在Pandas数据框中对分类数据进行排序时获得不必要的顺序

Question

When sorting columns in a pandas dataframe that contain text (and thus have datatype 'object'), the df.sort syntax works, and sorts apple, orange, banana in the correct order. However if I convert the fruit column to Categorical data type then try and sort it doesn't work.

I want to sort first by a datetime column, and then by a Categorical column, then by some numerical ones (float/int).

Code (where account is not categorical) sorts by month_date which is datetime object and account (AZ) correctly:

#data['month_name'] = pd.Categorical(data['month_name'],
#           categories=data.month_name.unique().tolist())
#data['account'] = pd.Categorical(data['account'],
#           categories=data.account.unique().tolist())

column_list = data.columns.values.tolist()
sorted_data = data.sort(["month_date","account"], ascending=True)
display(sorted_data)

Example:

• Apple
• Banana
• Carrot

Code (where account is Categorical) does not sort correctly (note pd.categorical data no longer commented out):

data['month_name'] = pd.Categorical(data['month_name'],
    categories=data.month_name.unique().tolist())
data['account'] = pd.Categorical(data['account'],
    categories=data.account.unique().tolist())
column_list = data.columns.values.tolist()
sorted_data = data.sort(["month_date","account"], ascending=True)
display(sorted_data)

Example

• Apple
• Carrot
• Banana

Answer 1

Your categories are themselves not in a guaranteed order. unique does not guarantee any order. They will be in the order listed (not clear what the values they have in your example)

In [7]: df = DataFrame({'A' : pd.Categorical(list('bbeebbaa'),categories=['e','a','b']), 'B' : np.arange(8) })

In [8]: df
Out[8]: 
   A  B
0  b  0
1  b  1
2  e  2
3  e  3
4  b  4
5  b  5
6  a  6
7  a  7

In [9]: df.dtypes
Out[9]: 
A    category
B       int64
dtype: object

In [10]: df.sort(['A','B'])
Out[10]: 
   A  B
2  e  2
3  e  3
6  a  6
7  a  7
0  b  0
1  b  1
4  b  4
5  b  5

In [11]: df.sort(['A','B'],ascending=False)
Out[11]: 
   A  B
5  b  5
4  b  4
1  b  1
0  b  0
7  a  7
6  a  6
3  e  3
2  e  2