REF/API: DatetimeTZDtype

* Remove magic constructor from string
* Remove Caching

The remaining changes in the DatetimeArray PR will be to

1. Inherit from ExtensionDtype
2. Implement construct_array_type
3. Register

pandas/core/arrays/datetimelike.py

@@ -978,16 +978,21 @@ def validate_tz_from_dtype(dtype, tz):
     ValueError : on tzinfo mismatch
     """
     if dtype is not None:
-        try:
-            dtype = DatetimeTZDtype.construct_from_string(dtype)
-            dtz = getattr(dtype, 'tz', None)
-            if dtz is not None:
-                if tz is not None and not timezones.tz_compare(tz, dtz):
-                    raise ValueError("cannot supply both a tz and a dtype"
-                                     " with a tz")
-                tz = dtz
-        except TypeError:
-            pass
+        if isinstance(dtype, compat.string_types):
+            try:
+                dtype = DatetimeTZDtype.construct_from_string(dtype)
+            except TypeError:
+                # Things like `datetime64[ns]`, which is OK for the
+                # constructors, but also nonsense, which should be validated
+                # but not by us. We *do* allow non-existent tz errors to
+                # go through
+                pass
+        dtz = getattr(dtype, 'tz', None)
+        if dtz is not None:
+            if tz is not None and not timezones.tz_compare(tz, dtz):
+                raise ValueError("cannot supply both a tz and a dtype"
+                                 " with a tz")
+            tz = dtz
     return tz
 
 

pandas/core/dtypes/common.py

@@ -1789,7 +1789,7 @@ def _coerce_to_dtype(dtype):
         ordered = getattr(dtype, 'ordered', False)
         dtype = CategoricalDtype(categories=categories, ordered=ordered)
     elif is_datetime64tz_dtype(dtype):
-        dtype = DatetimeTZDtype(dtype)
+        dtype = DatetimeTZDtype.construct_from_string(dtype)
     elif is_period_dtype(dtype):
         dtype = PeriodDtype(dtype)
     elif is_interval_dtype(dtype):

pandas/core/dtypes/dtypes.py

@@ -1,8 +1,8 @@
 """ define extension dtypes """
-
 import re
 
 import numpy as np
+import pytz
 
 from pandas._libs.interval import Interval
 from pandas._libs.tslibs import NaT, Period, Timestamp, timezones
@@ -483,99 +483,103 @@ class DatetimeTZDtype(PandasExtensionDtype):
     str = '|M8[ns]'
     num = 101
     base = np.dtype('M8[ns]')
+    na_value = NaT
     _metadata = ('unit', 'tz')
     _match = re.compile(r"(datetime64|M8)\[(?P<unit>.+), (?P<tz>.+)\]")
     _cache = {}
+    # TODO: restore caching? who cares though? It seems needlessly complex.
+    # np.dtype('datetime64[ns]') isn't a singleton
 
-    def __new__(cls, unit=None, tz=None):
-        """ Create a new unit if needed, otherwise return from the cache
+    def __init__(self, unit="ns", tz=None):
+        """
+        An ExtensionDtype for timezone-aware datetime data.
 
         Parameters
         ----------
-        unit : string unit that this represents, currently must be 'ns'
-        tz : string tz that this represents
-        """
+        unit : str, default "ns"
+            The precision of the datetime data. Currently limited
+            to ``"ns"``.
+        tz : str, int, or datetime.tzinfo
+            The timezone.
+
+        Raises
+        ------
+        pytz.UnknownTimeZoneError
+            When the requested timezone cannot be found.
 
+        Examples
+        --------
+        >>> pd.core.dtypes.dtypes.DatetimeTZDtype(tz='UTC')
+        datetime64[ns, UTC]
+
+        >>> pd.core.dtypes.dtypes.DatetimeTZDtype(tz='dateutil/US/Central')
+        datetime64[ns, tzfile('/usr/share/zoneinfo/US/Central')]
+        """
         if isinstance(unit, DatetimeTZDtype):
             unit, tz = unit.unit, unit.tz
 
-        elif unit is None:
-            # we are called as an empty constructor
-            # generally for pickle compat
-            return object.__new__(cls)
+        if unit != 'ns':
+            raise ValueError("DatetimeTZDtype only supports ns units")
 
+        if tz:
+            tz = timezones.maybe_get_tz(tz)
+        elif tz is not None:
+            raise pytz.UnknownTimeZoneError(tz)
         elif tz is None:
+            raise TypeError("A 'tz' is required.")
 
-            # we were passed a string that we can construct
-            try:
-                m = cls._match.search(unit)
-                if m is not None:
-                    unit = m.groupdict()['unit']
-                    tz = timezones.maybe_get_tz(m.groupdict()['tz'])
-            except TypeError:
-                raise ValueError("could not construct DatetimeTZDtype")
-
-        elif isinstance(unit, compat.string_types):
-
-            if unit != 'ns':
-                raise ValueError("DatetimeTZDtype only supports ns units")
+        self._unit = unit
+        self._tz = tz
 
-            unit = unit
-            tz = tz
-
-        if tz is None:
-            raise ValueError("DatetimeTZDtype constructor must have a tz "
-                             "supplied")
-
-        # hash with the actual tz if we can
-        # some cannot be hashed, so stringfy
-        try:
-            key = (unit, tz)
-            hash(key)
-        except TypeError:
-            key = (unit, str(tz))
+    @property
+    def unit(self):
+        """The precision of the datetime data."""
+        return self._unit
 
-        # set/retrieve from cache
-        try:
-            return cls._cache[key]
-        except KeyError:
-            u = object.__new__(cls)
-            u.unit = unit
-            u.tz = tz
-            cls._cache[key] = u
-            return u
+    @property
+    def tz(self):
+        """The timezone."""
+        return self._tz
 
     @classmethod
-    def construct_array_type(cls):
-        """Return the array type associated with this dtype
-
-        Returns
-        -------
-        type
+    def construct_from_string(cls, string):
         """
-        from pandas import DatetimeIndex
-        return DatetimeIndex
+        Construct a DatetimeTZDtype from a string.
 
-    @classmethod
-    def construct_from_string(cls, string):
-        """ attempt to construct this type from a string, raise a TypeError if
-        it's not possible
+        Parameters
+        ----------
+        string : str
+            The string alias for this DatetimeTZDtype.
+            Should be formatted like ``datetime64[ns, <tz>]``,
+            where ``<tz>`` is the timezone name.
+
+        Examples
+        --------
+        >>> DatetimeTZDtype.construct_from_string('datetime64[ns, UTC]')
+        datetime64[ns, UTC]
         """
+        msg = "could not construct DatetimeTZDtype"""
         try:
-            return cls(unit=string)
+            match = cls._match.match(string)
+            if match:
+                d = match.groupdict()
+                return cls(unit=d['unit'], tz=d['tz'])
+            else:
+                raise TypeError(msg)
         except ValueError:
-            raise TypeError("could not construct DatetimeTZDtype")
+            raise TypeError(msg)
 
     def __unicode__(self):
-        # format the tz
         return "datetime64[{unit}, {tz}]".format(unit=self.unit, tz=self.tz)
 
     @property
     def name(self):
+        """A string representation of the dtype."""
         return str(self)
 
     def __hash__(self):
         # make myself hashable
+        # TODO: update this.
         return hash(str(self))
 
     def __eq__(self, other):
@@ -586,6 +590,10 @@ def __eq__(self, other):
                 self.unit == other.unit and
                 str(self.tz) == str(other.tz))
 
+    def __getstate__(self):
+        # for pickle compat.
+        return self.__dict__
+
 
 class PeriodDtype(ExtensionDtype, PandasExtensionDtype):
     """

pandas/tests/dtypes/test_common.py

@@ -42,9 +42,12 @@ def test_numpy_string_dtype(self):
         'datetime64[ns, US/Eastern]',
         'datetime64[ns, Asia/Tokyo]',
         'datetime64[ns, UTC]'])
+    @pytest.mark.xfail(reason="dtype-caching", strict=True)
     def test_datetimetz_dtype(self, dtype):
-        assert com.pandas_dtype(dtype) is DatetimeTZDtype(dtype)
-        assert com.pandas_dtype(dtype) == DatetimeTZDtype(dtype)
+        assert (com.pandas_dtype(dtype) is
+                DatetimeTZDtype.construct_from_string(dtype))
+        assert (com.pandas_dtype(dtype) ==
+                DatetimeTZDtype.construct_from_string(dtype))
         assert com.pandas_dtype(dtype) == dtype
 
     def test_categorical_dtype(self):

pandas/tests/dtypes/test_dtypes.py

@@ -155,19 +155,20 @@ def test_hash_vs_equality(self):
         assert dtype == dtype2
         assert dtype2 == dtype
         assert dtype3 == dtype
-        assert dtype is dtype2
-        assert dtype2 is dtype
-        assert dtype3 is dtype
         assert hash(dtype) == hash(dtype2)
         assert hash(dtype) == hash(dtype3)
 
+        dtype4 = DatetimeTZDtype("ns", "US/Central")
+        assert dtype2 != dtype4
+        assert hash(dtype2) != hash(dtype4)
+
     def test_construction(self):
         pytest.raises(ValueError,
                       lambda: DatetimeTZDtype('ms', 'US/Eastern'))
 
     def test_subclass(self):
-        a = DatetimeTZDtype('datetime64[ns, US/Eastern]')
-        b = DatetimeTZDtype('datetime64[ns, CET]')
+        a = DatetimeTZDtype.construct_from_string('datetime64[ns, US/Eastern]')
+        b = DatetimeTZDtype.construct_from_string('datetime64[ns, CET]')
 
         assert issubclass(type(a), type(a))
         assert issubclass(type(a), type(b))
@@ -189,8 +190,6 @@ def test_compat(self):
         assert not is_datetime64_dtype('datetime64[ns, US/Eastern]')
 
     def test_construction_from_string(self):
-        result = DatetimeTZDtype('datetime64[ns, US/Eastern]')
-        assert is_dtype_equal(self.dtype, result)
         result = DatetimeTZDtype.construct_from_string(
             'datetime64[ns, US/Eastern]')
         assert is_dtype_equal(self.dtype, result)
@@ -255,14 +254,13 @@ def test_dst(self):
     def test_parser(self, tz, constructor):
         # pr #11245
         dtz_str = '{con}[ns, {tz}]'.format(con=constructor, tz=tz)
-        result = DatetimeTZDtype(dtz_str)
+        result = DatetimeTZDtype.construct_from_string(dtz_str)
         expected = DatetimeTZDtype('ns', tz)
         assert result == expected
 
     def test_empty(self):
-        dt = DatetimeTZDtype()
-        with pytest.raises(AttributeError):
-            str(dt)
+        with pytest.raises(TypeError, match="A 'tz' is required."):
+            DatetimeTZDtype()
 
 
 class TestPeriodDtype(Base):
@@ -795,34 +793,38 @@ def test_update_dtype_errors(self, bad_dtype):
             dtype.update_dtype(bad_dtype)
 
 
-@pytest.mark.parametrize(
-    'dtype',
-    [CategoricalDtype, IntervalDtype])
+@pytest.mark.parametrize('dtype', [
+    CategoricalDtype,
+    IntervalDtype,
+])
 def test_registry(dtype):
     assert dtype in registry.dtypes
 
 
-@pytest.mark.parametrize('dtype', [DatetimeTZDtype, PeriodDtype])
+@pytest.mark.parametrize('dtype', [
+    PeriodDtype,
+    DatetimeTZDtype,
+])
 def test_pandas_registry(dtype):
     assert dtype not in registry.dtypes
     assert dtype in _pandas_registry.dtypes
 
 
-@pytest.mark.parametrize(
-    'dtype, expected',
-    [('int64', None),
-     ('interval', IntervalDtype()),
-     ('interval[int64]', IntervalDtype()),
-     ('interval[datetime64[ns]]', IntervalDtype('datetime64[ns]')),
-     ('category', CategoricalDtype())])
+@pytest.mark.parametrize('dtype, expected', [
+    ('int64', None),
+    ('interval', IntervalDtype()),
+    ('interval[int64]', IntervalDtype()),
+    ('interval[datetime64[ns]]', IntervalDtype('datetime64[ns]')),
+    ('category', CategoricalDtype()),
+])
 def test_registry_find(dtype, expected):
     assert registry.find(dtype) == expected
 
 
-@pytest.mark.parametrize(
-    'dtype, expected',
-    [('period[D]', PeriodDtype('D')),
-     ('datetime64[ns, US/Eastern]', DatetimeTZDtype('ns', 'US/Eastern'))])
+@pytest.mark.parametrize('dtype, expected', [
+    ('period[D]', PeriodDtype('D')),
+    ('datetime64[ns, US/Eastern]', DatetimeTZDtype('ns', 'US/Eastern')),
+])
 def test_pandas_registry_find(dtype, expected):
     assert _pandas_registry.find(dtype) == expected
 

pandas/tests/dtypes/test_missing.py

@@ -322,7 +322,7 @@ def test_array_equivalent_str():
     # Datetime-like
     (np.dtype("M8[ns]"), NaT),
     (np.dtype("m8[ns]"), NaT),
-    (DatetimeTZDtype('datetime64[ns, US/Eastern]'), NaT),
+    (DatetimeTZDtype.construct_from_string('datetime64[ns, US/Eastern]'), NaT),
     (PeriodDtype("M"), NaT),
     # Integer
     ('u1', 0), ('u2', 0), ('u4', 0), ('u8', 0),