pandas的merge函数只能同时合并三个dataframe，如果涉及到合并多个dataframe就比较麻烦

这种情况下我们可以创建一个我们需要合并的列表，然后将他们一次性合并在一起

# merge any number of dataframes
from functools import reduce
df_groups = [df2, group1, group2, group3, group4, group5, group6, group7, group8]
df_merged = reduce(lambda left, right: pd.merge(left, right, on=['title']), df_groups)
df_merged.head()

reduce函数的用法：Python reduce() 函数

参考来源：Outer merge in pandas with more than two data frames [duplicate]