<p dir="auto">When I upgraded from 0.16.2 to 0.17.0, I was met with a nasty surprise when dropping duplicates.  It looks like DataFrame.drop_duplicates() is not working as I would expect it to based on the previous version.  I have a dataframe</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="test_ids = df['test_id'].unique()
print('N test ids: {}'.format(test_ids.shape))
print('N tests: {}'.format(df[['test_id', &lt;some other columns&gt;]].drop_duplicates().shape))"><pre class="notranslate"><code class="notranslate">test_ids = df['test_id'].unique()
print('N test ids: {}'.format(test_ids.shape))
print('N tests: {}'.format(df[['test_id', &lt;some other columns&gt;]].drop_duplicates().shape))
</code></pre></div>
<p dir="auto">the output is:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="N test ids: (341334,)
N tests: (237426, 10)"><pre class="notranslate"><code class="notranslate">N test ids: (341334,)
N tests: (237426, 10)
</code></pre></div>
<p dir="auto">when I run the same in 0.16.2 the output is:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="N test ids: (341334,)
N tests: (341334, 10)"><pre class="notranslate"><code class="notranslate">N test ids: (341334,)
N tests: (341334, 10)
</code></pre></div>
<p dir="auto">I don't think you should be able to get fewer rows than the number of unique entries in a single column.</p>
<p dir="auto">promotes consistency, see <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="11926460" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/3021" data-hovercard-type="pull_request" data-hovercard-url="/pandas-dev/pandas/pull/3021/hovercard" href="https://github.com/pandas-dev/pandas/pull/3021">#3021</a>, <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="11909589" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/3016" data-hovercard-type="issue" data-hovercard-url="/pandas-dev/pandas/issues/3016/hovercard" href="https://github.com/pandas-dev/pandas/issues/3016">#3016</a>, <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="51814091" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/9065" data-hovercard-type="issue" data-hovercard-url="/pandas-dev/pandas/issues/9065/hovercard" href="https://github.com/pandas-dev/pandas/issues/9065">#9065</a></p>