title: Merging the columns based on findingthe closest date in range
body: <p>I have a dataframe having multiple columns and another bigger dataframe with other multiple columns. </p>
  
  <p>The matching columns in df1 are symbol,m_date
  the matching columns in df2 are symbol, date</p>
  
  <p>I want to get the values for other columns from df2 to df1 such that </p>
  
  <pre><code>if m_date = date matches for a particular symbol, then copy the other columns 
  if m_date != date for a particular symbol, then we find the closest date to m_date in df2 and use their values to be added to the df1 
  </code></pre>
  
  <p>I tried </p>
  
  <pre><code>merged_left = pd.merge(left=df31, right=df2, how='left', left_on=['symbol','m_date'], right_on=['symbol','date'])
  </code></pre>
  
  <p>This merges the file, but the dates that are not available have corresponding values as blank. Can anyone advise as to how to achieve the same </p>
  
  <p>Thanks </p>