title: add datetime column to all rows in pandas dataframe
body: <p>how can I add a time_pinged to all rows in pandas dataframe?</p>
  <pre><code>import sqlalchemy, pandas as pd, datetime as dt
  engine = sqlalchemy.create_engine('sqlite:///school_stream.db')
  
  class_summary = client.get_account()
  classes = class_summary [&quot;scores&quot;]
  
  df=pd.DataFrame([classes])
  time_pinged = dt.datetime.utcnow().strftime('%Y-%m-%d %H-%M-%S.%f')[:-7]
  df['time_pinged'] = time_pinged
  pprint.pprint (df)
  </code></pre>
  <p>this is what I get as a result:</p>
  <pre><code>0  {'class': 'AA1', 'mark': '200.00000000', 'pending': '0'}   
  0  {'class': 'AA2', 'mark': '200.00000000', 'pending': '0'}   
  0  {'class': 'AA3', 'mark': '200.00000000', 'pending': '0'}   
  0  {'class': 'AA4', 'mark': '50.00000000',  'pending': '0'}   
  0  {'class': 'AA5', 'mark': '150.00000000', 'pending': '0', time_pinged: 2022-07-14 16-13-53 }  
  </code></pre>
  <p>but the goal is to get</p>
  <pre><code>0  {'class': 'AA1', 'mark': '200.00000000', 'pending': '0', time_pinged: 2022-07-14 16-13-53 }   
  0  {'class': 'AA2', 'mark': '200.00000000', 'pending': '0', time_pinged: 2022-07-14 16-13-53 }   
  0  {'class': 'AA3', 'mark': '200.00000000', 'pending': '0', time_pinged: 2022-07-14 16-13-53 }   
  0  {'class': 'AA4', 'mark': '50.00000000',  'pending': '0', time_pinged: 2022-07-14 16-13-53 }   
  0  {'class': 'AA5', 'mark': '150.00000000', 'pending': '0', time_pinged: 2022-07-14 16-13-53 } 
  </code></pre>