Consistent CategoricalDtype use in Categorical init

Get a valid instance of `CategoricalDtype` as early as possible, and use that
throughout.

pandas/core/categorical.py

@@ -234,6 +234,21 @@ class Categorical(PandasObject):
     def __init__(self, values, categories=None, ordered=None, dtype=None,
                  fastpath=False):
 
+        # Ways of specifying the dtype (prioritized ordered)
+        # 1. dtype is a CategoricalDtype
+        #    a.) with known categories, use dtype.categories
+        #    b.) else with Categorical values, use values.dtype
+        #    c.) else, infer from values
+        #    d.) specifying dtype=CategoricalDtype and categories is an error
+        # 2. dtype is a string 'category'
+        #    a.) use categories, ordered
+        #    b.) use values.dtype
+        #    c.) infer from values
+        # 3. dtype is None
+        #    a.) use categories, ordered
+        #    b.) use values.dtype
+        #    c.) infer from values
+
         if dtype is not None:
             if isinstance(dtype, compat.string_types):
                 if dtype == 'category':
@@ -247,20 +262,24 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
             categories = dtype.categories
             ordered = dtype.ordered
 
-        if ordered is None:
-            ordered = False
+        elif is_categorical(values):
+            dtype = values.dtype._from_categorical_dtype(values.dtype,
+                                                         categories, ordered)
+        else:
+            dtype = CategoricalDtype(categories, ordered)
+
+        # At this point, dtype is always a CategoricalDtype
+        # if dtype.categories is None, we are inferring
 
         if fastpath:
-            if dtype is None:
-                dtype = CategoricalDtype(categories, ordered)
             self._codes = coerce_indexer_dtype(values, categories)
             self._dtype = dtype
             return
 
         # sanitize input
         if is_categorical_dtype(values):
 
-            # we are either a Series, CategoricalIndex
+            # we are either a Series or a CategoricalIndex
             if isinstance(values, (ABCSeries, ABCCategoricalIndex)):
                 values = values._values
 
@@ -271,6 +290,7 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
             values = values.get_values()
 
         elif isinstance(values, (ABCIndexClass, ABCSeries)):
+            # we'll do inference later
             pass
 
         else:
@@ -288,12 +308,12 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
                 # "object" dtype to prevent this. In the end objects will be
                 # casted to int/... in the category assignment step.
                 if len(values) == 0 or isna(values).any():
-                    dtype = 'object'
+                    sanitize_dtype = 'object'
                 else:
-                    dtype = None
-                values = _sanitize_array(values, None, dtype=dtype)
+                    sanitize_dtype = None
+                values = _sanitize_array(values, None, dtype=sanitize_dtype)
 
-        if categories is None:
+        if dtype.categories is None:
             try:
                 codes, categories = factorize(values, sort=True)
             except TypeError:
@@ -310,7 +330,8 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
                 raise NotImplementedError("> 1 ndim Categorical are not "
                                           "supported at this time")
 
-            if dtype is None or isinstance(dtype, str):
+            if dtype.categories is None:
+                # we're inferring from values
                 dtype = CategoricalDtype(categories, ordered)
 
         else:
@@ -321,11 +342,6 @@ def __init__(self, values, categories=None, ordered=None, dtype=None,
             # - the new one, where each value is also in the categories array
             #   (or np.nan)
 
-            # make sure that we always have the same type here, no matter what
-            # we get passed in
-            if dtype is None or isinstance(dtype, str):
-                dtype = CategoricalDtype(categories, ordered)
-
             codes = _get_codes_for_values(values, dtype.categories)
 
             # TODO: check for old style usage. These warnings should be removes

pandas/core/dtypes/dtypes.py

@@ -160,9 +160,22 @@ def _from_fastpath(cls, categories=None, ordered=False):
         self._finalize(categories, ordered, fastpath=True)
         return self
 
+    @classmethod
+    def _from_categorical_dtype(cls, dtype, categories=None, ordered=None):
+        if categories is ordered is None:
+            return dtype
+        if categories is None:
+            categories = dtype.categories
+        if ordered is None:
+            ordered = dtype.ordered
+        return cls(categories, ordered)
+
     def _finalize(self, categories, ordered, fastpath=False):
         from pandas.core.indexes.base import Index
 
+        if ordered is None:
+            ordered = False
+
         if categories is not None:
             categories = Index(categories, tupleize_cols=False)
             # validation

pandas/tests/dtypes/test_dtypes.py

@@ -622,3 +622,30 @@ def test_mixed(self):
         a = CategoricalDtype(['a', 'b', 1, 2])
         b = CategoricalDtype(['a', 'b', '1', '2'])
         assert hash(a) != hash(b)
+
+    def test_from_categorical_dtype_identity(self):
+        c1 = Categorical([1, 2], categories=[1, 2, 3], ordered=True)
+        # Identity test for no changes
+        c2 = CategoricalDtype._from_categorical_dtype(c1)
+        assert c2 is c1
+
+    def test_from_categorical_dtype_categories(self):
+        c1 = Categorical([1, 2], categories=[1, 2, 3], ordered=True)
+        # override categories
+        result = CategoricalDtype._from_categorical_dtype(
+            c1, categories=[2, 3])
+        assert result == CategoricalDtype([2, 3], ordered=True)
+
+    def test_from_categorical_dtype_ordered(self):
+        c1 = Categorical([1, 2], categories=[1, 2, 3], ordered=True)
+        # override ordered
+        result = CategoricalDtype._from_categorical_dtype(
+            c1, ordered=False)
+        assert result == CategoricalDtype([1, 2, 3], ordered=False)
+
+    def test_from_categorical_dtype_both(self):
+        c1 = Categorical([1, 2], categories=[1, 2, 3], ordered=True)
+        # override ordered
+        result = CategoricalDtype._from_categorical_dtype(
+            c1, categories=[1, 2], ordered=False)
+        assert result == CategoricalDtype([1, 2], ordered=False)

pandas/tests/test_categorical.py

@@ -488,6 +488,37 @@ def test_constructor_str_unknown(self):
         with tm.assert_raises_regex(ValueError, "Unknown `dtype`"):
             Categorical([1, 2], dtype="foo")
 
+    def test_constructor_from_categorical_with_dtype(self):
+        dtype = CategoricalDtype(['a', 'b', 'c'], ordered=True)
+        values = Categorical(['a', 'b', 'd'])
+        result = Categorical(values, dtype=dtype)
+        # We use dtype.categories, not values.categories
+        expected = Categorical(['a', 'b', 'd'], categories=['a', 'b', 'c'],
+                               ordered=True)
+        tm.assert_categorical_equal(result, expected)
+
+    def test_constructor_from_categorical_with_unknown_dtype(self):
+        dtype = CategoricalDtype(None, ordered=True)
+        values = Categorical(['a', 'b', 'd'])
+        result = Categorical(values, dtype=dtype)
+        # We use values.categories, not dtype.categories
+        expected = Categorical(['a', 'b', 'd'], categories=['a', 'b', 'd'],
+                               ordered=True)
+        tm.assert_categorical_equal(result, expected)
+
+    def test_contructor_from_categorical_string(self):
+        values = Categorical(['a', 'b', 'd'])
+        # use categories, ordered
+        result = Categorical(values, categories=['a', 'b', 'c'], ordered=True,
+                             dtype='category')
+        expected = Categorical(['a', 'b', 'd'], categories=['a', 'b', 'c'],
+                               ordered=True)
+        tm.assert_categorical_equal(result, expected)
+
+        # No string
+        result = Categorical(values, categories=['a', 'b', 'c'], ordered=True)
+        tm.assert_categorical_equal(result, expected)
+
     def test_from_codes(self):
 
         # too few categories
@@ -932,6 +963,16 @@ def test_set_dtype_nans(self):
         tm.assert_numpy_array_equal(result.codes, np.array([0, -1, -1],
                                                            dtype='int8'))
 
+    def test_set_categories(self):
+        cat = Categorical(['a', 'b', 'c'], categories=['a', 'b', 'c', 'd'])
+        result = cat._set_categories(['a', 'b', 'c', 'd', 'e'])
+        expected = Categorical(['a', 'b', 'c'], categories=list('abcde'))
+        tm.assert_categorical_equal(result, expected)
+
+        # fastpath
+        result = cat._set_categories(['a', 'b', 'c', 'd', 'e'], fastpath=True)
+        tm.assert_categorical_equal(result, expected)
+
     @pytest.mark.parametrize('values, categories, new_categories', [
         # No NaNs, same cats, same order
         (['a', 'b', 'a'], ['a', 'b'], ['a', 'b'],),