在 Pandas 中，在 groupby 之后分组列消失了

Question

I have the following dataframe named ttm:

    usersidid   clienthostid    eventSumTotal   LoginDaysSum    score
0       12          1               60              3           1728
1       11          1               240             3           1331
3       5           1               5               3           125
4       6           1               16              2           216
2       10          3               270             3           1000
5       8           3               18              2           512

When i do

ttm.groupby(['clienthostid'], as_index=False, sort=False)['LoginDaysSum'].count()

I get what I expected (though I would've wanted the results to be under a new label named 'ratio'):

       clienthostid  LoginDaysSum
0             1          4
1             3          2

But when I do

ttm.groupby(['clienthostid'], as_index=False, sort=False)['LoginDaysSum'].apply(lambda x: x.iloc[0] / x.iloc[1])

I get:

0    1.0
1    1.5

1. Why did the labels go? I still also need the grouped need the 'clienthostid' and I need also the results of the apply to be under a label too
2. Sometimes when I do groupby some of the other columns still appear, why is that that sometimes columns disappear and sometime stays? is there a flag I'm missing that do those stuff?
3. In the example that I gave, when I did count the results showed on label 'LoginDaysSum', is there a why to add a new label for the results instead?

Thank you,

Answer 1

For return DataFrame after groupby are 2 possible solutions:

1. parameter as_index=False what works nice with count , sum , mean functions

2. reset_index for create new column from levels of index , more general solution

df = ttm.groupby(['clienthostid'], as_index=False, sort=False)['LoginDaysSum'].count()
print (df)
   clienthostid  LoginDaysSum
0             1             4
1             3             2

df = ttm.groupby(['clienthostid'], sort=False)['LoginDaysSum'].count().reset_index()
print (df)
   clienthostid  LoginDaysSum
0             1             4
1             3             2

---

For second need remove as_index=False and instead add reset_index :

#output is `Series`
a = ttm.groupby(['clienthostid'], sort=False)['LoginDaysSum'] \
         .apply(lambda x: x.iloc[0] / x.iloc[1])
print (a)
clienthostid
1    1.0
3    1.5
Name: LoginDaysSum, dtype: float64

print (type(a))
<class 'pandas.core.series.Series'>

print (a.index)
Int64Index([1, 3], dtype='int64', name='clienthostid')


df1 = ttm.groupby(['clienthostid'], sort=False)['LoginDaysSum']
         .apply(lambda x: x.iloc[0] / x.iloc[1]).reset_index(name='ratio')
print (df1)
   clienthostid  ratio
0             1    1.0
1             3    1.5

Why some columns are gone?

I think there can be problem automatic exclusion of nuisance columns :

#convert column to str
ttm.usersidid = ttm.usersidid.astype(str) + 'aa'
print (ttm)
  usersidid  clienthostid  eventSumTotal  LoginDaysSum  score
0      12aa             1             60             3   1728
1      11aa             1            240             3   1331
3       5aa             1              5             3    125
4       6aa             1             16             2    216
2      10aa             3            270             3   1000
5       8aa             3             18             2    512

#removed str column userid
a = ttm.groupby(['clienthostid'], sort=False).sum()
print (a)
              eventSumTotal  LoginDaysSum  score
clienthostid                                    
1                       321            11   3400
3                       288             5   1512

What is the difference between size and count in pandas?

Answer 2

count is a built in method for the groupby object and pandas knows what to do with it. There are two other things specified that goes into determining what the out put looks like.

#                         For a built in method, when
#                         you don't want the group column
#                         as the index, pandas keeps it in
#                         as a column.
#                             |----||||----|
ttm.groupby(['clienthostid'], as_index=False, sort=False)['LoginDaysSum'].count()

   clienthostid  LoginDaysSum
0             1             4
1             3             2

---

#                         For a built in method, when
#                         you do want the group column
#                         as the index, then...
#                             |----||||---|
ttm.groupby(['clienthostid'], as_index=True, sort=False)['LoginDaysSum'].count()
#                                                       |-----||||-----|
#                                                 the single brackets tells
#                                                 pandas to operate on a series
#                                                 in this case, count the series

clienthostid
1    4
3    2
Name: LoginDaysSum, dtype: int64

---

ttm.groupby(['clienthostid'], as_index=True, sort=False)[['LoginDaysSum']].count()
#                                                       |------||||------|
#                                             the double brackets tells pandas
#                                                to operate on the dataframe
#                                              specified by these columns and will
#                                                return a dataframe

              LoginDaysSum
clienthostid              
1                        4
3                        2

---

When you used apply pandas no longer knows what to do with the group column when you say as_index=False . It has to trust that if you use apply you want returned exactly what you say to return, so it will just throw it away. Also, you have single brackets around your column which says to operate on a series. Instead, use as_index=True to keep the grouping column information in the index. Then follow it up with a reset_index to transfer it from the index back into the dataframe. At this point, it will not have mattered that you used single brackets because after the reset_index you'll have a dataframe again.

ttm.groupby(['clienthostid'], as_index=True, sort=False)['LoginDaysSum'].apply(lambda x: x.iloc[0] / x.iloc[1])

0    1.0
1    1.5
dtype: float64

---

ttm.groupby(['clienthostid'], as_index=True, sort=False)['LoginDaysSum'].apply(lambda x: x.iloc[0] / x.iloc[1]).reset_index()

   clienthostid  LoginDaysSum
0             1           1.0
1             3           1.5

Answer 3

Reading the groupy documentarion , a found out that automatic exclusion of columns after groupby usually caused by the presence of null values in that columns excluded.

Try fill the 'null' with some value.

Like this:

df.fillna('')

Answer 4

You simply need this instead:

ttm.groupby(['clienthostid'], as_index=False, sort=False)[['LoginDaysSum']].apply(lambda x: x.iloc[0] / x.iloc[1]).reset_index()

The double [[]] will turn the output into a pd.Dataframe instead of a pd.Series.