为给定列中的每个唯一元素重复 DataFrame 的元素

Question

My question is quite straightforward and there is probably a really simple way to solve but has been taking too much of my patience when trying to find a solution.

I have the following data that I made up just to illustrate:

x1 = ['a','b','c']
x2 = [1,2,3,4]
x3 = ['y1','y2','y3','y4']

What I want to do is for each unique element of the first column, I want to repeat the rest of that dataframe for that specific unique value. Hence, obtaining the following:

   0  1   2
0  a  1  y1
1  a  2  y2
2  a  3  y3
3  a  4  y4
4  b  1  y1
5  b  2  y2
6  b  3  y3
7  b  4  y4
8  c  1  y1
9  c  2  y2
10 c  3  y3
11 c  4  y4

Any ideas how to overcome that?

Answer 1

Use itertools.product with zipped columns:

from itertools import product

df = pd.DataFrame([(a,b,c) for a, (b, c) in product(x1, zip(x2, x3))])
print (df)
    0  1   2
0   a  1  y1
1   a  2  y2
2   a  3  y3
3   a  4  y4
4   b  1  y1
5   b  2  y2
6   b  3  y3
7   b  4  y4
8   c  1  y1
9   c  2  y2
10  c  3  y3
11  c  4  y4

If input are DataFrames use cross join :

df1 = pd.DataFrame({'a':x1})
df2 = pd.DataFrame({'b':x2, 'c':x3})

df = df1.assign(val=1).merge(df2.assign(val=1), on='val').drop('val', axis=1)
print (df)
    a  b   c
0   a  1  y1
1   a  2  y2
2   a  3  y3
3   a  4  y4
4   b  1  y1
5   b  2  y2
6   b  3  y3
7   b  4  y4
8   c  1  y1
9   c  2  y2
10  c  3  y3
11  c  4  y4