value_counts转成DataFrame

import pandas as pd
data = {'created_at':['a','a','a','b','c']}
df = pd.DataFrame.from_dict(data)
df['b'] = df['created_at']
df1 = df.created_at.value_counts().rename_axis('created_at').reset_index(name='count')
df1
df = pd.merge(df,df1,how='left')
df