title: How to add a Column named Key into a dictionary of multiple dataframes
body: <p>Given a dictionary with multiple dataframes in it. How I can add a column to each dataframe with all the rows in that df filled with the key name'?</p>
  <p><a href="https://i.stack.imgur.com/tDSxA.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/tDSxA.png" alt="Dictionary Structure" /></a></p>
  <p>I tried this code:</p>
  <pre><code>for key, df in sheet_to_df_map.items():
      df['sheet_name'] = key
  </code></pre>
  <p>This code does add the key column in each dataframe inside the dictionary, but also creates an additional dataframe.</p>
  <p><a href="https://i.stack.imgur.com/s86ob.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/s86ob.png" alt="DF" /></a></p>
  <p>Can't this be done without creating an additional dataframe?</p>
  <p>Furthermore, I want to separate dataframes from the dictionary by number of columns. All the dataframes that have 10 columns concatenated, the ones with 9 concatenated and so on. I don't know how to do this.</p>