ENH: support min/max on timedelta64[ns] Series

RELEASE.rst

@@ -131,7 +131,7 @@ pandas 0.11.0
     - Support null checking on timedelta64, representing (and formatting) with NaT
     - Support setitem with np.nan value, converts to NaT
     - Support min/max ops in a Dataframe (abs not working, nor do we error on non-supported ops)
-    - Support idxmin/idxmax/abs in a Series (but with no NaT)
+    - Support idxmin/idxmax/abs/max/min in a Series (GH2989_, GH2982_)
 
   - Bug on in-place putmasking on an ``integer`` series that needs to be converted to ``float`` (GH2746_)
   - Bug in argsort of ``datetime64[ns]`` Series with ``NaT`` (GH2967_)
@@ -160,6 +160,7 @@ pandas 0.11.0
 .. _GH2973: https://github.com/pydata/pandas/issues/2973
 .. _GH2967: https://github.com/pydata/pandas/issues/2967
 .. _GH2982: https://github.com/pydata/pandas/issues/2982
+.. _GH2989: https://github.com/pydata/pandas/issues/2989
 
 
 pandas 0.10.1

doc/source/timeseries.rst

@@ -966,14 +966,30 @@ Some timedelta numeric like operations are supported.
 
 .. ipython:: python
 
-    s  = Series(date_range('2012-1-1', periods=3, freq='D'))
+    td - timedelta(minutes=5,seconds=5,microseconds=5)
+
+``min, max`` and the corresponding ``idxmin, idxmax`` operations are support on frames
+
+.. ipython:: python
+
     df = DataFrame(dict(A = s - Timestamp('20120101')-timedelta(minutes=5,seconds=5),
                         B = s - Series(date_range('2012-1-2', periods=3, freq='D'))))
     df
 
-    # timedelta arithmetic
-    td - timedelta(minutes=5,seconds=5,microseconds=5)
 
-    # min/max operations
     df.min()
     df.min(axis=1)
+
+    df.idxmin()
+    df.idxmax()
+
+``min, max`` operations are support on series, these return a single element ``timedelta64[ns]`` Series (this avoids
+having to deal with numpy timedelta64 issues). ``idxmin, idxmax`` are supported as well.
+
+.. ipython:: python
+
+    df.min().max()
+    df.min(axis=1).min()
+
+    df.min().idxmax()
+    df.min(axis=1).idxmin()

doc/source/v0.11.0.txt

@@ -258,8 +258,6 @@ Bug Fixes
         df = DataFrame(dict(A = s, B = td))
         df
         s - s.max()
-        s - datetime(2011,1,1,3,5)
-        s + timedelta(minutes=5)
         df['C'] = df['A'] + df['B']
         df
         df.dtypes
@@ -274,10 +272,11 @@ Bug Fixes
 
         # works on lhs too
    	s.max() - s
-        datetime(2011,1,1,3,5) - s
-        timedelta(minutes=5) + s
 
-  - Fix pretty-printing of infinite data structures, GH2978
+        # some timedelta numeric operations are supported
+        td - timedelta(minutes=5,seconds=5,microseconds=5)
+
+  - Fix pretty-printing of infinite data structures (closes GH2978_)
 
 
 See the `full release notes

pandas/core/common.py

@@ -955,10 +955,10 @@ def _possibly_cast_to_timedelta(value, coerce=True):
 def _possibly_cast_to_datetime(value, dtype, coerce = False):
     """ try to cast the array/value to a datetimelike dtype, converting float nan to iNaT """
 
-    if isinstance(dtype, basestring):
-        dtype = np.dtype(dtype)
-
     if dtype is not None:
+        if isinstance(dtype, basestring):
+            dtype = np.dtype(dtype)
+
         is_datetime64  = is_datetime64_dtype(dtype)
         is_timedelta64 = is_timedelta64_dtype(dtype)
 
@@ -984,21 +984,28 @@ def _possibly_cast_to_datetime(value, dtype, coerce = False):
                     except:
                         pass
 
-    elif dtype is None:
-        # we might have a array (or single object) that is datetime like, and no dtype is passed
-        # don't change the value unless we find a datetime set
-        v = value
-        if not is_list_like(v):
-            v = [ v ]
-        if len(v):
-            inferred_type = lib.infer_dtype(v)
-            if inferred_type == 'datetime':
-                try:
-                    value = tslib.array_to_datetime(np.array(v))
-                except:
-                    pass
-            elif inferred_type == 'timedelta':
-                value = _possibly_cast_to_timedelta(value)
+    else:
+
+        # only do this if we have an array and the dtype of the array is not setup already
+        # we are not an integer/object, so don't bother with this conversion
+        if isinstance(value, np.ndarray) and not (issubclass(value.dtype.type, np.integer) or value.dtype == np.object_):
+            pass
+
+        else:
+            # we might have a array (or single object) that is datetime like, and no dtype is passed
+            # don't change the value unless we find a datetime set
+            v = value
+            if not is_list_like(v):
+                v = [ v ]
+            if len(v):
+                inferred_type = lib.infer_dtype(v)
+                if inferred_type == 'datetime':
+                    try:
+                        value = tslib.array_to_datetime(np.array(v))
+                    except:
+                        pass
+                elif inferred_type == 'timedelta':
+                    value = _possibly_cast_to_timedelta(value)
 
     return value
 

pandas/core/nanops.py

@@ -146,7 +146,13 @@ def _wrap_results(result,dtype):
             result = result.view(dtype)
     elif issubclass(dtype.type, np.timedelta64):
         if not isinstance(result, np.ndarray):
-            pass
+
+            # this is a scalar timedelta result!
+            # we have series convert then take the element (scalar)
+            # as series will do the right thing in py3 (and deal with numpy 1.6.2
+            # bug in that it results dtype of timedelta64[us]
+            from pandas import Series
+            result = Series([result],dtype='timedelta64[ns]')
         else:
             result = result.view(dtype)
 

pandas/tests/test_series.py

@@ -1838,6 +1838,15 @@ def test_timedelta64_functions(self):
         result = (s1-s2).abs()
         assert_series_equal(result,expected)
 
+        # max/min
+        result = td.max()
+        expected = Series([timedelta(2)],dtype='timedelta64[ns]')
+        assert_series_equal(result,expected)
+
+        result = td.min()
+        expected = Series([timedelta(1)],dtype='timedelta64[ns]')
+        assert_series_equal(result,expected)
+
     def test_sub_of_datetime_from_TimeSeries(self): 
         from pandas.core import common as com
         from datetime import datetime