ENH: Allow conversion of datetime64 and timedelta64 to string in astype (GH 9757)

Author: evanpw

doc/source/whatsnew/v0.16.1.txt

@@ -30,6 +30,7 @@ Enhancements
     df = DataFrame(np.random.randn(3, 3), columns=['A', 'B', 'C'])
     df.drop(['A', 'X'], axis=1, errors='ignore')
 
+- Allow conversion of values with dtype ``datetime64`` or ``timedelta64`` to strings using ``astype(str)`` (:issue:`9757`)
 
 .. _whatsnew_0161.api:
 

pandas/core/common.py

@@ -2637,7 +2637,12 @@ def _astype_nansafe(arr, dtype, copy=True):
     if not isinstance(dtype, np.dtype):
         dtype = _coerce_to_dtype(dtype)
 
-    if is_datetime64_dtype(arr):
+    if issubclass(dtype.type, compat.text_type):
+        # in Py3 that's str, in Py2 that's unicode
+        return lib.astype_unicode(arr.ravel()).reshape(arr.shape)
+    elif issubclass(dtype.type, compat.string_types):
+        return lib.astype_str(arr.ravel()).reshape(arr.shape)
+    elif is_datetime64_dtype(arr):
         if dtype == object:
             return tslib.ints_to_pydatetime(arr.view(np.int64))
         elif dtype == np.int64:
@@ -2675,11 +2680,6 @@ def _astype_nansafe(arr, dtype, copy=True):
     elif arr.dtype == np.object_ and np.issubdtype(dtype.type, np.integer):
         # work around NumPy brokenness, #1987
         return lib.astype_intsafe(arr.ravel(), dtype).reshape(arr.shape)
-    elif issubclass(dtype.type, compat.text_type):
-        # in Py3 that's str, in Py2 that's unicode
-        return lib.astype_unicode(arr.ravel()).reshape(arr.shape)
-    elif issubclass(dtype.type, compat.string_types):
-        return lib.astype_str(arr.ravel()).reshape(arr.shape)
 
     if copy:
         return arr.astype(dtype)

pandas/tests/test_frame.py

@@ -4192,6 +4192,33 @@ def test_astype_cast_nan_int(self):
         df = DataFrame(data={"Values": [1.0, 2.0, 3.0, np.nan]})
         self.assertRaises(ValueError, df.astype, np.int64)
 
+    def test_astype_str(self):
+        # GH9757
+        dts = Series(date_range('2010-01-04', periods=5))
+        tds = Series([Timedelta(x, unit='d') for x in range(5)])
+        ns = Series(range(5))
+        fs = Series([0.0, 0.2, 0.4, 0.6, 0.8])
+
+        df = pd.DataFrame({
+            'dts' : dts,
+            'tds' : tds,
+            'ns' : ns,
+            'fs' : fs,
+            })
+
+        # Test str and unicode on python 2.x and just str on python 3.x
+        for tt in set([str, compat.text_type]):
+            result = df.astype(tt)
+
+            expected = pd.DataFrame({
+                'dts' : list(map(tt, dts.values)),
+                'tds' : list(map(tt, tds.values)),
+                'ns' : list(map(tt, ns.values)),
+                'fs' : list(map(tt, fs.values)),
+                })
+
+            assert_frame_equal(result, expected)
+
     def test_array_interface(self):
         result = np.sqrt(self.frame)
         tm.assert_isinstance(result, type(self.frame))

pandas/tests/test_series.py

@@ -5511,6 +5511,19 @@ def test_astype_str(self):
                 expec = s.map(compat.text_type)
                 assert_series_equal(res, expec)
 
+        # GH9757
+        # Test str and unicode on python 2.x and just str on python 3.x
+        for tt in set([str, compat.text_type]):
+            ts = Series([Timestamp('2010-01-04 00:00:00')])
+            s = ts.astype(tt)
+            expected = Series([tt(ts.values[0])])
+            assert_series_equal(s, expected)
+
+            td = Series([Timedelta(1, unit='d')])
+            s = td.astype(tt)
+            expected = Series([tt(td.values[0])])
+            assert_series_equal(s, expected)
+
     def test_astype_unicode(self):
 
         # GH7758