BUG: Ensure 'coerce' actually coerces datatypes

Changes behavior of convert objects so that passing 'coerce' will
ensure that data of the correct type is returned, even if all
values are null-types (NaN or NaT).

closes #9589

Author: Kevin Sheppard

pandas/core/common.py

@@ -34,6 +34,8 @@ class SettingWithCopyError(ValueError):
 class SettingWithCopyWarning(Warning):
     pass
 
+class RedundantSettingWarning(Warning):
+    pass
 
 class AmbiguousIndexError(PandasError, KeyError):
     pass
@@ -1888,60 +1890,58 @@ def _possibly_convert_objects(values, convert_dates=True,
                               convert_timedeltas=True):
     """ if we have an object dtype, try to coerce dates and/or numbers """
 
+    # If 1 flag is coerce, ensure 2 others are False
+    conversions = (convert_dates, convert_numeric, convert_timedeltas)
+    if 'coerce' in conversions:
+        coerce_count = sum([c == 'coerce' for c in conversions])
+        if coerce_count > 1:
+            raise ValueError("'coerce' can be used at most once.")
+
+        false_count = sum([not c for c in conversions])
+        if false_count != 2:
+            import warnings
+            warnings.warn("Soft conversion flags ignored when using 'coerce'",
+                          RedundantSettingWarning)
+
     # if we have passed in a list or scalar
     if isinstance(values, (list, tuple)):
         values = np.array(values, dtype=np.object_)
     if not hasattr(values, 'dtype'):
         values = np.array([values], dtype=np.object_)
 
-    # convert dates
-    if convert_dates and values.dtype == np.object_:
-
-        # we take an aggressive stance and convert to datetime64[ns]
-        if convert_dates == 'coerce':
-            new_values = _possibly_cast_to_datetime(
-                values, 'M8[ns]', coerce=True)
+    # If not object, do not convert
+    if values.dtype != np.object_:
+        return values
 
-            # if we are all nans then leave me alone
-            if not isnull(new_values).all():
-                values = new_values
-
-        else:
-            values = lib.maybe_convert_objects(
-                values, convert_datetime=convert_dates)
+    # Immediate return if coerce
+    if convert_dates == 'coerce':
+        return _possibly_cast_to_datetime(values, 'M8[ns]', coerce=True)
+    if convert_timedeltas == 'coerce':
+        from pandas.tseries.timedeltas import to_timedelta
+        return np.asanyarray(to_timedelta(values, coerce=True))
+    if convert_numeric == 'coerce':
+        return lib.maybe_convert_numeric(values, set(), coerce_numeric=True)
 
+    # convert dates
+    if convert_dates:
+        values = lib.maybe_convert_objects(values,
+                                           convert_datetime=convert_dates)
     # convert timedeltas
     if convert_timedeltas and values.dtype == np.object_:
-
-        if convert_timedeltas == 'coerce':
-            from pandas.tseries.timedeltas import to_timedelta
-            values = to_timedelta(values, coerce=True)
-
+        # Only if previous failed
+        values = lib.maybe_convert_objects(values,
+                                           convert_timedelta=convert_timedeltas)
+    # convert to numeric
+    if convert_numeric and values.dtype == np.object_:
+        # Only if previous failed
+        try:
+            new_values = lib.maybe_convert_numeric(values, set(),
+                                                   coerce_numeric=True)
             # if we are all nans then leave me alone
             if not isnull(new_values).all():
                 values = new_values
-
-        else:
-            values = lib.maybe_convert_objects(
-                values, convert_timedelta=convert_timedeltas)
-
-    # convert to numeric
-    if values.dtype == np.object_:
-        if convert_numeric:
-            try:
-                new_values = lib.maybe_convert_numeric(
-                    values, set(), coerce_numeric=True)
-
-                # if we are all nans then leave me alone
-                if not isnull(new_values).all():
-                    values = new_values
-
-            except:
-                pass
-        else:
-
-            # soft-conversion
-            values = lib.maybe_convert_objects(values)
+        except:
+            pass
 
     return values
 

pandas/core/internals.py

@@ -1484,8 +1484,10 @@ def convert(self, convert_dates=True, convert_numeric=True, convert_timedeltas=T
         else:
 
             values = com._possibly_convert_objects(
-                self.values.ravel(), convert_dates=convert_dates,
-                convert_numeric=convert_numeric
+                self.values.ravel(),
+                convert_dates=convert_dates,
+                convert_numeric=convert_numeric,
+                convert_timedeltas=convert_timedeltas
             ).reshape(self.values.shape)
             blocks.append(make_block(values,
                                      ndim=self.ndim, placement=self.mgr_locs))

pandas/tests/test_series.py

@@ -8,14 +8,15 @@
 from inspect import getargspec
 from itertools import product, starmap
 from distutils.version import LooseVersion
+import warnings
 
 import nose
-
 from numpy import nan, inf
 import numpy as np
 import numpy.ma as ma
-import pandas as pd
+import pandas.lib as lib
 
+import pandas as pd
 from pandas import (Index, Series, DataFrame, isnull, notnull, bdate_range,
                     date_range, period_range, timedelta_range)
 from pandas.core.index import MultiIndex
@@ -25,11 +26,8 @@
 from pandas.tseries.tdi import Timedelta, TimedeltaIndex
 import pandas.core.common as com
 import pandas.core.config as cf
-import pandas.lib as lib
-
 import pandas.core.datetools as datetools
 import pandas.core.nanops as nanops
-
 from pandas.compat import StringIO, lrange, range, zip, u, OrderedDict, long
 from pandas import compat
 from pandas.util.testing import (assert_series_equal,
@@ -39,6 +37,7 @@
 import pandas.util.testing as tm
 
 
+
 #------------------------------------------------------------------------------
 # Series test cases
 
@@ -3432,7 +3431,6 @@ def test_ops_datetimelike_align(self):
 
     def test_timedelta64_functions(self):
 
-        from datetime import timedelta
         from pandas import date_range
 
         # index min/max
@@ -5820,6 +5818,24 @@ def test_apply_dont_convert_dtype(self):
         self.assertEqual(result.dtype, object)
 
     def test_convert_objects(self):
+        # Tests: All to nans, coerce, true
+        # Test coercion returns correct type
+        s = Series(['a', 'b', 'c'])
+        results = s.convert_objects('coerce', False, False)
+        expected = Series([lib.NaT] * 3)
+        assert_series_equal(results, expected)
+
+        results = s.convert_objects(False, 'coerce', False)
+        expected = Series([np.nan] * 3)
+        assert_series_equal(results, expected)
+
+        results = s.convert_objects(False, False, 'coerce')
+        expected = Series([lib.NaT] * 3, dtype=np.dtype('m8[ns]'))
+        assert_series_equal(results, expected)
+
+        with warnings.catch_warnings(record=True) as w:
+            tm.assert_produces_warning(s.convert_objects(True, True, 'coerce'),
+                                       com.RedundantSettingWarning)
 
         s = Series([1., 2, 3], index=['a', 'b', 'c'])
         result = s.convert_objects(convert_dates=False, convert_numeric=True)
@@ -5875,23 +5891,28 @@ def test_convert_objects(self):
             [Timestamp(
                 '20010101'), Timestamp('20010102'), Timestamp('20010103'),
                           lib.NaT, lib.NaT, lib.NaT, Timestamp('20010104'), Timestamp('20010105')], dtype='M8[ns]')
-        result = s2.convert_objects(
-            convert_dates='coerce', convert_numeric=False)
+        result = s2.convert_objects(convert_dates='coerce',
+                                    convert_numeric=False,
+                                    convert_timedeltas=False)
         assert_series_equal(result, expected)
-        result = s2.convert_objects(
-            convert_dates='coerce', convert_numeric=True)
+        result = s2.convert_objects(convert_dates='coerce',
+                                    convert_numeric=False,
+                                    convert_timedeltas=False)
         assert_series_equal(result, expected)
 
         # preserver all-nans (if convert_dates='coerce')
         s = Series(['foo', 'bar', 1, 1.0], dtype='O')
-        result = s.convert_objects(
-            convert_dates='coerce', convert_numeric=False)
-        assert_series_equal(result, s)
+        result = s.convert_objects(convert_dates='coerce',
+                                    convert_numeric=False,
+                                    convert_timedeltas=False)
+        expected = Series([lib.NaT]*4)
+        assert_series_equal(result, expected)
 
         # preserver if non-object
         s = Series([1], dtype='float32')
-        result = s.convert_objects(
-            convert_dates='coerce', convert_numeric=False)
+        result = s.convert_objects(convert_dates='coerce',
+                                   convert_numeric=False,
+                                   convert_timedeltas=False)
         assert_series_equal(result, s)
 
         #r = s.copy()
@@ -5900,13 +5921,14 @@ def test_convert_objects(self):
         #self.assertEqual(result.dtype, 'M8[ns]')
 
         # dateutil parses some single letters into today's value as a date
+        expected = Series([lib.NaT])
         for x in 'abcdefghijklmnopqrstuvwxyz':
             s = Series([x])
             result = s.convert_objects(convert_dates='coerce')
-            assert_series_equal(result, s)
+            assert_series_equal(result, expected)
             s = Series([x.upper()])
             result = s.convert_objects(convert_dates='coerce')
-            assert_series_equal(result, s)
+            assert_series_equal(result, expected)
 
     def test_convert_objects_preserve_bool(self):
         s = Series([1, True, 3, 5], dtype=object)