title: Select columns from dataframe on condition they exist
body: <p>I have a pandas DataFrame with multiple columns (columns names are numbers; 1, 2, ...) and I want to copy some of them if they do exist.</p>
  
  <p>For example <code>df1 = df[[1,2,3,4]]</code>
  But it might happen that some columns do not exist in df, eg df might only have columns 1, 2, and 4 or columns 1, and 2 etc</p>