Joining pandas dataframes by column names

you need to make county_ID as index for the right frame:

frame_2.join ( frame_1.set_index( [ 'county_ID' ], verify_integrity=True ),
               on=[ 'countyid' ], how='left' )

for your information, in pandas left join breaks when the right frame has non unique values on the joining column. see this bug.

so you need to verify integrity before joining by , verify_integrity=True

You can use the left_on and right_on options of pd.merge as follows:

pd.merge(frame_1, frame_2, left_on='county_ID', right_on='countyid')

Or equivalently with DataFrame.merge:

frame_1.merge(frame_2, left_on='county_ID', right_on='countyid')

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I was not sure from the question if you only wanted to merge if the key was in the left hand DataFrame. If that is the case then the following will do that (the above will in effect do a many to many merge)

pd.merge(frame_1, frame_2, how='left', left_on='county_ID', right_on='countyid')

Or

frame_1.merge(frame_2, how='left', left_on='county_ID', right_on='countyid')