根据前一行值对 Pandas Dataframe 进行排序

Question

I have a dataframe that looks like:

Name      Previous Name
Alice     NaN
Charlie   Bob
Bob       Alice
Fred      Eddy
Danny     Charlie
Eddy      Dan

I would like to sort the dataframe so that is looks like:

Name      Previous Name
Alice     NaN
Bob      Alice
Charlie  Bob
Danny    Charlie
Eddy     Danny
Fred     Eddy

I know the boolean test involves something like

dataframe['Value'] = dataframe['PreviousValue'].shift(1)

But how can I use that as a sort criteria?

EDIT: Changed example from letters to names

Answer 1

Sort of values, then shift:

df = df.sort_values('Value')
df['Previous Value'] = df['Value'].shift()

Output:

  Value Previous Value
0     A            NaN
2     B              A
1     C              B
4     D              C
5     E              D
3     F              E