CLN: assorted comments

pandas/_libs/internals.pyx

@@ -7,6 +7,7 @@ from cython cimport Py_ssize_t
 
 
 cdef extern from "Python.h":
+    # TODO(cython3): from cpython.pyport cimport PY_SSIZE_T_MAX
     Py_ssize_t PY_SSIZE_T_MAX
 
 import numpy as np

pandas/_libs/lib.pyi

@@ -217,7 +217,6 @@ def count_level_2d(
     mask: np.ndarray,  # ndarray[uint8_t, ndim=2, cast=True],
     labels: np.ndarray,  # const intp_t[:]
     max_bin: int,
-    axis: int,
 ) -> np.ndarray: ...  # np.ndarray[np.int64, ndim=2]
 def get_level_sorter(
     label: np.ndarray,  # const int64_t[:]

pandas/_libs/lib.pyx

@@ -921,29 +921,19 @@ def get_level_sorter(
 def count_level_2d(ndarray[uint8_t, ndim=2, cast=True] mask,
                    const intp_t[:] labels,
                    Py_ssize_t max_bin,
-                   int axis):
+                   ):
     cdef:
         Py_ssize_t i, j, k, n
         ndarray[int64_t, ndim=2] counts
 
-    assert (axis == 0 or axis == 1)
     n, k = (<object>mask).shape
 
-    if axis == 0:
-        counts = np.zeros((max_bin, k), dtype="i8")
-        with nogil:
-            for i in range(n):
-                for j in range(k):
-                    if mask[i, j]:
-                        counts[labels[i], j] += 1
-
-    else:  # axis == 1
-        counts = np.zeros((n, max_bin), dtype="i8")
-        with nogil:
-            for i in range(n):
-                for j in range(k):
-                    if mask[i, j]:
-                        counts[i, labels[j]] += 1
+    counts = np.zeros((n, max_bin), dtype="i8")
+    with nogil:
+        for i in range(n):
+            for j in range(k):
+                if mask[i, j]:
+                    counts[i, labels[j]] += 1
 
     return counts
 
@@ -1710,6 +1700,7 @@ cdef class Validator:
 
     cdef bint is_valid_null(self, object value) except -1:
         return value is None or value is C_NA or util.is_nan(value)
+        # TODO: include decimal NA?
 
     cdef bint is_array_typed(self) except -1:
         return False

pandas/_libs/parsers.pyx

@@ -848,6 +848,9 @@ cdef class TextReader:
         with nogil:
             status = tokenize_nrows(self.parser, nrows, self.encoding_errors)
 
+        self._check_tokenize_status(status)
+
+    cdef _check_tokenize_status(self, int status):
         if self.parser.warn_msg != NULL:
             print(PyUnicode_DecodeUTF8(
                 self.parser.warn_msg, strlen(self.parser.warn_msg),
@@ -879,15 +882,7 @@ cdef class TextReader:
             with nogil:
                 status = tokenize_all_rows(self.parser, self.encoding_errors)
 
-            if self.parser.warn_msg != NULL:
-                print(PyUnicode_DecodeUTF8(
-                    self.parser.warn_msg, strlen(self.parser.warn_msg),
-                    self.encoding_errors), file=sys.stderr)
-                free(self.parser.warn_msg)
-                self.parser.warn_msg = NULL
-
-            if status < 0:
-                raise_parser_error("Error tokenizing data", self.parser)
+            self._check_tokenize_status(status)
 
         if self.parser_start >= self.parser.lines:
             raise StopIteration

pandas/core/frame.py

@@ -7814,7 +7814,7 @@ def combine(
         if self.empty and len(other) == other_idxlen:
             return other.copy()
 
-        # sorts if possible
+        # sorts if possible; otherwise align above ensures that these are set-equal
         new_columns = this.columns.union(other.columns)
         do_fill = fill_value is not None
         result = {}

pandas/core/groupby/generic.py

@@ -1424,7 +1424,7 @@ def _wrap_applied_output_series(
         values: list[Series],
         not_indexed_same: bool,
         first_not_none,
-        key_index,
+        key_index: Index | None,
         is_transform: bool,
     ) -> DataFrame | Series:
         kwargs = first_not_none._construct_axes_dict()

pandas/core/groupby/groupby.py

@@ -1043,7 +1043,9 @@ def _concat_objects(
             # when the ax has duplicates
             # so we resort to this
             # GH 14776, 30667
+            # TODO: can we re-use e.g. _reindex_non_unique?
             if ax.has_duplicates and not result.axes[self.axis].equals(ax):
+                # e.g. test_category_order_transformer
                 target = algorithms.unique1d(ax._values)
                 indexer, _ = result.index.get_indexer_non_unique(target)
                 result = result.take(indexer, axis=self.axis)
@@ -1460,6 +1462,7 @@ def _agg_py_fallback(
         NotImplementedError.
         """
         # We get here with a) EADtypes and b) object dtype
+        assert alt is not None
 
         if values.ndim == 1:
             # For DataFrameGroupBy we only get here with ExtensionArray
@@ -1775,7 +1778,7 @@ def hfunc(bvalues: ArrayLike) -> ArrayLike:
             else:
                 masked = mask & ~isna(bvalues)
 
-            counted = lib.count_level_2d(masked, labels=ids, max_bin=ngroups, axis=1)
+            counted = lib.count_level_2d(masked, labels=ids, max_bin=ngroups)
             if is_series:
                 assert counted.ndim == 2
                 assert counted.shape[0] == 1

pandas/core/indexes/base.py

@@ -78,7 +78,6 @@
     LossySetitemError,
     can_hold_element,
     common_dtype_categorical_compat,
-    ensure_dtype_can_hold_na,
     find_result_type,
     infer_dtype_from,
     maybe_cast_pointwise_result,
@@ -351,6 +350,7 @@ def _left_indexer_unique(self: _IndexT, other: _IndexT) -> npt.NDArray[np.intp]:
         # can_use_libjoin assures sv and ov are ndarrays
         sv = cast(np.ndarray, sv)
         ov = cast(np.ndarray, ov)
+        # similar but not identical to ov.searchsorted(sv)
         return libjoin.left_join_indexer_unique(sv, ov)
 
     @final
@@ -3132,7 +3132,7 @@ def union(self, other, sort=None):
         if not is_dtype_equal(self.dtype, other.dtype):
             if (
                 isinstance(self, ABCMultiIndex)
-                and not is_object_dtype(unpack_nested_dtype(other))
+                and not is_object_dtype(_unpack_nested_dtype(other))
                 and len(other) > 0
             ):
                 raise NotImplementedError(
@@ -3213,6 +3213,8 @@ def _union(self, other: Index, sort):
             result_dups = algos.union_with_duplicates(self, other)
             return _maybe_try_sort(result_dups, sort)
 
+        # The rest of this method is analogous to Index._intersection_via_get_indexer
+
         # Self may have duplicates; other already checked as unique
         # find indexes of things in "other" that are not in "self"
         if self._index_as_unique:
@@ -3800,7 +3802,7 @@ def _should_partial_index(self, target: Index) -> bool:
                 return False
             # See https://github.com/pandas-dev/pandas/issues/47772 the commented
             # out code can be restored (instead of hardcoding `return True`)
-            # once that issue if fixed
+            # once that issue is fixed
             # "Index" has no attribute "left"
             # return self.left._should_compare(target)  # type: ignore[attr-defined]
             return True
@@ -4778,6 +4780,9 @@ def _join_monotonic(
         assert other.dtype == self.dtype
 
         if self.equals(other):
+            # This is a convenient place for this check, but its correctness
+            #  does not depend on monotonicity, so it could go earlier
+            #  in the calling method.
             ret_index = other if how == "right" else self
             return ret_index, None, None
 
@@ -5762,6 +5767,9 @@ def get_indexer_non_unique(
             that = target.astype(dtype, copy=False)
             return this.get_indexer_non_unique(that)
 
+        # TODO: get_indexer has fastpaths for both Categorical-self and
+        #  Categorical-target. Can we do something similar here?
+
         # Note: _maybe_promote ensures we never get here with MultiIndex
         #  self and non-Multi target
         tgt_values = target._get_engine_target()
@@ -5922,7 +5930,7 @@ def _get_indexer_non_comparable(
             If doing an inequality check, i.e. method is not None.
         """
         if method is not None:
-            other = unpack_nested_dtype(target)
+            other = _unpack_nested_dtype(target)
             raise TypeError(f"Cannot compare dtypes {self.dtype} and {other.dtype}")
 
         no_matches = -1 * np.ones(target.shape, dtype=np.intp)
@@ -5998,16 +6006,6 @@ def _find_common_type_compat(self, target) -> DtypeObj:
         Implementation of find_common_type that adjusts for Index-specific
         special cases.
         """
-        if is_valid_na_for_dtype(target, self.dtype):
-            # e.g. setting NA value into IntervalArray[int64]
-            dtype = ensure_dtype_can_hold_na(self.dtype)
-            if is_dtype_equal(self.dtype, dtype):
-                raise NotImplementedError(
-                    "This should not be reached. Please report a bug at "
-                    "github.com/pandas-dev/pandas"
-                )
-            return dtype
-
         target_dtype, _ = infer_dtype_from(target, pandas_dtype=True)
 
         # special case: if one dtype is uint64 and the other a signed int, return object
@@ -6040,7 +6038,7 @@ def _should_compare(self, other: Index) -> bool:
             #  respectively.
             return False
 
-        other = unpack_nested_dtype(other)
+        other = _unpack_nested_dtype(other)
         dtype = other.dtype
         return self._is_comparable_dtype(dtype) or is_object_dtype(dtype)
 
@@ -6052,6 +6050,8 @@ def _is_comparable_dtype(self, dtype: DtypeObj) -> bool:
             return dtype.kind == "b"
         elif is_numeric_dtype(self.dtype):
             return is_numeric_dtype(dtype)
+        # TODO: this was written assuming we only get here with object-dtype,
+        #  which is nom longer correct. Can we specialize for EA?
         return True
 
     @final
@@ -7141,7 +7141,7 @@ def get_unanimous_names(*indexes: Index) -> tuple[Hashable, ...]:
     return names
 
 
-def unpack_nested_dtype(other: _IndexT) -> _IndexT:
+def _unpack_nested_dtype(other: Index) -> Index:
     """
     When checking if our dtype is comparable with another, we need
     to unpack CategoricalDtype to look at its categories.dtype.
@@ -7155,12 +7155,10 @@ def unpack_nested_dtype(other: _IndexT) -> _IndexT:
     Index
     """
     dtype = other.dtype
-    if is_categorical_dtype(dtype):
+    if isinstance(dtype, CategoricalDtype):
         # If there is ever a SparseIndex, this could get dispatched
         #  here too.
-        # error: Item  "dtype[Any]"/"ExtensionDtype" of "Union[dtype[Any],
-        # ExtensionDtype]" has no attribute "categories"
-        return dtype.categories  # type: ignore[union-attr]
+        return dtype.categories
     return other
 
 

pandas/core/indexes/multi.py

@@ -2746,6 +2746,7 @@ def _get_loc_single_level_index(self, level_index: Index, key: Hashable) -> int:
         Index.get_loc : The get_loc method for (single-level) index.
         """
         if is_scalar(key) and isna(key):
+            # TODO: need is_valid_na_for_dtype(key, level_index.dtype)
             return -1
         else:
             return level_index.get_loc(key)
@@ -2818,6 +2819,8 @@ def _maybe_to_slice(loc):
             )
 
         if keylen == self.nlevels and self.is_unique:
+            # TODO: what if we have an IntervalIndex level?
+            #  i.e. do we need _index_as_unique on that level?
             try:
                 return self._engine.get_loc(key)
             except TypeError:
@@ -3853,6 +3856,8 @@ def maybe_droplevels(index: Index, key) -> Index:
     # drop levels
     original_index = index
     if isinstance(key, tuple):
+        # Caller is responsible for ensuring the key is not an entry in the first
+        #  level of the MultiIndex.
         for _ in key:
             try:
                 index = index._drop_level_numbers([0])

pandas/core/indexing.py

@@ -1779,6 +1779,7 @@ def _setitem_with_indexer(self, indexer, value, name: str = "iloc"):
                             self.obj[key] = empty_value
 
                         else:
+                            # FIXME: GH#42099#issuecomment-864326014
                             self.obj[key] = infer_fill_value(value)
 
                         new_indexer = convert_from_missing_indexer_tuple(
@@ -1866,6 +1867,8 @@ def _setitem_with_indexer_split_path(self, indexer, value, name: str):
                 self._setitem_with_indexer_frame_value(indexer, value, name)
 
             elif np.ndim(value) == 2:
+                # TODO: avoid np.ndim call in case it isn't an ndarray, since
+                #  that will construct an ndarray, which will be wasteful
                 self._setitem_with_indexer_2d_value(indexer, value)
 
             elif len(ilocs) == 1 and lplane_indexer == len(value) and not is_scalar(pi):

pandas/core/internals/blocks.py

@@ -429,6 +429,7 @@ def _maybe_downcast(
             return blocks
 
         if self.dtype == _dtype_obj:
+            # TODO: does it matter that self.dtype might not match blocks[i].dtype?
             # GH#44241 We downcast regardless of the argument;
             #  respecting 'downcast=None' may be worthwhile at some point,
             #  but ATM it breaks too much existing code.
@@ -1817,39 +1818,38 @@ def _unwrap_setitem_indexer(self, indexer):
         """
         # TODO: ATM this doesn't work for iget/_slice, can we change that?
 
-        if isinstance(indexer, tuple):
+        if isinstance(indexer, tuple) and len(indexer) == 2:
             # TODO(EA2D): not needed with 2D EAs
             #  Should never have length > 2.  Caller is responsible for checking.
             #  Length 1 is reached vis setitem_single_block and setitem_single_column
             #  each of which pass indexer=(pi,)
-            if len(indexer) == 2:
-                if all(isinstance(x, np.ndarray) and x.ndim == 2 for x in indexer):
-                    # GH#44703 went through indexing.maybe_convert_ix
-                    first, second = indexer
-                    if not (
-                        second.size == 1 and (second == 0).all() and first.shape[1] == 1
-                    ):
-                        raise NotImplementedError(
-                            "This should not be reached. Please report a bug at "
-                            "github.com/pandas-dev/pandas/"
-                        )
-                    indexer = first[:, 0]
-
-                elif lib.is_integer(indexer[1]) and indexer[1] == 0:
-                    # reached via setitem_single_block passing the whole indexer
-                    indexer = indexer[0]
-
-                elif com.is_null_slice(indexer[1]):
-                    indexer = indexer[0]
-
-                elif is_list_like(indexer[1]) and indexer[1][0] == 0:
-                    indexer = indexer[0]
-
-                else:
+            if all(isinstance(x, np.ndarray) and x.ndim == 2 for x in indexer):
+                # GH#44703 went through indexing.maybe_convert_ix
+                first, second = indexer
+                if not (
+                    second.size == 1 and (second == 0).all() and first.shape[1] == 1
+                ):
                     raise NotImplementedError(
                         "This should not be reached. Please report a bug at "
                         "github.com/pandas-dev/pandas/"
                     )
+                indexer = first[:, 0]
+
+            elif lib.is_integer(indexer[1]) and indexer[1] == 0:
+                # reached via setitem_single_block passing the whole indexer
+                indexer = indexer[0]
+
+            elif com.is_null_slice(indexer[1]):
+                indexer = indexer[0]
+
+            elif is_list_like(indexer[1]) and indexer[1][0] == 0:
+                indexer = indexer[0]
+
+            else:
+                raise NotImplementedError(
+                    "This should not be reached. Please report a bug at "
+                    "github.com/pandas-dev/pandas/"
+                )
         return indexer
 
     @property

pandas/core/internals/concat.py

@@ -596,9 +596,6 @@ def _concatenate_join_units(join_units: list[JoinUnit], copy: bool) -> ArrayLike
 
     elif any(is_1d_only_ea_dtype(t.dtype) for t in to_concat):
         # TODO(EA2D): special case not needed if all EAs used HybridBlocks
-        # NB: we are still assuming here that Hybrid blocks have shape (1, N)
-        # concatting with at least one EA means we are concatting a single column
-        # the non-EA values are 2D arrays with shape (1, n)
 
         # error: No overload variant of "__getitem__" of "ExtensionArray" matches
         # argument type "Tuple[int, slice]"