NaT.isoformat() returns 'NaT'.

NaT.isoformat() returning 'NaT' allows users to use Timestamp to
rehydrate any _Timestamp object from its isoformat. This means that it
is safe to use _Timestamp.isoformat() as a serialization format for
timestamps where NaT is a valid value.

Author: Joe Jevnik <joe@quantopian.com>

Closes #12300 from llllllllll/nat-isoformat and squashes the following commits:

eb34f07 [Joe Jevnik] ENH: NaT.isoformat() returns 'NaT'.

Author: Joe Jevnik

doc/source/whatsnew/v0.18.0.txt

@@ -437,6 +437,8 @@ Backwards incompatible API changes
 - ``Series.head(0)`` and ``Series.tail(0)`` return empty series, rather than ``self``.  (:issue:`11937`)
 - ``to_msgpack`` and ``read_msgpack`` encoding now defaults to ``'utf-8'``. (:issue:`12170`)
 - the order of keyword arguments to text file parsing functions (``.read_csv()``, ``.read_table()``, ``.read_fwf()``) changed to group related arguments. (:issue:`11555`)
+- ``NaTType.isoformat`` now returns the string ``'NaT`` to allow the result to
+  be passed to the constructor of ``Timestamp``. (:issue:`12300`)
 
 NaT and Timedelta operations
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -507,6 +509,11 @@ Subtraction by ``Timedelta`` in a ``Series`` by a ``Timestamp`` works (:issue:`1
    pd.Timestamp('2012-01-01') - ser
 
 
+``NaT.isoformat()`` now returns ``'NaT'``. This change allows allows
+``pd.Timestamp`` to rehydrate any timestamp like object from its isoformat
+(:issue:`12300`).
+
+
 Signature change for .rank
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 

pandas/tests/test_format.py

@@ -10,6 +10,7 @@
 from pandas.compat import range, zip, lrange, StringIO, PY3, lzip, u
 import pandas.compat as compat
 import itertools
+from operator import methodcaller
 import os
 import sys
 from textwrap import dedent
@@ -4083,6 +4084,10 @@ def test_tz_dateutil(self):
         dt_datetime_us = datetime(2013, 1, 2, 12, 1, 3, 45, tzinfo=utc)
         self.assertEqual(str(dt_datetime_us), str(Timestamp(dt_datetime_us)))
 
+    def test_nat_representations(self):
+        for f in (str, repr, methodcaller('isoformat')):
+            self.assertEqual(f(pd.NaT), 'NaT')
+
 
 if __name__ == '__main__':
     nose.runmodule(argv=[__file__, '-vvs', '-x', '--pdb', '--pdb-failure'],

pandas/tseries/tests/test_timeseries.py

@@ -973,9 +973,9 @@ def test_NaT_methods(self):
         # GH 9513
         raise_methods = ['astimezone', 'combine', 'ctime', 'dst',
                          'fromordinal', 'fromtimestamp', 'isocalendar',
-                         'isoformat', 'strftime', 'strptime', 'time',
-                         'timestamp', 'timetuple', 'timetz', 'toordinal',
-                         'tzname', 'utcfromtimestamp', 'utcnow', 'utcoffset',
+                         'strftime', 'strptime', 'time', 'timestamp',
+                         'timetuple', 'timetz', 'toordinal', 'tzname',
+                         'utcfromtimestamp', 'utcnow', 'utcoffset',
                          'utctimetuple']
         nat_methods = ['date', 'now', 'replace', 'to_datetime', 'today']
         nan_methods = ['weekday', 'isoweekday']
@@ -992,6 +992,9 @@ def test_NaT_methods(self):
             if hasattr(NaT, method):
                 self.assertIs(getattr(NaT, method)(), NaT)
 
+        # GH 12300
+        self.assertEqual(NaT.isoformat(), 'NaT')
+
     def test_to_datetime_types(self):
 
         # empty string

pandas/tslib.pyx

@@ -673,6 +673,10 @@ class NaTType(_NaT):
     def __str__(self):
         return 'NaT'
 
+    def isoformat(self, sep='T'):
+        # This allows Timestamp(ts.isoformat()) to always correctly roundtrip.
+        return 'NaT'
+
     def __hash__(self):
         return NPY_NAT
 
@@ -736,7 +740,7 @@ _nat_methods = ['date', 'now', 'replace', 'to_datetime', 'today']
 
 _nan_methods = ['weekday', 'isoweekday', 'total_seconds']
 
-_implemented_methods = ['to_datetime64']
+_implemented_methods = ['to_datetime64', 'isoformat']
 _implemented_methods.extend(_nat_methods)
 _implemented_methods.extend(_nan_methods)