title: Append rows to a pandas DataFrame without making a new copy
body: <p>The pandas <a href="http://pandas.pydata.org/pandas-docs/stable/merging.html">documentation</a> includes a note:</p>
  
  <blockquote>
    <p>Note Unlike list.append method, which appends to the original list and returns nothing, append here does not modify df1 and returns its copy with df2 appended.</p>
  </blockquote>
  
  <p>How can I append to an existing DataFrame without making a copy? Or in the terms of the note how can I modify df1 in place by appending df2 and return nothing?</p>