title: Check missing row in specific column and then add it to the dataframe
body: <p>I want the code to check if the dataframe has all elements of period_list. If not I want to add that element to the dataframe, the values associated with the element will be zero. I wrote this and it didn't work
  Dataframe: test_1</p>
  <pre><code>    Period       A     B    C
  0   2018 - Q2    1     0    1
  1   2018 - Q3    1     1    1
  2   2018 - Q4    0     1    1
  3   2019 - Q1    0     0    0
  4   2019 - Q2    0     0    1
  5   2019 - Q3    1     0    1
  6   2019 - Q4    0     1    1
  7   2020 - Q1    1     0    1
  8   2020 - Q2    0     0    0
  </code></pre>
  <p>my code</p>
  <pre><code>period_list = ['2018 - Q1', '2018 - Q2', '2018 - Q3', '2018 - Q4', '2019 - Q1', '2019 - Q2', '2019 - Q3', '2019 - Q4', '2020 - Q1', '2020 - Q2', '2020 - Q3', '2020 - Q4', '2021 - Q1']
  
  for row in period_list:
      if row not in test_1.iloc[:, 0]:
          test_1.append(row)
  </code></pre>