<p>使用<a href="http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.combine_first.html" rel="nofollow noreferrer">^{<cd1>}</a>,必要时先使用<a href="http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.set_index.html" rel="nofollow noreferrer">^{<cd2>}</a>:</p>
<pre><code>df11 = df1.set_index('subject_id')
df22 = df2.set_index('subject_id')
df3 = df22.combine_first(df11).reset_index()
print (df3)
subject_id first_name last_name
0 1 Alex Anderson
1 2 Amy Ackerman
2 3 Allen Ali
3 4 Billy Bonder
4 5 Brian Black
5 6 Bran Balwner
6 7 Bryce Brice
7 8 Betty Btisan
df3 = df11.combine_first(df22).reset_index()
print (df3)
subject_id first_name last_name
0 1 Alex Anderson
1 2 Amy Ackerman
2 3 Allen Ali
3 4 Alice Aoni
4 5 Ayoung Atiches
5 6 Bran Balwner
6 7 Bryce Brice
7 8 Betty Btisan
</code></pre>