按日期对 Pandas dataframe 列索引进行排序

Question

I want to sort dataframe by column index. The issue is my columns are 'dates' dd/mm/yyyy directly imported from my excel. For ex:

    10/08/20  12/08/20 11/08/20
0   2.0        6.0       15.0
1   6.0        11.0      8.0
2   4.0        7.0       3.0
3   7.0        12.0      2.0
4   12.0       5.0       7.0

The output I want is:

    10/08/20  11/08/20 12/08/20
0   2.0        15.0      6.0
1   6.0        8.0       11.0
2   4.0        3.0       7.0
3   7.0        2.0       12.0
4   12.0       7.0       5.0

I am using

df.sort_index(axis=1)

It is giving me following error:

TypeError: '<' not supported between instances of 'datetime.datetime' and 'str'

I want to do it in panda dataframe. Any help will be appreciated. Thanks

Answer 1

First remove the '.' at the end of date from the data shource sheet. the for this data

    10-08-2020  12-08-2020  11-08-2020
0   2           6           15
1   6           11          8
2   4           7           3
3   7           12          2
4   12          5           7

try this

import datetime as dt
df.columns=pd.Series(df.columns).apply(lambda d: dt.datetime(d, dt.datetime.strptime(d, '%d/%m/%Y')))
df.sort_index(axis = 1)

Answer 2

First:

df.columns = df.columns.str.replace(".", "")

Then:

df.sort_index(axis = 1)

Update : as Ch3steR mentioned in the comments. For removing "."

df.columns = df.columns.str.rstrip(".")

Using str.rstrip to generalize, as day.month.year is valid format, using str.replace would replace every .

Example:

s = pd.Series(["1.2.2020."])
pd.to_datetime(s.str.replace('.', ''))
# 0   2020-12-20         # Interpeted wrong
# dtype: datetime64[ns]

pd.to_datetime(s.str.rstrip('.'))
# 0   2020-01-02
# dtype: datetime64[ns]

Answer 3

Your error comes from the fact that you are mixing string types with date types . Either all your column names are strings or all are dates but you cannot have both mixed.

For example

l=[[2.0, 6.0, 15.0],
   [6.0, 11.0, 8.0],
   [4.0, 7.0, 3.0],
   [7.0, 12.0, 2.0],
   [12.0, 5.0, 7.0]]

d = pd.DataFrame(l, columns =['10/08/20',  '12/08/20', '11/08/20']) # column names are strings

yields

   10/08/20  12/08/20  11/08/20
0       2.0       6.0      15.0
1       6.0      11.0       8.0
2       4.0       7.0       3.0
3       7.0      12.0       2.0
4      12.0       5.0       7.0

Now if I want to sort by column names I type

d.sort_index(axis = 1)

   10/08/20  11/08/20  12/08/20
0       2.0      15.0       6.0
1       6.0       8.0      11.0
2       4.0       3.0       7.0
3       7.0       2.0      12.0
4      12.0       7.0       5.0

If on the other hand, column names were dates as in

from dateutil.parser import parse
d = pd.DataFrame(l, columns =[parse('10/08/20'),  parse('12/08/20'), parse('11/08/20')])

we will have

   2020-10-08  2020-12-08  2020-11-08   #now column names are dates
0         2.0         6.0        15.0
1         6.0        11.0         8.0
2         4.0         7.0         3.0
3         7.0        12.0         2.0
4        12.0         5.0         7.0

Again you can sort them using the same

details.sort_index(axis = 1)
  
   2020-10-08  2020-11-08  2020-12-08
0         2.0        15.0         6.0
1         6.0         8.0        11.0
2         4.0         3.0         7.0
3         7.0         2.0        12.0
4        12.0         7.0         5.0

and will give you no error.