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

df = ...
key_col = "brand"
count_col = "brand_count"

result = (
    df.join(
        df[key_col].value_counts().rename(count_col), 
        how="left", 
        on=key_col)
)

If you need to join the counts to a different dataframe remember to fill NaNs with zeros:

df = ...
other = ...
key_col = "brand"
count_col = "brand_count"

result = (
    other.join(
        df[key_col].value_counts().rename(count_col), 
        how="left", 
        on=key_col)
    .fillna({count_col: 0})
)