How to keep index when using pandas merge

by 
In [5]: a.reset_index().merge(b, how="left").set_index('index')
Out[5]:
       col1  to_merge_on  col2
index
a         1            1     1
b         2            3     2
c         3            4   NaN

Note that for some left merge operations, you may end up with more rows than in a when there are multiple matches between a and b. In this case, you may need to drop duplicates.

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