Closed
Description
# This code is failing
df = pd.DataFrame({'timestamp': [np.datetime64('2017-01-01 01:00:00'), np.datetime64('2017-01-01 02:00:00')],
'value': ['a', 'b']})
df.loc[:, 'day'] = df.loc[:, 'timestamp'].values.astype('datetime64[D]')Error
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-91-7e5c52b6ed28> in <module>()
----> 1 df.loc[:, 'day'] = df.loc[:, 'timestamp'].values.astype('datetime64[D]')
/home/homola/env/venv/local/lib/python2.7/site-packages/pandas/core/indexing.pyc in __setitem__(self, key, value)
188 key = com.apply_if_callable(key, self.obj)
189 indexer = self._get_setitem_indexer(key)
--> 190 self._setitem_with_indexer(indexer, value)
191
192 def _validate_key(self, key, axis):
/home/homola/env/venv/local/lib/python2.7/site-packages/pandas/core/indexing.pyc in _setitem_with_indexer(self, indexer, value)
360
361 # add a new item with the dtype setup
--> 362 self.obj[key] = _infer_fill_value(value)
363
364 new_indexer = convert_from_missing_indexer_tuple(
/home/homola/env/venv/local/lib/python2.7/site-packages/pandas/core/frame.pyc in __setitem__(self, key, value)
3368 else:
3369 # set column
-> 3370 self._set_item(key, value)
3371
3372 def _setitem_slice(self, key, value):
/home/homola/env/venv/local/lib/python2.7/site-packages/pandas/core/frame.pyc in _set_item(self, key, value)
3443
3444 self._ensure_valid_index(value)
-> 3445 value = self._sanitize_column(key, value)
3446 NDFrame._set_item(self, key, value)
3447
/home/homola/env/venv/local/lib/python2.7/site-packages/pandas/core/frame.pyc in _sanitize_column(self, key, value, broadcast)
3652 # upcast
3653 value = cast_scalar_to_array(len(self.index), value)
-> 3654 value = maybe_cast_to_datetime(value, infer_dtype)
3655
3656 # return internal types directly
/home/homola/env/venv/local/lib/python2.7/site-packages/pandas/core/dtypes/cast.pyc in maybe_cast_to_datetime(value, dtype, errors)
979 else:
980 raise TypeError("cannot convert datetimelike to "
--> 981 "dtype [{dtype}]".format(dtype=dtype))
982 elif is_datetime64tz:
983
TypeError: cannot convert datetimelike to dtype [datetime64[D]]
# This code is ok
df = pd.DataFrame({'timestamp': [np.datetime64('2017-01-01 01:00:00'), np.datetime64('2017-01-01 02:00:00')],
'value': ['a', 'b']})
df['day'] = df.loc[:, 'timestamp'].values.astype('datetime64[D]')Problem description
When assigning to a column and using pandas.DataFrame.loc, it fails due to 'TypeError: cannot convert datetimelike to dtype [datetime64[D]]'. When using simple assignment, it doesn't fail.
If it is a bad practice what is done above in the first example, the error message should be different.
Expected Outcome
Assignment is successful for both below.
df.loc[:, 'day'] = df.loc[:, 'timestamp'].values.astype('datetime64[D]')
df['day'] = df.loc[:, 'timestamp'].values.astype('datetime64[D]')Output of pd.show_versions()
INSTALLED VERSIONS
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-51-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: en_US.utf8
LANG: en_US.UTF-8
LOCALE: None.None
pandas: 0.24.2
numpy: 1.16.4
...