title: Python: Create a new column of date from an existing column of date by subtracting consecutive rows
body: <p><strong>Code:</strong></p>
  
  <pre><code>import pandas as pd
  df = pd.read_csv('xyz.csv', usecols=['transaction_date', 'amount'])
  df=pd.concat(g for _, g in df.groupby("amount") if len(g) &gt; 3)
  df=df.reset_index(drop=True)
  print(df)
  </code></pre>
  
  <p><strong>Output:</strong></p>
  
  <pre><code>    transaction_date    amount
  0         2016-06-02      50.0
  1         2016-06-02      50.0
  2         2016-06-02      50.0
  3         2016-06-02      50.0
  4         2016-06-02      50.0
  5         2016-06-02      50.0
  6         2016-07-04      50.0
  7         2016-07-04      50.0
  8         2016-09-29     225.0
  9         2016-10-29     225.0
  10        2016-11-29     225.0
  11        2016-12-30     225.0
  12        2017-01-30     225.0
  13        2016-05-16    1000.0
  14        2016-05-20    1000.0
  </code></pre>
  
  <p>I need to add another column next to the amount column which gives the difference between corresponding rows of transaction_date
  e.g.</p>
  
  <pre><code>     transaction_date   amount  delta(days)
  0         2016-06-02      50.0     -
  1         2016-06-02      50.0     0
  2         2016-06-02      50.0     0
  3         2016-06-02      50.0     0
  4         2016-06-02      50.0     0
  5         2016-06-02      50.0     0
  6         2016-07-04      50.0    32
  7         2016-07-04      50.0    .
  8         2016-09-29     225.0    .
  9         2016-10-29     225.0    .
  10        2016-11-29     225.0
  </code></pre>