Confusing error message on frame.__set_item__ with duplicate index entries #7432
Description
Struggled with this for a while to work out why I was getting TypeError: incompatible index of inserted column with frame index when doing df['new_col'] = my_series.
Turns out the the series had duplicates in its index. This occured because I was importing a CSV and then setting the index. Perhaps we need a warning when setting an index with duplicate values?
In [2]: df = pd.DataFrame({'foo':['a', 'b', 'c'], 'bar':[1,2,3], 'baz':['d','e','f']}).set_index('foo')
In [3]: df
Out[3]:
bar baz
foo
a 1 d
b 2 e
c 3 f
[3 rows x 2 columns]
In [5]: ser = pd.DataFrame({'foo':['a', 'b', 'c', 'a'], 'fiz':['g','h','i','j']}).set_index('foo')
In [6]: ser
Out[6]:
fiz
foo
a g
b h
c i
a j
[4 rows x 1 columns]
In [8]: df['newcol'] = ser
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-8-a8d535085de2> in <module>()
----> 1 df['newcol'] = ser
C:\WinPython-64bit-2.7.5.2\python-2.7.5.amd64\lib\site-packages\pandas-0.13.1-py2.7-win-amd64.egg\pandas\core\frame.pyc in __setitem__(self, key, value)
1885 else:
1886 # set column
-> 1887 self._set_item(key, value)
1888
1889 def _setitem_slice(self, key, value):
C:\WinPython-64bit-2.7.5.2\python-2.7.5.amd64\lib\site-packages\pandas-0.13.1-py2.7-win-amd64.egg\pandas\core\frame.pyc in _set_item(self, key, value)
1965 is_existing = key in self.columns
1966 self._ensure_valid_index(value)
-> 1967 value = self._sanitize_column(key, value)
1968 NDFrame._set_item(self, key, value)
1969
C:\WinPython-64bit-2.7.5.2\python-2.7.5.amd64\lib\site-packages\pandas-0.13.1-py2.7-win-amd64.egg\pandas\core\frame.pyc in _sanitize_column(self, key, value)
2008 value = value.reindex(self.index).values
2009 except:
-> 2010 raise TypeError('incompatible index of inserted column '
2011 'with frame index')
2012
TypeError: incompatible index of inserted column with frame index