title: Function resolution order in pandas
body: <p>I'm not sure if MRO is the correct term for this but let's take the following example:</p>
  <pre><code>summary_df = df.groupby(['provider', 'master_id']).agg(
      content_type_id     =('content_type_id', 'sum')
  )
  </code></pre>
  <p>The <code>sum</code> will run when it is listed in strings. Yet it will also return if it's not in a string, for example:</p>
  <pre><code>summary_df = df.groupby(['provider', 'master_id']).agg(
      content_type_id     =('content_type_id', sum)
  )
  </code></pre>
  <p>What functions may I pass here? Locally defined functions? Globally defined functions? etc. What is the 'lookup order' for the function?</p>