python - 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，我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中。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 中，我如何将 df_2 的行附加到 df_1 df_1?

期望的结果:

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

您需要一个外部连接:

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