Revert "REF: pre-compute JoinUnit.needs_filling (pandas-dev#43590)"

This reverts commit 7036de3.

Author: jorisvandenbossche

pandas/core/internals/concat.py

@@ -296,21 +296,9 @@ def _get_mgr_concatenation_plan(mgr: BlockManager, indexers: dict[int, np.ndarra
 
     assert 0 not in indexers
 
-    needs_filling = False
-    if 1 in indexers:
-        # indexers[1] is shared by all the JoinUnits, so we can save time
-        #  by only doing this check once
-        if (indexers[1] == -1).any():
-            needs_filling = True
-
     if mgr.is_single_block:
         blk = mgr.blocks[0]
-        return [
-            (
-                blk.mgr_locs,
-                JoinUnit(blk, mgr_shape, indexers, needs_filling=needs_filling),
-            )
-        ]
+        return [(blk.mgr_locs, JoinUnit(blk, mgr_shape, indexers))]
 
     blknos = mgr.blknos
     blklocs = mgr.blklocs
@@ -356,17 +344,15 @@ def _get_mgr_concatenation_plan(mgr: BlockManager, indexers: dict[int, np.ndarra
         # Assertions disabled for performance
         # assert blk._mgr_locs.as_slice == placements.as_slice
         # assert blk.shape[0] == shape[0]
-        unit = JoinUnit(blk, shape, join_unit_indexers, needs_filling=needs_filling)
+        unit = JoinUnit(blk, shape, join_unit_indexers)
 
         plan.append((placements, unit))
 
     return plan
 
 
 class JoinUnit:
-    def __init__(
-        self, block: Block, shape: Shape, indexers=None, *, needs_filling: bool = False
-    ):
+    def __init__(self, block: Block, shape: Shape, indexers=None):
         # Passing shape explicitly is required for cases when block is None.
         # Note: block is None implies indexers is None, but not vice-versa
         if indexers is None:
@@ -376,11 +362,28 @@ def __init__(
         self.indexers = indexers
         self.shape = shape
 
-        self.needs_filling = needs_filling
-
     def __repr__(self) -> str:
         return f"{type(self).__name__}({repr(self.block)}, {self.indexers})"
 
+    @cache_readonly
+    def needs_filling(self) -> bool:
+        for indexer in self.indexers.values():
+            # FIXME: cache results of indexer == -1 checks.
+            if (indexer == -1).any():
+                return True
+
+        return False
+
+    @cache_readonly
+    def dtype(self):
+        blk = self.block
+        if blk.values.dtype.kind == "V":
+            raise AssertionError("Block is None, no dtype")
+
+        if not self.needs_filling:
+            return blk.dtype
+        return ensure_dtype_can_hold_na(blk.dtype)
+
     @cache_readonly
     def is_na(self) -> bool:
         blk = self.block
@@ -538,12 +541,12 @@ def _get_empty_dtype(join_units: Sequence[JoinUnit]) -> DtypeObj:
         empty_dtype = join_units[0].block.dtype
         return empty_dtype
 
-    needs_can_hold_na = any(unit.is_na or unit.needs_filling for unit in join_units)
+    has_none_blocks = any(unit.is_na for unit in join_units)
 
-    dtypes = [unit.block.dtype for unit in join_units if not unit.is_na]
+    dtypes = [unit.dtype for unit in join_units if not unit.is_na]
 
     dtype = find_common_type(dtypes)
-    if needs_can_hold_na:
+    if has_none_blocks:
         dtype = ensure_dtype_can_hold_na(dtype)
     return dtype
 
@@ -606,13 +609,7 @@ def _trim_join_unit(join_unit: JoinUnit, length: int) -> JoinUnit:
     extra_shape = (join_unit.shape[0] - length,) + join_unit.shape[1:]
     join_unit.shape = (length,) + join_unit.shape[1:]
 
-    # extra_indexers does not introduce any -1s, so we can inherit needs_filling
-    return JoinUnit(
-        block=extra_block,
-        indexers=extra_indexers,
-        shape=extra_shape,
-        needs_filling=join_unit.needs_filling,
-    )
+    return JoinUnit(block=extra_block, indexers=extra_indexers, shape=extra_shape)
 
 
 def _combine_concat_plans(plans, concat_axis: int):