如何计算python数据帧中的计数规则？(类似于SQL中的count if )

import pandas as pd

df = pd.DataFrame({'col1': [1, 2, 3, 4, 5], 'col2': ['a', 'b', 'a', 'b', 'a']})

filtered_df = df[df['col2'] == 'a']

count = len(filtered_df)

import pandas as pd

df = pd.DataFrame({'col1': [1, 2, 3, 4, 5], 'col2': ['a', 'b', 'a', 'b', 'a']})

filtered_df = df[df['col2'] == 'a']
count = len(filtered_df)

print(count)