Python - Combinig pandas dataframes

Question

I have 3 dataframes that I'd like to combine. They look like this:

df1       |df2        |df3
col1 col2 |col1 col2  |col1 col3
1    5     2    9      1    some
                       2    data

I'd like the first two df-s to be merged into the third df based on col1, so the desired output is

df3
col1 col3 col2
1    some 5
2    data 9

How can I achieve this? I'm trying:

df3['col2'] = df1[df1.col1 == df3.col1].col2 if df1[df1.col1 == df3.col1].col2 is not None else df2[df2.col1 == df3.col1].col2

For this I get ValueError: Series lengths must match to compare

It is guaranteed, that df3's col1 values are present either in df1 or df2. What's the way to do this? PLEASE NOTE, that a simple concat will not work, since there is other data in df3, not just col1.

IIUC you can do df3.merge(df1, on='col1', how='outer').merge(df2, on='col1', how='outer') — EdChum
– EdChum, Commented Mar 14, 2017 at 14:50

akuiper · Accepted Answer · 2017-03-14 14:53:17Z

1

If df1 and df2 don't have duplicates in col1, you can try this:

pd.concat([df1, df2]).merge(df3)

Data:

df1 = pd.DataFrame({'col1': [1], 'col2': [5]})
df2 = pd.DataFrame({'col1': [2], 'col2': [9]})
df3 = pd.DataFrame({'col1': [1,2], 'col3': ['some', 'data']})

answered Mar 14, 2017 at 14:53

akuiper

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