title: Concat two dataframe from two different source. Unable to handle situation if dataframe missing
body: <p>I would like to concat two dataframes from two sources, both of them are having identical in structure but only difference in rows entry. And the problem I am facing is to handle on cases like sometime #input might be single, sometime none and sometime both.</p>
  <p>My function is like this:</p>
  <pre><code>def CombiTwoDf(df1,df2):
  if isinstance(df1, pd.DataFrame) and isinstance(df2, pd.DataFrame):
      frames = [df1, df2]
      result = pd.concat(frames)
      result.drop_duplicates(subset =None, keep = 'first', inplace = True)
      return result
  if isinstance(df1, pd.DataFrame) and df2 is None:
      return df1
  if df2 is None and isinstance(df2, pd.DataFrame):
      return df2
  if df1 is None and df2 is None:
      return None
  </code></pre>