How to merge pandas value_counts() to dataframe or use it to subset a dataframe

Question

I used pandas df.value_counts() to find the number of occurrences of particular brands. I want to merge those value counts with the respective brands in the initial dataframe.

Answer 1

You want to use transform.

import numpy as np
import pandas as pd

np.random.seed(0)

# Create dummy data.
df = pd.DataFrame({'brands': ['brand{0}'.format(n) 
                   for n in np.random.random_integers(0, 5, 10)]})

df['brand_count'] = \
    df.groupby('brands', as_index=False)['brands'].transform(lambda s: s.count())

>>> df
   brands brand_count
0  brand4           1
1  brand5           2
2  brand0           1
3  brand3           4
4  brand3           4
5  brand3           4
6  brand1           1
7  brand3           4
8  brand5           2
9  brand2           1

For reference:

>>> df.brands.value_counts()
brand3    4
brand5    2
brand4    1
brand0    1
brand1    1
brand2    1
Name: brands, dtype: int64