如何将r个数据框的多列组合成一个单列，即一个列表

import pandas as pd

def combine_columns(dataframes):
    combined_list = []
    for df in dataframes:
        num_columns = len(df.columns)
        for i in range(num_columns):
            column_data = df.iloc[:, i].tolist()
            combined_list.extend(column_data)
    return combined_list

# 示例数据框
df1 = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]})
df2 = pd.DataFrame({'C': [7, 8, 9], 'D': [10, 11, 12]})
df3 = pd.DataFrame({'E': [13, 14, 15], 'F': [16, 17, 18]})

# 调用函数进行列的组合
combined_columns = combine_columns([df1, df2, df3])

# 打印组合后的列表
print(combined_columns)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]