REF: catch more specific exceptions in Series.__setitem__ (#45194)

Author: jbrockmendel

pandas/core/series.py

@@ -1081,7 +1081,9 @@ def __setitem__(self, key, value) -> None:
 
         try:
             self._set_with_engine(key, value)
-        except (KeyError, ValueError):
+        except KeyError:
+            # We have a scalar (or for MultiIndex or object-dtype, scalar-like)
+            #  key that is not present in self.index.
             if is_integer(key) and self.index.inferred_type != "integer":
                 # positional setter
                 if not self.index._should_fallback_to_positional:
@@ -1102,9 +1104,15 @@ def __setitem__(self, key, value) -> None:
                 # GH#12862 adding a new key to the Series
                 self.loc[key] = value
 
-        except (InvalidIndexError, TypeError) as err:
+        except (TypeError, ValueError):
+            # The key was OK, but we cannot set the value losslessly
+            indexer = self.index.get_loc(key)
+            self._set_values(indexer, value)
+
+        except InvalidIndexError as err:
             if isinstance(key, tuple) and not isinstance(self.index, MultiIndex):
                 # cases with MultiIndex don't get here bc they raise KeyError
+                # e.g. test_basic_getitem_setitem_corner
                 raise KeyError(
                     "key of type tuple not found and not a MultiIndex"
                 ) from err
@@ -1163,17 +1171,19 @@ def _set_with(self, key, value):
             # Without this, the call to infer_dtype will consume the generator
             key = list(key)
 
-        key_type = lib.infer_dtype(key, skipna=False)
+        if not self.index._should_fallback_to_positional:
+            # Regardless of the key type, we're treating it as labels
+            self._set_labels(key, value)
 
-        # Note: key_type == "boolean" should not occur because that
-        #  should be caught by the is_bool_indexer check in __setitem__
-        if key_type == "integer":
-            if not self.index._should_fallback_to_positional:
-                self._set_labels(key, value)
-            else:
-                self._set_values(key, value)
         else:
-            self.loc[key] = value
+            # Note: key_type == "boolean" should not occur because that
+            #  should be caught by the is_bool_indexer check in __setitem__
+            key_type = lib.infer_dtype(key, skipna=False)
+
+            if key_type == "integer":
+                self._set_values(key, value)
+            else:
+                self._set_labels(key, value)
 
     def _set_labels(self, key, value) -> None:
         key = com.asarray_tuplesafe(key)