PERF: Improve perf of to_datetime with ISO format

Author: chris-b1

doc/source/whatsnew/v0.17.0.txt

@@ -328,6 +328,7 @@ Performance Improvements
 - Significantly improved performance of indexing ``MultiIndex`` with slicers (:issue:`10287`)
 - Improved performance of ``Series.isin`` for datetimelike/integer Series (:issue:`10287`)
 - 20x improvement in ``concat`` of Categoricals when categories are identical (:issue:`10587`)
+- Improved performance of ``to_datetime`` when specified format string is ISO8601 (:issue:`10178`)
 
 
 .. _whatsnew_0170.bug_fixes:

pandas/tseries/tests/test_timeseries.py

@@ -919,8 +919,8 @@ def test_to_datetime_with_apply(self):
         assert_series_equal(result, expected)
 
         td = pd.Series(['May 04', 'Jun 02', ''], index=[1,2,3])
-        self.assertRaises(ValueError, lambda : pd.to_datetime(td,format='%b %y'))
-        self.assertRaises(ValueError, lambda : td.apply(pd.to_datetime, format='%b %y'))
+        self.assertRaises(ValueError, lambda : pd.to_datetime(td,format='%b %y', errors='raise'))
+        self.assertRaises(ValueError, lambda : td.apply(pd.to_datetime, format='%b %y', errors='raise'))
         expected = pd.to_datetime(td, format='%b %y', coerce=True)
 
         result = td.apply(lambda x: pd.to_datetime(x, format='%b %y', coerce=True))
@@ -4197,6 +4197,20 @@ def test_to_datetime_format_YYYYMMDD(self):
         expected = Series(['20121231','20141231','NaT'],dtype='M8[ns]')
         assert_series_equal(result, expected)
 
+    # GH 10178
+    def test_to_datetime_format_integer(self):
+        s = Series([2000, 2001, 2002])
+        expected = Series([ Timestamp(x) for x in s.apply(str) ])
+
+        result = to_datetime(s,format='%Y')
+        assert_series_equal(result, expected)
+
+        s = Series([200001, 200105, 200206])
+        expected = Series([ Timestamp(x[:4] + '-' + x[4:]) for x in s.apply(str) ])
+
+        result = to_datetime(s,format='%Y%m')
+        assert_series_equal(result, expected)
+
     def test_to_datetime_format_microsecond(self):
         val = '01-Apr-2011 00:00:01.978'
         format = '%d-%b-%Y %H:%M:%S.%f'
@@ -4524,9 +4538,9 @@ def test_day_not_in_month_coerce_false_raise(self):
 
     def test_day_not_in_month_coerce_false_ignore(self):
         self.assertEqual(to_datetime('2015-02-29', errors='ignore', coerce=False), '2015-02-29')
-        self.assertRaises(ValueError, to_datetime, '2015-02-29', errors='ignore', format="%Y-%m-%d", coerce=False)
-        self.assertRaises(ValueError, to_datetime, '2015-02-32', errors='ignore', format="%Y-%m-%d", coerce=False)
-        self.assertRaises(ValueError, to_datetime, '2015-04-31', errors='ignore', format="%Y-%m-%d", coerce=False)
+        self.assertEqual(to_datetime('2015-02-29', errors='ignore', format="%Y-%m-%d", coerce=False), '2015-02-29')
+        self.assertEqual(to_datetime('2015-02-32', errors='ignore', format="%Y-%m-%d", coerce=False), '2015-02-32')
+        self.assertEqual(to_datetime('2015-04-31', errors='ignore', format="%Y-%m-%d", coerce=False), '2015-04-31')
 
 if __name__ == '__main__':
     nose.runmodule(argv=[__file__, '-vvs', '-x', '--pdb', '--pdb-failure'],

pandas/tseries/tools.py

@@ -296,21 +296,24 @@ def _convert_listlike(arg, box, format):
             return result
 
         arg = com._ensure_object(arg)
+        require_iso8601 = False
 
         if infer_datetime_format and format is None:
             format = _guess_datetime_format_for_array(arg, dayfirst=dayfirst)
 
-            if format is not None:
-                # There is a special fast-path for iso8601 formatted
-                # datetime strings, so in those cases don't use the inferred
-                # format because this path makes process slower in this
-                # special case
-                format_is_iso8601 = (
-                    '%Y-%m-%dT%H:%M:%S.%f'.startswith(format) or
-                    '%Y-%m-%d %H:%M:%S.%f'.startswith(format)
-                )
-                if format_is_iso8601:
-                    format = None
+        if format is not None:
+            # There is a special fast-path for iso8601 formatted
+            # datetime strings, so in those cases don't use the inferred
+            # format because this path makes process slower in this
+            # special case
+            format_is_iso8601 = (
+                ('%Y-%m-%dT%H:%M:%S.%f'.startswith(format) or
+                '%Y-%m-%d %H:%M:%S.%f'.startswith(format)) and
+				format != '%Y'
+            )
+            if format_is_iso8601:
+                require_iso8601 = not infer_datetime_format
+                format = None
 
         try:
             result = None
@@ -334,16 +337,20 @@ def _convert_listlike(arg, box, format):
                             raise
                         result = arg
                     except ValueError:
-                        # Only raise this error if the user provided the
-                        # datetime format, and not when it was inferred
+                        # if format was inferred, try falling back
+                        # to array_to_datetime - terminate here
+                        # for specified formats
                         if not infer_datetime_format:
-                            raise
+                            if errors == 'raise':
+                                raise
+                            result = arg
 
             if result is None and (format is None or infer_datetime_format):
-                result = tslib.array_to_datetime(arg, raise_=errors == 'raise',
+                result = tslib.array_to_datetime(arg, raise_=errors=='raise',
                                                  utc=utc, dayfirst=dayfirst,
                                                  yearfirst=yearfirst, freq=freq,
-                                                 coerce=coerce, unit=unit)
+                                                 coerce=coerce, unit=unit,
+                                                 require_iso8601=require_iso8601)
 
             if com.is_datetime64_dtype(result) and box:
                 result = DatetimeIndex(result, tz='utc' if utc else None)

pandas/tslib.pyx

@@ -1808,7 +1808,8 @@ cpdef object _get_rule_month(object source, object default='DEC'):
 
 cpdef array_to_datetime(ndarray[object] values, raise_=False,
                         dayfirst=False, yearfirst=False, freq=None,
-                        format=None, utc=None, coerce=False, unit=None):
+                        format=None, utc=None, coerce=False, unit=None,
+                        require_iso8601=False):
     cdef:
         Py_ssize_t i, n = len(values)
         object val, py_dt
@@ -1908,6 +1909,17 @@ cpdef array_to_datetime(ndarray[object] values, raise_=False,
                     iresult[i] = value
                     _check_dts_bounds(&dts)
                 except ValueError:
+                    # if requiring iso8601 strings, skip trying other formats
+                    if require_iso8601:
+                        if coerce:
+                            iresult[i] = iNaT
+                            continue
+                        elif raise_:
+                            raise ValueError("time data %r does match format specified" %
+                                             (val,))
+                        else:
+                            return values
+
                     try:
                         py_dt = parse_datetime_string(val, dayfirst=dayfirst,
                                                       yearfirst=yearfirst, freq=freq)
@@ -1971,7 +1983,7 @@ cpdef array_to_datetime(ndarray[object] values, raise_=False,
                     continue
                 try:
                     oresult[i] = parse_datetime_string(val, dayfirst=dayfirst,
-                                                       yearfirst=yearfirst, freq=freq)
+                                                    yearfirst=yearfirst, freq=freq)
                     _pydatetime_to_dts(oresult[i], &dts)
                     _check_dts_bounds(&dts)
                 except Exception:

vb_suite/timeseries.py

@@ -157,6 +157,10 @@ def date_range(start=None, end=None, periods=None, freq=None):
     Benchmark('to_datetime(strings)', setup,
               start_date=datetime(2012, 7, 11))
 
+timeseries_to_datetime_iso8601_format = \
+    Benchmark("to_datetime(strings, format='%Y-%m-%d %H:%M:%S')", setup,
+              start_date=datetime(2012, 7, 11))
+
 setup = common_setup + """
 rng = date_range('1/1/2000', periods=10000, freq='D')
 strings = Series(rng.year*10000+rng.month*100+rng.day,dtype=np.int64).apply(str)