<p>我有以下数据框:</p>
<pre class="lang-py prettyprint-override"><code> key1 key2
0 a one
1 a two
2 b one
3 b two
4 a one
5 c two
</code></pre>
<p>现在,我要按<code>key1</code>对数据帧进行分组,并使用值<code>"one"</code>对列<code>key2</code>进行计数,以获得以下结果:</p>
<pre class="lang-py prettyprint-override"><code> key1
0 a 2
1 b 1
2 c 0
</code></pre>
<p>我只是按惯例数数:</p>
<pre class="lang-py prettyprint-override"><code>df.groupby(['key1']).size()
</code></pre>
<p>但我不知道如何插入条件。</p>
<p>我尝试过这样的事情:</p>
<pre class="lang-py prettyprint-override"><code>df.groupby(['key1']).apply(df[df['key2'] == 'one'])
</code></pre>
<p>但我不能再继续下去了。我该怎么做?</p>