pandas - how to get last n groups of a groupby object and combine them as a dataframe

Question

How to get last 'n' groups after df.groupby() and combine them as a dataframe.

data = pd.read_sql_query(sql=sqlstr, con=sql_conn, index_col='SampleTime')
grouped = data.groupby(data.index.date,sort=False)

After doing grouped.ngroups i am getting total number of groups 277. I want to combine last 12 groups and generate a dataframe.

Grandma

Answer 1

Pandas GroupBy objects are iterables. To extract the last n elements of an iterable, there's generally no need to create a list from the iterable and slice the last n elements. This will be memory-expensive.

Instead, you can use either itertools.islice (as suggested by @mtraceur) or collections.deque. Both work in O(n) time.

itertools.islice

Unlike a generator, a Pandas GroupBy object is an iterable which can be reused. Therefore, you can calculate the number of groups via len(g) for a GroupBy object g and then slice g via islice. Or, perhaps more idiomatic, you can use GroupBy.ngroups. Then use pd.concat to concatenate an iterable of dataframes:

from operator import itemgetter

g = data.groupby(data.index.date, sort=False)
res = pd.concat(islice(map(itemgetter(1), g), max(0, g.ngroups-12), None))

collections.deque

Alternatively, you can use collections.deque and specify maxlen, then concatenate as before.

from collections import deque

grouped = data.groupby(data.index.date, sort=False)
res = pd.concat(deque(map(itemgetter(1), grouped), maxlen=12))

As described in the collections docs:

Once a bounded length deque is full, when new items are added, a corresponding number of items are discarded from the opposite end.... They are also useful for tracking transactions and other pools of data where only the most recent activity is of interest.

Fidelis

in your example where can it be seen that you recover the last 5? – Liverpudlian

Great catch using deques, but you are still iterating over all groups. So the advantage is to save memory in this case, am I right? Good catch anyway – Edinburgh

@RafaelC, Yes, the advantage is principally lower memory usage. You can't avoid iterating over all groups. – Fidelis

@Yuca, maxlen=12 here. – Fidelis

I'm curious, did you also consider/evaluate doing itertools.islice(map(...), len(grouped) - 12, len(grouped) instead of a collections.deque(map(...), maxlen=12)? My intuition is to prefer islice in this case, but maybe I'm missing something? – Intramundane

@mtraceur, I've gone ahead and added the islice solution (but using ngroups), hope that's ok. – Fidelis

Answer 2

Assuming you know the order of grouped

grouped = zip(*df.groupby(data.index.date,sort=False))
pd.concat(list(grouped)[1][-12:])

Edinburgh

Answer 3

use pd.concat on lists comprehension and groupby.get_group

pd.concat([grouped.get_group(x) for x in list(grouped.groups.keys())[-12:]])

Liverpudlian

I just tried this on a dataframe I was working on, and this seemed to be what OP asked for? ed: the concat didn't work, but .tail(12) returned the final 12 groups – Innate

new version should be aligned to what OP wants :) (although it doesn't provide much vs rahlf23's version) – Liverpudlian

@MathewSavage that is really interesting, doc says it return the last rows from each group, not the last groups. Maybe I should test it too – Liverpudlian

Answer 4

You could pass a list comprehension to pd.concat():

import pandas as pd

df = pd.DataFrame([
['A',1,2],
['A',7,6],
['B',1,3],
['B',9,9],
['C',1,8],
['A',4,3],
['C',7,6],
['D',4,2]],
columns=['Var','Val1','Val2'])

last_n = 2
grouped = df.groupby('Var')

pd.concat([grouped.get_group(group) for i, group in enumerate(grouped.groups) if i>=len(grouped)-last_n])

Yields:

  Var  Val1  Val2
4   C     1     8
6   C     7     6
7   D     4     2

Officer

Answer 5

Get last n keys and filter dataframe with those keys

n = 12
grouped = data.groupby(data.index.date)
keys = list(grouped.groups.keys())
last_n_groups = data.loc[data[data.index.date].isin(keys[-n:])]

Lowgrade