Given same value in one column, concatenate remaining rows?

Question

Given the pandas DataFrame:

name   hobby   since
paul   A       1995 
john   A       2005 
paul   B       2015
mary   G       2013
chris  E       2005
chris  D       2001
paul   C       1986

I would like to get:

name   hobby1   since1    hobby2   since2    hobby3   since3
paul   A        1995      B        2015      C        1986 
john   A        2005      NaN      NaN       NaN      NaN
mary   G        2013      NaN      NaN       NaN      NaN
chris  E        2005      D        2001      NaN      NaN

I.e. I would like to have one row per name. The maximum number of hobbies a person can have, say 3 in this case, is something I know in advance. What would be the most elegant/short way to do this?

Answer 1

You can first melt and then , groupby.cumcount() to add to the variable and then pivot using pivot_table():

m=df.melt('name')
(m.assign(variable=m.variable+(m.groupby(['name','variable']).cumcount()+1).astype(str))
 .pivot_table(index='name',columns='variable',values='value',aggfunc='first')
.rename_axis(None,axis=1))

---

      hobby1 hobby2 hobby3 since1 since2 since3
name                                           
chris      E      D    NaN   2005   2001    NaN
john       A    NaN    NaN   2005    NaN    NaN
mary       G    NaN    NaN   2013    NaN    NaN
paul       A      B      C   1995   2015   1986

Answer 2

Use cumcount and unstack. Finally, use multiindex.map to join 2-level columns to one level

df1 = df.set_index(['name', df.groupby('name').cumcount().add(1)]) \
        .unstack().sort_index(1,level=1)

df1.columns = df1.columns.map('{0[0]}{0[1]}'.format)


Out[812]:
      hobby1  since1 hobby2  since2 hobby3  since3
name
chris      E  2005.0      D  2001.0    NaN     NaN
john       A  2005.0    NaN     NaN    NaN     NaN
mary       G  2013.0    NaN     NaN    NaN     NaN
paul       A  1995.0      B  2015.0      C  1986.0