABCDataFrame,
     ABCDatetimeArray,
     ABCDatetimeIndex,
     ABCPeriodArray,
@@ -95,12 +96,13 @@ def maybe_downcast_to_dtype(result, dtype):
     """ try to cast to the specified dtype (e.g. convert back to bool/int
     or could be an astype of float64->float32
     """
+    do_round = False
 
     if is_scalar(result):
         return result
-
-    def trans(x):
-        return x
+    elif isinstance(result, ABCDataFrame):
+        # occurs in pivot_table doctest
+        return result
 
     if isinstance(dtype, str):
         if dtype == "infer":
@@ -118,83 +120,115 @@ def trans(x):
             elif inferred_type == "floating":
                 dtype = "int64"
                 if issubclass(result.dtype.type, np.number):
-
-                    def trans(x):  # noqa
-                        return x.round()
+                    do_round = True
 
             else:
                 dtype = "object"
 
-    if isinstance(dtype, str):
         dtype = np.dtype(dtype)
 
-    try:
+    converted = maybe_downcast_numeric(result, dtype, do_round)
+    if converted is not result:
+        return converted
+
+    # a datetimelike
+    # GH12821, iNaT is casted to float
+    if dtype.kind in ["M", "m"] and result.dtype.kind in ["i", "f"]:
+        try:
+            result = result.astype(dtype)
+        except Exception:
+            if dtype.tz:
+                # convert to datetime and change timezone
+                from pandas import to_datetime
+
+                result = to_datetime(result).tz_localize("utc")
+                result = result.tz_convert(dtype.tz)
+
+    elif dtype.type is Period:
+        # TODO(DatetimeArray): merge with previous elif
+        from pandas.core.arrays import PeriodArray
 
+        try:
+            return PeriodArray(result, freq=dtype.freq)
+        except TypeError:
+            # e.g. TypeError: int() argument must be a string, a
+            #  bytes-like object or a number, not 'Period
+            pass
+
+    return result
+
+
+def maybe_downcast_numeric(result, dtype, do_round: bool = False):
+    """
+    Subset of maybe_downcast_to_dtype restricted to numeric dtypes.
+
+    Parameters
+    ----------
+    result : ndarray or ExtensionArray
+    dtype : np.dtype or ExtensionDtype
+    do_round : bool
+
+    Returns
+    -------
+    ndarray or ExtensionArray
+    """
+    if not isinstance(dtype, np.dtype):
+        # e.g. SparseDtype has no itemsize attr
+        return result
+
+    if isinstance(result, list):
+        # reached via groupoby.agg _ohlc; really this should be handled
+        #  earlier
+        result = np.array(result)
+
+    def trans(x):
+        if do_round:
+            return x.round()
+        return x
+
+    if dtype.kind == result.dtype.kind:
         # don't allow upcasts here (except if empty)
-        if dtype.kind == result.dtype.kind:
-            if result.dtype.itemsize <= dtype.itemsize and np.prod(result.shape):
-                return result
+        if result.dtype.itemsize <= dtype.itemsize and result.size:
+            return result
 
-        if is_bool_dtype(dtype) or is_integer_dtype(dtype):
+    if is_bool_dtype(dtype) or is_integer_dtype(dtype):
 
+        if not result.size:
             # if we don't have any elements, just astype it
-            if not np.prod(result.shape):
-                return trans(result).astype(dtype)
+            return trans(result).astype(dtype)
 
-            # do a test on the first element, if it fails then we are done
-            r = result.ravel()
-            arr = np.array([r[0]])
+        # do a test on the first element, if it fails then we are done
+        r = result.ravel()
+        arr = np.array([r[0]])
 
+        if isna(arr).any() or not np.allclose(arr, trans(arr).astype(dtype), rtol=0):
             # if we have any nulls, then we are done
-            if isna(arr).any() or not np.allclose(
-                arr, trans(arr).astype(dtype), rtol=0
-            ):
-                return result
+            return result
 
+        elif not isinstance(r[0], (np.integer, np.floating, np.bool, int, float, bool)):
             # a comparable, e.g. a Decimal may slip in here
-            elif not isinstance(
-                r[0], (np.integer, np.floating, np.bool, int, float, bool)
-            ):
-                return result
+            return result
 
-            if (
-                issubclass(result.dtype.type, (np.object_, np.number))
-                and notna(result).all()
-            ):
-                new_result = trans(result).astype(dtype)
-                try:
-                    if np.allclose(new_result, result, rtol=0):
-                        return new_result
-                except Exception:
-
-                    # comparison of an object dtype with a number type could
-                    # hit here
-                    if (new_result == result).all():
-                        return new_result
-        elif issubclass(dtype.type, np.floating) and not is_bool_dtype(result.dtype):
-            return result.astype(dtype)
-
-        # a datetimelike
-        # GH12821, iNaT is casted to float
-        elif dtype.kind in ["M", "m"] and result.dtype.kind in ["i", "f"]:
+        if (
+            issubclass(result.dtype.type, (np.object_, np.number))
+            and notna(result).all()
+        ):
+            new_result = trans(result).astype(dtype)
             try:
-                result = result.astype(dtype)
+                if np.allclose(new_result, result, rtol=0):
+                    return new_result
             except Exception:
-                if dtype.tz:
-                    # convert to datetime and change timezone
-                    from pandas import to_datetime
-
-                    result = to_datetime(result).tz_localize("utc")
-                    result = result.tz_convert(dtype.tz)
-
-        elif dtype.type == Period:
-            # TODO(DatetimeArray): merge with previous elif
-            from pandas.core.arrays import PeriodArray
-
-            return PeriodArray(result, freq=dtype.freq)
-
-    except Exception:
-        pass
+                # comparison of an object dtype with a number type could
+                #  hit here
+                if (new_result == result).all():
+                    return new_result
+
+    elif (
+        issubclass(dtype.type, np.floating)
+        and not is_bool_dtype(result.dtype)
+        and not is_string_dtype(result.dtype)
+    ):
+        return result.astype(dtype)
 
     return result