Pythonic way to put specified value at last position when sorting

Question

For series pd.Series(['d', 'b', 'a', 'b', 'c', 'a', 'd']) , how to sort it so that:

• a is the last item of the series
• all other items are ascending

So desired outcome will be: pd.Series(['b', 'b', 'c', 'd', 'd', 'a', 'a'])

Thanks

Answer 1

Sort in two steps:

s = pd.Series(['d', 'b', 'a', 'b', 'c', 'a', 'd'])

s.sort_values().pipe(lambda x: x.iloc[x.eq('a').values.argsort(kind='mergesort')])
# use mergesort to make sure the sorting is stable so the second sort doesn't change 
# the first sorting order when values are a and not a separately

#1    b
#3    b
#4    c
#0    d
#6    d
#2    a
#5    a
#dtype: object

---

Or create a dummpy data frame with an extra column indicating whether the values are equal to a , then sort by the two columns:

pd.concat([s.rename('s'), s.eq('a').rename('a')], axis=1).sort_values(['a', 's']).s

#1    b
#3    b
#4    c
#0    d
#6    d
#2    a
#5    a
#Name: s, dtype: object

Answer 2

您可以通过对不是'a'的值进行排序，然后附加所有'a'值来形成排序后的序列。

sorted_series = ser[ser != 'a'].append(ser[ser == 'a'])