ENH: Parametrized CategoricalDtype

We extended the CategoricalDtype to accept optional categories and ordered
argument.

```python
pd.CategoricalDtype(categories=['a', 'b'], ordered=True
```

CategoricalDtype is now part of the public API. This allows users to
specify the desired categories and orderedness of an operation ahead of time.
The current behavior, which is still possible with categories=None, the
default, is to infer the categories from whatever is present.

This change will make it easy to implement support for specifying categories
that are know ahead of time in other places e.g. .astype, .read_csv, and the
Series constructor.

Closes pandas-dev#14711
Closes pandas-dev#15078
Closes pandas-dev#14676

doc/source/advanced.rst

@@ -638,9 +638,11 @@ and allows efficient indexing and storage of an index with a large number of dup
 
 .. ipython:: python
 
+   from pandas.api.types import CategoricalDtype
+
    df = pd.DataFrame({'A': np.arange(6),
                       'B': list('aabbca')})
-   df['B'] = df['B'].astype('category', categories=list('cab'))
+   df['B'] = df['B'].astype(CategoricalDtype(list('cab')))
    df
    df.dtypes
    df.B.cat.categories

doc/source/api.rst

@@ -646,7 +646,10 @@ strings and apply several methods to it. These can be accessed like
 Categorical
 ~~~~~~~~~~~
 
-If the Series is of dtype ``category``, ``Series.cat`` can be used to change the the categorical
+.. autoclass:: api.types.CategoricalDtype
+   :members: categories, ordered
+
+If the Series is of dtype ``CategoricalDtype``, ``Series.cat`` can be used to change the categorical
 data. This accessor is similar to the ``Series.dt`` or ``Series.str`` and has the
 following usable methods and properties:
 

doc/source/categorical.rst

@@ -89,12 +89,22 @@ By passing a :class:`pandas.Categorical` object to a `Series` or assigning it to
     df["B"] = raw_cat
     df
 
-You can also specify differently ordered categories or make the resulting data ordered, by passing these arguments to ``astype()``:
+Anywhere above we passed a keyword ``dtype='category'``, we used the default behavior of
+
+1. categories are inferred from the data
+2. categories are unordered.
+
+To control those behaviors, instead of passing ``'category'``, use an instance
+of :class:`~pandas.api.types.CategoricalDtype`.
 
 .. ipython:: python
 
-    s = pd.Series(["a","b","c","a"])
-    s_cat = s.astype("category", categories=["b","c","d"], ordered=False)
+    from pandas.api.types import CategoricalDtype
+
+    s = pd.Series(["a", "b", "c", "a"])
+    cat_type = CategoricalDtype(categories=["b", "c", "d"],
+                                ordered=True)
+    s_cat = s.astype(cat_type)
     s_cat
 
 Categorical data has a specific ``category`` :ref:`dtype <basics.dtypes>`:
@@ -133,6 +143,70 @@ constructor to save the factorize step during normal constructor mode:
     splitter = np.random.choice([0,1], 5, p=[0.5,0.5])
     s = pd.Series(pd.Categorical.from_codes(splitter, categories=["train", "test"]))
 
+.. _categorical.categoricaldtype:
+
+CategoricalDtype
+----------------
+
+.. versionchanged:: 0.21.0
+
+A categorical's type is fully described by
+
+1. its categories: a sequence of unique values and no missing values
+2. its orderedness: a boolean
+
+This information can be stored in a :class:`~pandas.api.types.CategoricalDtype`.
+The ``categories`` argument is optional, which implies that the actual categories
+should be inferred from whatever is present in the data when the
+:class:`pandas.Categorical` is created.
+
+.. ipython:: python
+
+   from pandas.api.types import CategoricalDtype
+
+   CategoricalDtype(['a', 'b', 'c'])
+   CategoricalDtype(['a', 'b', 'c'], ordered=True)
+   CategoricalDtype()
+
+A :class:`~pandas.api.types.CategoricalDtype` can be used in any place pandas
+expects a `dtype`. For example :func:`pandas.read_csv`,
+:func:`pandas.DataFrame.astype`, or in the Series constructor.
+
+As a convenience, you can use the string ``'category'`` in place of a
+:class:`~pandas.api.types.CategoricalDtype` when you want the default behavior of
+the categories being unordered, and equal to the set values present in the
+array. In other words, ``dtype='category'`` is equivalent to
+``dtype=CategoricalDtype()``.
+
+Equality Semantics
+~~~~~~~~~~~~~~~~~~
+
+Two instances of :class:`~pandas.api.types.CategoricalDtype` compare equal
+whenever they have the same categories and orderedness. When comparing two
+unordered categoricals, the order of the ``categories`` is not considered
+
+.. ipython:: python
+
+   c1 = CategoricalDtype(['a', 'b', 'c'], ordered=False)
+
+   # Equal, since order is not considered when ordered=False
+   c1 == CategoricalDtype(['b', 'c', 'a'], ordered=False)
+
+   # Unequal, since the second CategoricalDtype is ordered
+   c1 == CategoricalDtype(['a',  'b', 'c'], ordered=True)
+
+All instances of ``CategoricalDtype`` compare equal to the string ``'category'``
+
+.. ipython:: python
+
+   c1 == 'category'
+
+.. warning::
+
+   Since ``dtype='category'`` is essentially ``CategoricalDtype(None, False)``,
+   and since all instances ``CategoricalDtype`` compare equal to ``'`category'``,
+   all instances of ``CategoricalDtype`` compare equal to a ``CategoricalDtype(None)``
+
 Description
 -----------
 
@@ -182,7 +256,7 @@ It's also possible to pass in the categories in a specific order:
 
     .. ipython:: python
 
-         s = pd.Series(list('babc')).astype('category', categories=list('abcd'))
+         s = pd.Series(list('babc')).astype(CategoricalDtype(list('abcd')))
          s
 
          # categories
@@ -295,7 +369,9 @@ meaning and certain operations are possible. If the categorical is unordered, ``
 
     s = pd.Series(pd.Categorical(["a","b","c","a"], ordered=False))
     s.sort_values(inplace=True)
-    s = pd.Series(["a","b","c","a"]).astype('category', ordered=True)
+    s = pd.Series(["a","b","c","a"]).astype(
+        CategoricalDtype(ordered=True)
+    )
     s.sort_values(inplace=True)
     s
     s.min(), s.max()
@@ -395,9 +471,15 @@ categories or a categorical with any list-like object, will raise a TypeError.
 
 .. ipython:: python
 
-    cat = pd.Series([1,2,3]).astype("category", categories=[3,2,1], ordered=True)
-    cat_base = pd.Series([2,2,2]).astype("category", categories=[3,2,1], ordered=True)
-    cat_base2 = pd.Series([2,2,2]).astype("category", ordered=True)
+    cat = pd.Series([1,2,3]).astype(
+        CategoricalDtype([3, 2, 1], ordered=True)
+    )
+    cat_base = pd.Series([2,2,2]).astype(
+        CategoricalDtype([3, 2, 1], ordered=True)
+    )
+    cat_base2 = pd.Series([2,2,2]).astype(
+        CategoricalDtype(ordered=True)
+    )
 
     cat
     cat_base

doc/source/merging.rst

@@ -830,8 +830,10 @@ The left frame.
 
 .. ipython:: python
 
+   from pandas.api.types import CategoricalDtype
+
    X = pd.Series(np.random.choice(['foo', 'bar'], size=(10,)))
-   X = X.astype('category', categories=['foo', 'bar'])
+   X = X.astype(CategoricalDtype(categories=['foo', 'bar']))
 
    left = pd.DataFrame({'X': X,
                         'Y': np.random.choice(['one', 'two', 'three'], size=(10,))})
@@ -842,8 +844,11 @@ The right frame.
 
 .. ipython:: python
 
-   right = pd.DataFrame({'X': pd.Series(['foo', 'bar']).astype('category', categories=['foo', 'bar']),
-                         'Z': [1, 2]})
+   right = pd.DataFrame({
+        'X': pd.Series(['foo', 'bar'],
+                       dtype=CategoricalDtype(['foo', 'bar'])),
+        'Z': [1, 2]
+   })
    right
    right.dtypes
 

doc/source/whatsnew/v0.21.0.txt

@@ -10,6 +10,8 @@ users upgrade to this version.
 Highlights include:
 
 - Integration with `Apache Parquet <https://parquet.apache.org/>`__, including a new top-level :func:`read_parquet` and :func:`DataFrame.to_parquet` method, see :ref:`here <io.parquet>`.
+- New user-facing :class:`pandas.api.types.CategoricalDtype` for specifying
+  categoricals independent of the data (:issue:`14711`, :issue:`15078`)
 
 Check the :ref:`API Changes <whatsnew_0210.api_breaking>` and :ref:`deprecations <whatsnew_0210.deprecations>` before updating.
 
@@ -88,6 +90,30 @@ This does not raise any obvious exceptions, but also does not create a new colum
 
 Setting a list-like data structure into a new attribute now raise a ``UserWarning`` about the potential for unexpected behavior. See :ref:`Attribute Access <indexing.attribute_access>`.
 
+.. _whatsnew_0210.enhancements.categorical_dtype:
+
+``CategoricalDtype`` for specifying categoricals
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+:class:`pandas.api.types.CategoricalDtype` has been added to the public API and
+expanded to include the ``categories`` and ``ordered`` attributes. A
+``CategoricalDtype`` can be used to specify the set of categories and
+orderedness of an array, independent of the data themselves. This can be useful,
+e.g., when converting string data to a ``Categorical``:
+
+.. ipython:: python
+
+   from pandas.api.types import CategoricalDtype
+
+   s = pd.Series(['a', 'b', 'c', 'a'])  # strings
+   dtype = CategoricalDtype(categories=['a', 'b', 'c', 'd'], ordered=True)
+   s.astype(dtype)
+
+The ``.dtype`` property of a ``Categorical``, ``CategoricalIndex`` or a
+``Series`` with categorical type will now return an instance of ``CategoricalDtype``.
+
+See the :ref:`CategoricalDtype docs <categorical.categoricaldtype>` for more.
+
 .. _whatsnew_0210.enhancements.other:
 
 Other Enhancements