pandas groupby and aggregate into new column

Question

did some searching but nothing yields the desired result, which is grouping the data by date and counting the frequency. I am able to do this with aggregate but I'm not sure how to create a new column with the results, thanks.

data in file:

Domain  Dates
twitter.com 2016-08-08
google.com  2016-08-09
apple.com   2016-08-09
linkedin.com    2016-08-09
microsoft.com   2016-08-09
slack.com   2016-08-12
instagram.com   2016-08-12
ibm.com 2016-08-12

code

import pandas as pd
import matplotlib.pyplot as plt
import datetime
import numpy as np

df = pd.read_csv('domains.tsv', sep='\t')
df = df.groupby([pd.to_datetime(df.Dates).dt.date]).agg({'Dates':'size'})
print(df)

yields

            Dates
Dates
2016-08-08      1
2016-08-09      4
2016-08-12      3

Ideally, I would like the count column to be 'count' and then I will save as a new csv.

Answer 1

import pandas as pd


df = pd.read_csv('domains.tsv', sep='\t')
counter = df.groupby('Dates').count().rename(columns={'Domain': 'count'})
counter.to_csv('count.csv')

You will get count.csv including following result on your current dir.

Dates,count
2016-08-08,1
2016-08-09,4
2016-08-12,3

Answer 2

df['count'] = df.groupby(['Dates']).transform('count')