title: Groupby multiple columns & Sum - Create new column with added If Condition
body: <p>I need to groupby multiple columns &amp; then get Sum in a new column with added If condition. I tried the next code and it worked great with grouping by single column:</p>
  <pre><code>df['new column'] = (
      df['value'].where(df['value'] &gt; 0).groupby(df['column1']).transform('sum')
  )
  </code></pre>
  <p>However, when I try to group by multiple columns I get an error.</p>
  <pre><code>df['new_column'] = (
          df['value'].where(df['value'] &gt; 0).groupby(df['column1', 'column2']).transform('sum')
      )
  </code></pre>
  <p>Error:</p>
  <pre><code>-&gt;return self._engine.get_loc(casted_key) 
  The above exception was the direct cause of the following exception: 
  -&gt;indexer = self.columns.get_loc(key) 
  -&gt;raise KeyError(key) from err 
  -&gt;if is_scalar(key) and isna(key) and not self.hasnans: ('column1', 'column2')
  </code></pre>
  <p>Could you please advise how I should change the code to get the same result but grouping by multiple columns?</p>
  <p>Thank you</p>