擅长:python、mysql、java
<p><code>unstack</code>和<code>stack</code>返回<code>dropna=False</code>:</p>
<pre><code>days = ['Mon','Tue','Wed','Thu','Fri']
out = (df.set_index(['Person','Day'])['Value'].unstack()
.reindex(days ,axis=1)
.stack(dropna=False).reset_index(name='Value'))
</code></pre>
<hr/>
<pre><code> Person Day Value
0 1 Mon 32.0
1 1 Tue 30.0
2 1 Wed 34.0
3 1 Thu 22.0
4 1 Fri 43.0
5 2 Mon 22.0
6 2 Tue NaN
7 2 Wed 14.0
8 2 Thu NaN
9 2 Fri 11.0
10 3 Mon NaN
11 3 Tue 13.0
12 3 Wed 22.0
13 3 Thu 23.0
14 3 Fri NaN
</code></pre>