title: Does "df['var'].map(df2)" and "df.var.map(df2)" always produce the same result?
body: <p>I have a dataframe <code>df</code> with a column <code>var</code>, and another dataframe <code>df2</code> with columns <code>var</code> and <code>var2</code>. Both columns <code>var</code> in 2 dataframes are exactly the same.</p>
  <p>In my example, <code>df['var'].map(df2)</code> and <code>df.var.map(df2)</code> yield the same result. I would like to ask if this is just a coincidence in my particular dataset, or it always holds.</p>
  <p>Thank you so much!</p>
  <p><strong>Update:</strong> In my example, these below codes also produce the same result.</p>
  <pre><code>df.groupby('parent_id')['parent_id'].transform('count').tolist()
  </code></pre>
  <p>and</p>
  <pre><code>df.groupby('parent_id').parent_id.transform('count').tolist()
  </code></pre>
  <p>This gives me a feeling that <code>df.groupby('parent_id')['parent_id']</code> and <code>df.groupby('parent_id').parent_id</code> produce the same result.</p>