Filter Pandas Dataframe using list, but making sure count of elements matches count in list

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so I have a list

my_list = [1,1,2,3,4,4]

I have a dataframe that looks like this

col_1    col_2
a        1
b        1
c        2
d        3
e        3
f        4
g        4
h        4

I basically want a final dataframe like

col_1    col_2
a        1
b        1
c        2
d        3
f        4
g        4

Basically I cant use

my_df[my_df['col_2'].isin(my_list)]

since this will include all the rows. I want the first row that matches with each item on the list, but all the same count of rows.

pandas python-3.x python
1 Answers

Use GroupBy.cumcount for counter with original and helper DataFrame and filter by inner join in DataFrame.merge:

my_list = [1,1,2,3,4,4]

df1 = pd.DataFrame({'col_2':my_list})
df1['g'] = df1.groupby('col_2').cumcount()
my_df['g'] = my_df.groupby('col_2').cumcount()

df = my_df.merge(df1).drop('g', axis=1)

print (df)
  col_1  col_2
0     a      1
1     b      1
2     c      2
3     d      3
4     f      4
5     g      4
user_29010020
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