Merging two pandas dataframes many-to-one

Question

How do I merge the following datasets:

df = A
date abc
1    a
1    b
1    c
2    d
2    dd
3    ee
3    df

df = B
date ZZZ
1    a
2    b
3    c

I want to get smth like this:

date abc  ZZZ
1    a     a
1    b     a
1    c     a
2    d     b
2    dd    b
3    ee    c
3    df    c

I tried this code:

aa = pd.merge(A, B, left_on="date", right_on="date", how="left", validate="m:1")

But I have the following mistake:

TypeError: merge() got an unexpected keyword argument 'validate'

I update my pandas using (conda update pandas), but still get the same error

Please, advise me this issue.

Answer 1

According to df.merge docs validate was added in version 0.21.0. You are using an older version so you should update the version of pandas you are using.

Answer 2

As @DeepSpace mentioned, you may need to upgrade your pandas.

To replicate the check in earlier versions, you can do something like this:

import pandas as pd

df1 = pd.DataFrame(index=['a', 'a', 'b', 'b', 'c'])
df2 = pd.DataFrame(index=['a', 'b', 'c'])

x = [i for i in df2.index if i in set(df1.index)]
len(x) == len(set(x))  # True


df1 = pd.DataFrame(index=['a', 'a', 'b', 'b', 'c'])
df2 = pd.DataFrame(index=['a', 'b', 'c', 'a'])

y = [i for i in df2.index if i in set(df1.index)]
len(y) == len(set(y))  # False