on在對(duì)in 的工作方式產(chǎn)生嚴(yán)重誤解之后join（劇透：與onin非常不同merge），這是我的示例代碼。import pandas as pdindex1 = pd.MultiIndex.from_product([["variables"], ["number", "fruit"]])df1 = pd.DataFrame([["one", "apple"], ["two", "banana"]], columns=index1)index2 = pd.MultiIndex.from_product([["variables"], ["fruit", "color"]])df2 = pd.DataFrame([["banana", "yellow"]], columns=index2)print(df1.merge(df2, on="fruit", how="left"))我得到一個(gè)KeyError. 我如何variables.fruit在這里正確引用？要理解我的目的，請(qǐng)考慮沒有多重索引的相同問題：import pandas as pddf1 = pd.DataFrame([["one", "apple"], ["two", "banana"]], columns=["number", "fruit"])df2 = pd.DataFrame([["banana", "yellow"]], columns=["fruit", "color"])# this is obviously incorrect as it uses indexes on `df1` as well as `df2`:print(df1.join(df2, rsuffix="_"))# this is *also* incorrect, although I initially thought it should work, but it uses the index on `df2`:print(df1.join(df2, on="fruit", rsuffix="_"))# this is correct:print(df1.merge(df2, on="fruit", how="left"))預(yù)期和想要的結(jié)果是這樣的：  number   fruit   color0    one   apple     NaN1    two  banana  yellowfruit當(dāng)是多重索引的一部分時(shí)，如何獲得相同的結(jié)果？