Duplicate rows when merging dataframes with repetitions

Question

Say I have the following dataframes:

>>> df1 = pd.DataFrame({'fruit':['apple','orange','orange'],'taste':['sweet','sweet','sour']})
>>> df1
    fruit  taste
0   apple  sweet
1  orange  sweet
2  orange   sour
>>> df2 = pd.DataFrame({'fruit':['apple','orange','orange'],'price':['high','low','low']})
>>> df2
    fruit price
0   apple  high
1  orange   low
2  orange   low

When I do df3=df1.merge(df2,on='fruit'), I got the following result:

    fruit  taste price
0   apple  sweet  high
1  orange  sweet   low
2  orange  sweet   low
3  orange   sour   low
4  orange   sour   low

Here it looks like 2 duplicate rows were created; instead, I would expect something like

    fruit  taste price
0   apple  sweet  high
1  orange  sweet   low
3  orange   sour   low

How should I understand this behavior and how to obtain the result I was looking for?

Answer 1

you should remove duplicates rows before merge:

df3 = df1.merge(df2.drop_duplicates(), on='fruit')

    fruit  taste price
0   apple  sweet  high
1  orange  sweet   low
3  orange   sour   low

Answer 2

if you want to merge row1,row2,row3 from df1 with row1,row2,row3 from df2 then the code below works.

import pandas as pd

df1 = pd.DataFrame({'fruit':['apple','orange','orange'],'taste':['sweet','sweet','sour']})
df2 = pd.DataFrame({'fruit':['apple','orange','orange'],'price':['high','low','low']})

df3=df1.copy()
df3["price"]=None
df3.update(df2, join="left")

print(df3)

the reason why you get duplicated rows usingdf3=df1.merge(df2,on='fruit') is because merge uses df1 cross join df2 (aka df1 X df2). More information on that if you research sql cross join.