如何将包含其他列中的值出现次数的列附加到现有的Dataframe？

import pandas as pd

data = {'A': [1, 2, 3, 4, 5],
        'B': ['a', 'b', 'a', 'b', 'a'],
        'C': ['x', 'y', 'x', 'y', 'x']}
df = pd.DataFrame(data)

counts = df['B'].value_counts()

df_merged = pd.merge(df, counts, left_on='B', right_index=True)

import pandas as pd

data = {'A': [1, 2, 3, 4, 5],
        'B': ['a', 'b', 'a', 'b', 'a'],
        'C': ['x', 'y', 'x', 'y', 'x']}
df = pd.DataFrame(data)

counts = df['B'].value_counts()
df_merged = pd.merge(df, counts, left_on='B', right_index=True)

print(df_merged)