TYP: fix MultiIndex._names type (#46919)

Author: iasoon

pandas/core/frame.py

@@ -10420,12 +10420,14 @@ def _count_level(self, level: Level, axis: int = 0, numeric_only: bool = False):
  else:
  mask = index_mask.reshape(-1, 1) & values_mask
 
- if isinstance(level, str):
- level = count_axis._get_level_number(level)
+ if isinstance(level, int):
+ level_number = level
+ else:
+ level_number = count_axis._get_level_number(level)
 
- level_name = count_axis._names[level]
- level_index = count_axis.levels[level]._rename(name=level_name)
- level_codes = ensure_platform_int(count_axis.codes[level])
+ level_name = count_axis._names[level_number]
+ level_index = count_axis.levels[level_number]._rename(name=level_name)
+ level_codes = ensure_platform_int(count_axis.codes[level_number])
  counts = lib.count_level_2d(mask, level_codes, len(level_index), axis=axis)
 
  if axis == 1:

pandas/core/indexes/multi.py

@@ -287,7 +287,7 @@ class MultiIndex(Index):
 
  # initialize to zero-length tuples to make everything work
  _typ = "multiindex"
- _names = FrozenList()
+ _names: list[Hashable | None] = []
  _levels = FrozenList()
  _codes = FrozenList()
  _comparables = ["names"]
@@ -326,9 +326,7 @@ def __new__(
  result._set_levels(levels, copy=copy, validate=False)
  result._set_codes(codes, copy=copy, validate=False)
 
- # Incompatible types in assignment (expression has type "List[None]",
- # variable has type "FrozenList") [assignment]
- result._names = [None] * len(levels) # type: ignore[assignment]
+ result._names = [None] * len(levels)
  if names is not None:
  # handles name validation
  result._set_names(names)
@@ -1476,8 +1474,7 @@ def _set_names(self, names, *, level=None, validate: bool = True):
  raise TypeError(
  f"{type(self).__name__}.name must be a hashable type"
  )
- # error: Cannot determine type of '__setitem__'
- self._names[lev] = name # type: ignore[has-type]
+ self._names[lev] = name
 
  # If .levels has been accessed, the names in our cache will be stale.
  self._reset_cache()