How can I “unpivot” specific columns from a pandas DataFrame?

This can be done with pd.melt():

# value_name is 'value' by default, but setting it here to make it clear
pd.melt(x, id_vars=['farm', 'fruit'], var_name="year", value_name="value")

Result:

  farm  fruit  year  value
0    A  apple  2014     10
1    B  apple  2014     12
2    A   pear  2014      6
3    B   pear  2014      8
4    A  apple  2015     11
5    B  apple  2015     13
6    A   pear  2015      7
7    B   pear  2015      9

[8 rows x 4 columns]

I’m not sure how common “melt” is as the name for this kind of operation, but that’s what it’s called in R’s reshape2 package, which probably inspired the name here.