合并中的 Pandas 不返回正确数据集的索引

Question

Is it bug?

work (GOOD)

df.merge(label,left_on='ID',right_on='REQ_ID',how='inner',left_index=True).index

Int64Index([102315, 102316, 142966,  21285,  21283,   1062,  61823,  21274,
             82044,  21280,
            ...
            101105,  80927, 141854, 162123,  80937,  80878, 141842, 121531,
            162107, 162117],
           dtype='int64', length=72231)

work (GOOD)

label.merge(df,left_on='REQ_ID',right_on='ID',how='inner',left_index=True).index

Int64Index([109713, 109584, 109665, 110019, 109877, 110295, 110232, 110195,
            110249, 110397,
            ...
            996301, 788659, 996305, 995726, 995358, 994281, 927879, 980877,
            895347, 995278],
           dtype='int64', length=72231)

does not work (BAD)

df.merge(label,left_on='ID',right_on='REQ_ID',how='inner',right_index=True).index

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-120-ba8a8ccff1bd> in <module>
----> 1 df.merge(label,left_on='ID',right_on='REQ_ID',how='inner',right_index=True).index

~/anaconda3/lib/python3.8/site-packages/pandas/core/frame.py in merge(self, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy, indicator, validate)
   7948         from pandas.core.reshape.merge import merge
   7949 
-> 7950         return merge(
   7951             self,
   7952             right,

~/anaconda3/lib/python3.8/site-packages/pandas/core/reshape/merge.py in merge(left, right, how, on, left_on, right_on, left_index, right_index, sort, suffixes, copy, indicator, validate)
     72     validate=None,
     73 ) -> "DataFrame":
---> 74     op = _MergeOperation(
     75         left,
     76         right,

~/anaconda3/lib/python3.8/site-packages/pandas/core/reshape/merge.py in __init__(self, left, right, how, on, left_on, right_on, axis, left_index, right_index, sort, suffixes, copy, indicator, validate)
    654         # validate the merge keys dtypes. We may need to coerce
    655         # to avoid incompatible dtypes
--> 656         self._maybe_coerce_merge_keys()
    657 
    658         # If argument passed to validate,

~/anaconda3/lib/python3.8/site-packages/pandas/core/reshape/merge.py in _maybe_coerce_merge_keys(self)
   1163                     inferred_right in string_types and inferred_left not in string_types
   1164                 ):
-> 1165                     raise ValueError(msg)
   1166 
   1167             # datetimelikes must match exactly

ValueError: You are trying to merge on object and int64 columns. If you wish to proceed you should use pd.concat

df.dtypes

ID                          object

label.dtypes

REQ_ID     object

Answer 1

Please check documentation https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.merge.html : right_index - Use the index from the right DataFrame as the join key. Same caveats as left_index.

so you actually try to merge on index of your label dataframe, which is of int64