title: How to add values from one dataframe into another ignoring the row indices
body: <p>I have a pandas dataframe called <code>trg_data</code> to collect data that I am producing in batches.  Each batch is produced by a sub-routine as a smaller dataframe <code>df</code> with the same number of columns but less rows and I want to insert the values from <code>df</code> into <code>trg_data</code> at a new row position each time.</p>
  
  <p>However, when I use the following statement df is always inserted at the top. (i.e. rows 0 to len(df)).</p>
  
  <pre><code>trg_data.iloc[trg_pt:(trg_pt + len(df))] = df
  </code></pre>
  
  <p>I'm guessing but I think the reason may be that even though the slice indicates the desired rows, it is using the index in df to decide where to put the data.</p>
  
  <p>As a test I found that I can insert an ndarray at the right position no problem:</p>
  
  <pre><code>trg_data.iloc[trg_pt:(trg_pt + len(df))] = np.ones(df.shape)
  </code></pre>
  
  <p>How do I get it to ignore the index in df and insert the data where I want it?  Or is there an entirely different way of achieving this?  At the end of the day I just want to create the dataframe trg_data and then save to file at the end.  I went down this route because there didn't seem to be a way of easily appending to an existing dataframe.</p>
  
  <p>I've been working at this for over an hour and I can't figure out what to google to find the right answer! </p>