title: Python list comprehension vs. nested loop, conciseness/efficiency
body: <p>The following 2 methods do the same thing. Which one is more efficient in terms of time/space complexity?</p>
  
  <pre><code>** Method A**
  for student in group.students:
      for grade in student.grades:
          some_operation(grade)
  
  ** Method B**
  for grade in [grade for student in group.students for grade in student.grades]
      some_operation(grade)
  </code></pre>