REF: Store metadata in attrs dict

This aids in the implementation of
pandas-dev#28394. Over there, I'm having
issues with using `NDFrame.__finalize__` to copy attributes, in part
because getattribute on NDFrame is so complicated.

This simplifies this because we only need to look in NDFrame.attrs,
which is just a plain dictionary.

Aside from the addition of a public NDFrame.attrs dictionary, there
aren't any user-facing API changes.

doc/source/whatsnew/v1.0.0.rst

@@ -204,7 +204,6 @@ Deprecations
 - ``Index.set_value`` has been deprecated. For a given index ``idx``, array ``arr``,
   value in ``idx`` of ``idx_val`` and a new value of ``val``, ``idx.set_value(arr, idx_val, val)``
   is equivalent to ``arr[idx.get_loc(idx_val)] = val``, which should be used instead (:issue:`28621`).
--
 
 .. _whatsnew_1000.prior_deprecations:
 

pandas/core/base.py

@@ -4,7 +4,7 @@
 import builtins
 from collections import OrderedDict
 import textwrap
-from typing import Dict, FrozenSet, Optional
+from typing import Dict, FrozenSet, Hashable, Optional
 import warnings
 
 import numpy as np
@@ -30,6 +30,7 @@
     is_timedelta64_ns_dtype,
 )
 from pandas.core.dtypes.generic import ABCDataFrame, ABCIndexClass, ABCSeries
+from pandas.core.dtypes.inference import is_hashable
 from pandas.core.dtypes.missing import isna
 
 from pandas.core import algorithms, common as com
@@ -663,6 +664,16 @@ class IndexOpsMixin:
         ]
     )  # type: FrozenSet[str]
 
+    @property
+    def name(self) -> Optional[Hashable]:
+        return self.attrs.get("name", None)
+
+    @name.setter
+    def name(self, value: Hashable) -> None:
+        if not is_hashable(value):
+            raise TypeError("Series.name must be a hashable type")
+        self.attrs["name"] = value
+
     def transpose(self, *args, **kwargs):
         """
         Return the transpose, which is by definition self.

pandas/core/generic.py

@@ -14,6 +14,7 @@
     FrozenSet,
     Hashable,
     List,
+    Mapping,
     Optional,
     Sequence,
     Set,
@@ -197,6 +198,7 @@ def __init__(
         axes: Optional[List[Index]] = None,
         copy: bool = False,
         dtype: Optional[Dtype] = None,
+        attrs: Mapping[Hashable, Any] = None,
         fastpath: bool = False,
     ):
 
@@ -213,6 +215,11 @@ def __init__(
         object.__setattr__(self, "_is_copy", None)
         object.__setattr__(self, "_data", data)
         object.__setattr__(self, "_item_cache", {})
+        if attrs is None:
+            attrs = {}
+        else:
+            attrs = dict(attrs)
+        object.__setattr__(self, "_attrs", attrs)
 
     def _init_mgr(self, mgr, axes=None, dtype=None, copy=False):
         """ passed a manager and a axes dict """
@@ -233,6 +240,19 @@ def _init_mgr(self, mgr, axes=None, dtype=None, copy=False):
 
     # ----------------------------------------------------------------------
 
+    @property
+    def attrs(self):
+        """
+        Dictionary of global attributes on this object.
+        """
+        if self._attrs is None:
+            self._attrs = {}
+        return self._attrs
+
+    @attrs.setter
+    def attrs(self, value: Mapping[Hashable, Any]) -> None:
+        self._attrs = dict(value)
+
     @property
     def is_copy(self):
         """
@@ -2029,7 +2049,13 @@ def to_dense(self):
 
     def __getstate__(self):
         meta = {k: getattr(self, k, None) for k in self._metadata}
-        return dict(_data=self._data, _typ=self._typ, _metadata=self._metadata, **meta)
+        return dict(
+            _data=self._data,
+            _typ=self._typ,
+            _metadata=self._metadata,
+            attrs=self.attrs,
+            **meta
+        )
 
     def __setstate__(self, state):
 
@@ -2038,6 +2064,8 @@ def __setstate__(self, state):
         elif isinstance(state, dict):
             typ = state.get("_typ")
             if typ is not None:
+                attrs = state.get("_attrs", {})
+                object.__setattr__(self, "_attrs", attrs)
 
                 # set in the order of internal names
                 # to avoid definitional recursion
@@ -5213,6 +5241,9 @@ def __finalize__(self, other, method=None, **kwargs):
 
         """
         if isinstance(other, NDFrame):
+            for name in other.attrs:
+                self.attrs[name] = other.attrs[name]
+            # For subclasses using _metadata.
             for name in self._metadata:
                 object.__setattr__(self, name, getattr(other, name, None))
         return self

pandas/core/indexes/base.py

@@ -1,7 +1,7 @@
 from datetime import datetime
 import operator
 from textwrap import dedent
-from typing import FrozenSet, Union
+from typing import Any, FrozenSet, Hashable, Mapping, Union
 import warnings
 
 import numpy as np
@@ -266,6 +266,7 @@ def __new__(
         name=None,
         fastpath=None,
         tupleize_cols=True,
+        attrs: Mapping[Hashable, Any] = None,
         **kwargs
     ) -> "Index":
 

pandas/core/series.py

@@ -29,7 +29,6 @@
     is_dict_like,
     is_extension_array_dtype,
     is_extension_type,
-    is_hashable,
     is_integer,
     is_iterator,
     is_list_like,
@@ -173,7 +172,7 @@ class Series(base.IndexOpsMixin, generic.NDFrame):
         Copy input data.
     """
 
-    _metadata = ["name"]
+    _metadata = []
     _accessors = {"dt", "cat", "str", "sparse"}
     _deprecations = (
         base.IndexOpsMixin._deprecations
@@ -324,7 +323,6 @@ def __init__(
                 data = SingleBlockManager(data, index, fastpath=True)
 
         generic.NDFrame.__init__(self, data, fastpath=True)
-
         self.name = name
         self._set_axis(0, index, fastpath=True)
 
@@ -457,19 +455,6 @@ def _update_inplace(self, result, **kwargs):
         # we want to call the generic version and not the IndexOpsMixin
         return generic.NDFrame._update_inplace(self, result, **kwargs)
 
-    @property
-    def name(self):
-        """
-        Return name of the Series.
-        """
-        return self._name
-
-    @name.setter
-    def name(self, value):
-        if value is not None and not is_hashable(value):
-            raise TypeError("Series.name must be a hashable type")
-        object.__setattr__(self, "_name", value)
-
     # ndarray compatibility
     @property
     def dtype(self):