How to keep index when using pandas merge中提出了一个类似的问题,但它不适用于 MultiIndexes,即
a = DataFrame(np.array([1,2,3,4,1,2,3,3]).reshape((4,2)), columns=['col1','to_merge_on'], index=['a','b','a','b'])
id = pd.MultiIndex.from_arrays([[1,1,2,2],['a','b','a','b']], names =['id1','id2'])
a.index = id
In [207]: a
Out[207]:
col1 to_merge_on
id1 id2
1 a 1 2
b 3 4
2 a 1 2
b 3 4
b=DataFrame(data={"col2": [1,2,3], 'to_merge_on' : [1,3,5]})
In [209]: b
Out[209]:
col2 to_merge_on
0 1 1
1 2 3
2 3 5
a.reset_index().merge(b, how="left").set_index('index')
In [208]: a.reset_index().merge(b, how="left").set_index('index')
------------------------------------------------------------
Traceback (most recent call last):
File "<ipython console>", line 1, in <module>
File "C:\Python27\lib\site-packages\pandas\core\frame.py", line 2054, in set_index
level = frame[col]
File "C:\Python27\lib\site-packages\pandas\core\frame.py", line 1458, in __getitem__
return self._get_item_cache(key)
File "C:\Python27\lib\site-packages\pandas\core\generic.py", line 294, in _get_item_cache
values = self._data.get(item)
File "C:\Python27\lib\site-packages\pandas\core\internals.py", line 625, in get
_, block = self._find_block(item)
File "C:\Python27\lib\site-packages\pandas\core\internals.py", line 715, in _find_block
self._check_have(item)
File "C:\Python27\lib\site-packages\pandas\core\internals.py", line 722, in _check_have
raise KeyError('no item named %s' % str(item))
KeyError: 'no item named index'
如何在左侧数据框中保留 MultiIndex 的同时进行合并?