合并一个数据框中与另一个数据框中的特定列不匹配的行 Python Pandas

Question

我有两个数据框，想将一个数据框中没有一组匹配列的所有行合并到另一个数据框。下面是一个例子：

import pandas

df_1 = pandas.DataFrame({"question":["ABC", "ABC", "EFG", "EFG"],
                         "answer":["abc", "bcd", "efg", "fgh"],
                         "grade":[ "A", "B", "F", "A"],
                         "system":[2,1,1,2]})

df_2 = pandas.DataFrame({"question":["ABC", "ABC", "EFG", "EFG"],
                         "answer":["abc", "jkl", "efg", "qrs"]})

print(f"df_1:\n{df_1}")
print(f"\ndf_2:\n{df_2}")


# <---console output-->

df_1:
  question answer grade  system
0      ABC    abc     A       2
1      ABC    bcd     B       1
2      EFG    efg     F       1
3      EFG    fgh     A       2

df_2:
  question answer
0      ABC    abc
1      ABC    jkl
2      EFG    efg
3      EFG    qrs

如果df_2['question', 'answer'] 不在df_1 中，我如何将df_2 的行附加到df_1？

想要的结果：

question    answer    grade    system
ABC         abc       A        2
ABC         bcd       B        1
EFG         efg       F        1
EFG         fgh       A        2
ABC         jkl       NaN      NaN
EFG         qrs       NaN      NaN

Answer 1

您需要outer 加入：

df_1.merge(df_2, how = "outer")