Unpivot Pandas Data
I currently have a DataFrame laid out as:
Jan Feb Mar Apr ...
2001 1 12 12 19
2002 9 ...
2003 ...
and I would like to "unpivot" the data to look like:
Date Value
Jan 2001 1
Feb 2001 1
Mar 2001 12
...
Jan 2002 9
What is the best way to accomplish this using pandas/NumPy?
You just have to do df.unstack() and that will create a MultiIndexed Series with month as a first level and the year as the second level index. If you want them to be columns then just call reset_index() after that.
>>> df
Jan Feb
2001 3 4
2002 2 7
>>> df.unstack()
Jan 2001 3
2002 2
Feb 2001 4
2002 7
>>> df = df.unstack().reset_index(name='value')
>>> df
level_0 level_1 value
0 Jan 2001 3
1 Jan 2002 2
2 Feb 2001 4
3 Feb 2002 7
>>> df.rename(columns={'level_0': 'month', 'level_1': 'year'}, inplace=True)
>>> df
month year value
0 Jan 2001 3
1 Jan 2002 2
2 Feb 2001 4
3 Feb 2002 7
Another solution would be to use pandas.melt to avoid unnecessary creation of a MultiIndex, though this isn't that expensive if your frame is small and with my solution you still have to create a temporary for the "molten" data. The guts of melt suggest that both id_vars and value are copied since id_vars creation uses tile and value creation uses df.values.ravel('F') which I believe makes a copy if your data are not in Fortran order.
EDIT: I'm not exactly sure when a copy is made when ravel is called since the order parameter only indicates how you want your data read and the docstring says a copy is made only when needed.
In [99]: mons
Out[99]:
['Jan',
'Feb',
'Mar',
'Apr',
'May',
'Jun',
'Jul',
'Aug',
'Sep',
'Oct',
'Nov',
'Dec']
In [100]: df = DataFrame(randn(201, len(mons)), columns=mons, index=map(str, arange(1901, 2102)))
In [101]: df.head()
Out[101]:
Jan Feb Mar Apr May Jun Jul Aug Sep Oct \
1901 1.141 -0.270 0.329 0.214 -1.030 0.324 -1.448 2.003 -0.061 0.477
1902 0.136 0.151 0.447 -0.493 1.329 1.410 0.020 -0.705 0.870 0.478
1903 -0.000 0.689 1.768 -0.057 -1.471 0.515 -0.315 0.703 2.511 0.592
1904 1.199 1.246 -0.255 0.182 -0.454 -0.452 1.074 0.178 2.495 -0.543
1905 1.073 1.375 -1.837 1.048 -0.139 -0.273 -0.958 -1.164 -1.012 0.950
Nov Dec
1901 0.102 0.122
1902 2.941 0.654
1903 0.347 -1.636
1904 -0.047 0.457
1905 1.277 -0.284
In [102]: df.reset_index(inplace=True)
In [103]: df.head()
Out[103]:
index Jan Feb Mar Apr May Jun Jul Aug Sep Oct \
0 1901 1.141 -0.270 0.329 0.214 -1.030 0.324 -1.448 2.003 -0.061 0.477
1 1902 0.136 0.151 0.447 -0.493 1.329 1.410 0.020 -0.705 0.870 0.478
2 1903 -0.000 0.689 1.768 -0.057 -1.471 0.515 -0.315 0.703 2.511 0.592
3 1904 1.199 1.246 -0.255 0.182 -0.454 -0.452 1.074 0.178 2.495 -0.543
4 1905 1.073 1.375 -1.837 1.048 -0.139 -0.273 -0.958 -1.164 -1.012 0.950
Nov Dec
0 0.102 0.122
1 2.941 0.654
2 0.347 -1.636
3 -0.047 0.457
4 1.277 -0.284
In [104]: res = pd.melt(df, id_vars=['index'], var_name=['months'])
In [105]: res['date'] = res['months'] + ' ' + res['index']
In [106]: res.head()
Out[106]:
index months value date
0 1901 Jan 1.141 Jan 1901
1 1902 Jan 0.136 Jan 1902
2 1903 Jan -0.000 Jan 1903
3 1904 Jan 1.199 Jan 1904
4 1905 Jan 1.073 Jan 1905
df.reset_index().melt(...) should unpivot it (credit):
>>> df = pd.DataFrame({
... 'foo': ['one', 'one', 'one', 'two', 'two', 'two'],
... 'bar': ['A', 'B', 'C', 'A', 'B', 'C'],
... 'baz': [1, 2, 3, 4, 5, 6],
... })
>>> df
foo bar baz
0 one A 1
1 one B 2
2 one C 3
3 two A 4
4 two B 5
5 two C 6
>>> df_piv = df.pivot(index='foo', columns='bar', values='baz')
>>> df_piv
bar A B C
foo
one 1 2 3
two 4 5 6
>>> def_piv_unpiv = df_piv.reset_index().melt(id_vars='foo', var_name='bar', value_name='baz')
>>> def_piv_unpiv
foo bar baz
0 one A 1
1 two A 4
2 one B 2
3 two B 5
4 one C 3
5 two C 6
Edit: I later realized this is basically a more concise version of Phillip Cloud's answer.