How to separate a comma delimited string into multiple lines in R and python

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The problem

df<- data.frame(id = c(1,2,3), value = c('x,y', 'z,w', 'a'))
df

#> id value
#> 1 1 x,y
#> 2 2 z,w
#> 3 3 a

we want to put x,y in the …