DEPR: Index.format (pandas-dev#55439)

* REF: implement Index._format_flat, _format_multi

* de-duplicate, change keyword

* DEPR: Index.format

* add formatter kwd

* Post-merge fixup

doc/source/whatsnew/v2.2.0.rst

@@ -234,6 +234,7 @@ For example:
 Other Deprecations
 ^^^^^^^^^^^^^^^^^^
 - Changed :meth:`Timedelta.resolution_string` to return ``h``, ``min``, ``s``, ``ms``, ``us``, and ``ns`` instead of ``H``, ``T``, ``S``, ``L``, ``U``, and ``N``, for compatibility with respective deprecations in frequency aliases (:issue:`52536`)
+- Deprecated :meth:`Index.format`, use ``index.astype(str)`` or ``index.map(formatter)`` instead (:issue:`55413`)
 - Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_clipboard`. (:issue:`54229`)
 - Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_csv` except ``path_or_buf``. (:issue:`54229`)
 - Deprecated allowing non-keyword arguments in :meth:`DataFrame.to_dict`. (:issue:`54229`)
@@ -261,6 +262,7 @@ Other Deprecations
 - Deprecated the extension test classes ``BaseNoReduceTests``, ``BaseBooleanReduceTests``, and ``BaseNumericReduceTests``, use ``BaseReduceTests`` instead (:issue:`54663`)
 - Deprecated the option ``mode.data_manager`` and the ``ArrayManager``; only the ``BlockManager`` will be available in future versions (:issue:`55043`)
 - Deprecating downcasting the results of :meth:`DataFrame.fillna`, :meth:`Series.fillna`, :meth:`DataFrame.ffill`, :meth:`Series.ffill`, :meth:`DataFrame.bfill`, :meth:`Series.bfill` in object-dtype cases. To opt in to the future version, use ``pd.set_option("future.no_silent_downcasting", True)`` (:issue:`54261`)
+-
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_220.performance:

pandas/core/indexes/base.py

@@ -1375,6 +1375,14 @@ def format(
         """
         Render a string representation of the Index.
         """
+        warnings.warn(
+            # GH#55413
+            f"{type(self).__name__}.format is deprecated and will be removed "
+            "in a future version. Convert using index.astype(str) or "
+            "index.map(formatter) instead.",
+            FutureWarning,
+            stacklevel=find_stack_level(),
+        )
         header = []
         if name:
             header.append(
@@ -1388,6 +1396,31 @@ def format(
 
         return self._format_with_header(header=header, na_rep=na_rep)
 
+    _default_na_rep = "NaN"
+
+    @final
+    def _format_flat(
+        self,
+        *,
+        include_name: bool,
+        formatter: Callable | None = None,
+    ) -> list[str_t]:
+        """
+        Render a string representation of the Index.
+        """
+        header = []
+        if include_name:
+            header.append(
+                pprint_thing(self.name, escape_chars=("\t", "\r", "\n"))
+                if self.name is not None
+                else ""
+            )
+
+        if formatter is not None:
+            return header + list(self.map(formatter))
+
+        return self._format_with_header(header=header, na_rep=self._default_na_rep)
+
     def _format_with_header(self, *, header: list[str_t], na_rep: str_t) -> list[str_t]:
         from pandas.io.formats.format import format_array
 

pandas/core/indexes/datetimelike.py

@@ -14,6 +14,7 @@
     cast,
     final,
 )
+import warnings
 
 import numpy as np
 
@@ -42,6 +43,7 @@
     cache_readonly,
     doc,
 )
+from pandas.util._exceptions import find_stack_level
 
 from pandas.core.dtypes.common import (
     is_integer,
@@ -187,6 +189,7 @@ def _convert_tolerance(self, tolerance, target):
 
     # --------------------------------------------------------------------
     # Rendering Methods
+    _default_na_rep = "NaT"
 
     def format(
         self,
@@ -198,6 +201,14 @@ def format(
         """
         Render a string representation of the Index.
         """
+        warnings.warn(
+            # GH#55413
+            f"{type(self).__name__}.format is deprecated and will be removed "
+            "in a future version. Convert using index.astype(str) or "
+            "index.map(formatter) instead.",
+            FutureWarning,
+            stacklevel=find_stack_level(),
+        )
         header = []
         if name:
             header.append(

pandas/core/indexes/multi.py

@@ -1430,6 +1430,15 @@ def format(
         sparsify=None,
         adjoin: bool = True,
     ) -> list:
+        warnings.warn(
+            # GH#55413
+            f"{type(self).__name__}.format is deprecated and will be removed "
+            "in a future version. Convert using index.astype(str) or "
+            "index.map(formatter) instead.",
+            FutureWarning,
+            stacklevel=find_stack_level(),
+        )
+
         if name is not None:
             names = name
 
@@ -1492,6 +1501,69 @@ def format(
         else:
             return result_levels
 
+    def _format_multi(
+        self,
+        *,
+        include_names: bool,
+        sparsify: bool | None | lib.NoDefault,
+        formatter: Callable | None = None,
+    ) -> list:
+        if len(self) == 0:
+            return []
+
+        stringified_levels = []
+        for lev, level_codes in zip(self.levels, self.codes):
+            na = _get_na_rep(lev.dtype)
+
+            if len(lev) > 0:
+                taken = formatted = lev.take(level_codes)
+                formatted = taken._format_flat(include_name=False, formatter=formatter)
+
+                # we have some NA
+                mask = level_codes == -1
+                if mask.any():
+                    formatted = np.array(formatted, dtype=object)
+                    formatted[mask] = na
+                    formatted = formatted.tolist()
+
+            else:
+                # weird all NA case
+                formatted = [
+                    pprint_thing(na if isna(x) else x, escape_chars=("\t", "\r", "\n"))
+                    for x in algos.take_nd(lev._values, level_codes)
+                ]
+            stringified_levels.append(formatted)
+
+        result_levels = []
+        for lev, lev_name in zip(stringified_levels, self.names):
+            level = []
+
+            if include_names:
+                level.append(
+                    pprint_thing(lev_name, escape_chars=("\t", "\r", "\n"))
+                    if lev_name is not None
+                    else ""
+                )
+
+            level.extend(np.array(lev, dtype=object))
+            result_levels.append(level)
+
+        if sparsify is None:
+            sparsify = get_option("display.multi_sparse")
+
+        if sparsify:
+            sentinel: Literal[""] | bool | lib.NoDefault = ""
+            # GH3547 use value of sparsify as sentinel if it's "Falsey"
+            assert isinstance(sparsify, bool) or sparsify is lib.no_default
+            if sparsify is lib.no_default:
+                sentinel = sparsify
+            # little bit of a kludge job for #1217
+            result_levels = sparsify_labels(
+                result_levels, start=int(include_names), sentinel=sentinel
+            )
+
+        return result_levels
+
     # --------------------------------------------------------------------
     # Names Methods
 

pandas/io/formats/excel.py

@@ -623,8 +623,8 @@ def _format_header_mi(self) -> Iterable[ExcelCell]:
             return
 
         columns = self.columns
-        level_strs = columns.format(
-            sparsify=self.merge_cells, adjoin=False, names=False
+        level_strs = columns._format_multi(
+            sparsify=self.merge_cells, include_names=False
         )
         level_lengths = get_level_lengths(level_strs)
         coloffset = 0
@@ -813,8 +813,8 @@ def _format_hierarchical_rows(self) -> Iterable[ExcelCell]:
 
             if self.merge_cells:
                 # Format hierarchical rows as merged cells.
-                level_strs = self.df.index.format(
-                    sparsify=True, adjoin=False, names=False
+                level_strs = self.df.index._format_multi(
+                    sparsify=True, include_names=False
                 )
                 level_lengths = get_level_lengths(level_strs)
 

pandas/io/formats/format.py

@@ -313,8 +313,14 @@ def to_string(self) -> str:
         if len(series) == 0:
             return f"{type(self.series).__name__}([], {footer})"
 
-        have_header = _has_names(series.index)
-        fmt_index = self.tr_series.index.format(name=True)
+        index = series.index
+        have_header = _has_names(index)
+        if isinstance(index, MultiIndex):
+            fmt_index = index._format_multi(include_names=True, sparsify=None)
+            adj = printing.get_adjustment()
+            fmt_index = adj.adjoin(2, *fmt_index).split("\n")
+        else:
+            fmt_index = index._format_flat(include_name=True)
         fmt_values = self._get_formatted_values()
 
         if self.is_truncated_vertically:
@@ -777,7 +783,7 @@ def _get_formatted_column_labels(self, frame: DataFrame) -> list[list[str]]:
         columns = frame.columns
 
         if isinstance(columns, MultiIndex):
-            fmt_columns = columns.format(sparsify=False, adjoin=False)
+            fmt_columns = columns._format_multi(sparsify=False, include_names=False)
             fmt_columns = list(zip(*fmt_columns))
             dtypes = self.frame.dtypes._values
 
@@ -802,7 +808,7 @@ def space_format(x, y):
 
             str_columns = [list(x) for x in zip(*str_columns)]
         else:
-            fmt_columns = columns.format()
+            fmt_columns = columns._format_flat(include_name=False)
             dtypes = self.frame.dtypes
             need_leadsp = dict(zip(fmt_columns, map(is_numeric_dtype, dtypes)))
             str_columns = [
@@ -821,14 +827,15 @@ def _get_formatted_index(self, frame: DataFrame) -> list[str]:
         fmt = self._get_formatter("__index__")
 
         if isinstance(index, MultiIndex):
-            fmt_index = index.format(
+            fmt_index = index._format_multi(
                 sparsify=self.sparsify,
-                adjoin=False,
-                names=self.show_row_idx_names,
+                include_names=self.show_row_idx_names,
                 formatter=fmt,
             )
         else:
-            fmt_index = [index.format(name=self.show_row_idx_names, formatter=fmt)]
+            fmt_index = [
+                index._format_flat(include_name=self.show_row_idx_names, formatter=fmt)
+            ]
 
         fmt_index = [
             tuple(

pandas/io/formats/html.py

@@ -282,7 +282,7 @@ def _write_col_header(self, indent: int) -> None:
                 sentinel = lib.no_default
             else:
                 sentinel = False
-            levels = self.columns.format(sparsify=sentinel, adjoin=False, names=False)
+            levels = self.columns._format_multi(sparsify=sentinel, include_names=False)
             level_lengths = get_level_lengths(levels, sentinel)
             inner_lvl = len(level_lengths) - 1
             for lnum, (records, values) in enumerate(zip(level_lengths, levels)):
@@ -437,7 +437,8 @@ def _write_regular_rows(
             if fmt is not None:
                 index_values = self.fmt.tr_frame.index.map(fmt)
             else:
-                index_values = self.fmt.tr_frame.index.format()
+                # only reached with non-Multi index
+                index_values = self.fmt.tr_frame.index._format_flat(include_name=False)
 
         row: list[str] = []
         for i in range(nrows):
@@ -480,13 +481,13 @@ def _write_hierarchical_rows(
         nrows = len(frame)
 
         assert isinstance(frame.index, MultiIndex)
-        idx_values = frame.index.format(sparsify=False, adjoin=False, names=False)
+        idx_values = frame.index._format_multi(sparsify=False, include_names=False)
         idx_values = list(zip(*idx_values))
 
         if self.fmt.sparsify:
             # GH3547
             sentinel = lib.no_default
-            levels = frame.index.format(sparsify=sentinel, adjoin=False, names=False)
+            levels = frame.index._format_multi(sparsify=sentinel, include_names=False)
 
             level_lengths = get_level_lengths(levels, sentinel)
             inner_lvl = len(level_lengths) - 1
@@ -579,7 +580,7 @@ def _write_hierarchical_rows(
                     )
 
                 idx_values = list(
-                    zip(*frame.index.format(sparsify=False, adjoin=False, names=False))
+                    zip(*frame.index._format_multi(sparsify=False, include_names=False))
                 )
                 row = []
                 row.extend(idx_values[i])
@@ -606,7 +607,8 @@ def _get_formatted_values(self) -> dict[int, list[str]]:
         return {i: self.fmt.format_col(i) for i in range(self.ncols)}
 
     def _get_columns_formatted_values(self) -> list[str]:
-        return self.columns.format()
+        # only reached with non-Multi Index
+        return self.columns._format_flat(include_name=False)
 
     def write_style(self) -> None:
         # We use the "scoped" attribute here so that the desired

pandas/io/formats/style_render.py

@@ -1652,9 +1652,9 @@ def _get_level_lengths(
         Result is a dictionary of (level, initial_position): span
     """
     if isinstance(index, MultiIndex):
-        levels = index.format(sparsify=lib.no_default, adjoin=False)
+        levels = index._format_multi(sparsify=lib.no_default, include_names=False)
     else:
-        levels = index.format()
+        levels = index._format_flat(include_name=False)
 
     if hidden_elements is None:
         hidden_elements = []

pandas/tests/indexes/base_class/test_formats.py

@@ -4,6 +4,7 @@
 import pandas._config.config as cf
 
 from pandas import Index
+import pandas._testing as tm
 
 
 class TestIndexRendering:
@@ -133,7 +134,9 @@ def test_summary_bug(self):
     def test_index_repr_bool_nan(self):
         # GH32146
         arr = Index([True, False, np.nan], dtype=object)
-        exp1 = arr.format()
+        msg = "Index.format is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            exp1 = arr.format()
         out1 = ["True", "False", "NaN"]
         assert out1 == exp1
 
@@ -145,4 +148,6 @@ def test_format_different_scalar_lengths(self):
         # GH#35439
         idx = Index(["aaaaaaaaa", "b"])
         expected = ["aaaaaaaaa", "b"]
-        assert idx.format() == expected
+        msg = r"Index\.format is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert idx.format() == expected

pandas/tests/indexes/categorical/test_formats.py

@@ -4,14 +4,17 @@
 import pandas._config.config as cf
 
 from pandas import CategoricalIndex
+import pandas._testing as tm
 
 
 class TestCategoricalIndexRepr:
     def test_format_different_scalar_lengths(self):
         # GH#35439
         idx = CategoricalIndex(["aaaaaaaaa", "b"])
         expected = ["aaaaaaaaa", "b"]
-        assert idx.format() == expected
+        msg = r"CategoricalIndex\.format is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert idx.format() == expected
 
     def test_string_categorical_index_repr(self):
         # short

pandas/tests/indexes/datetimes/test_datetimelike.py

@@ -1,10 +1,13 @@
 """ generic tests from the Datetimelike class """
 from pandas import date_range
+import pandas._testing as tm
 
 
 class TestDatetimeIndex:
     def test_format(self):
         # GH35439
         idx = date_range("20130101", periods=5)
         expected = [f"{x:%Y-%m-%d}" for x in idx]
-        assert idx.format() == expected
+        msg = r"DatetimeIndex\.format is deprecated"
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            assert idx.format() == expected