Python for Programmers: Terminating Loops

• Inside of a loop, you can use continue to abort a single iteration. Execution immediately jumps back up to the loop's condition, which is checked again as usual.

• >

  data = [10, -1, 3]
  result = 0
  
  while len(data) > 0:
    item = data.pop(0)
    if item < 0:
      continue
    result += item
  
  result

  Result:

  13

• The break statement terminates the entire loop immediately. Execution moves to the next statement after the loop.

• >

  data = [10, -1, 3]
  result = 0
  
  while len(data) > 0:
    item = data.pop(0)
    if item < 0:
      break
    result += item
  
  result

  Result:

  10

• Be careful when using break! Normally, when we reach the end of a while loop, we know that its condition is now false.

• >

  data = [20, 3, 2, -5, 3, 2]
  while len(data) > 0:
    data.pop()
  len(data)

  Result:

  0

• In that example, the condition was len(data) > 0. At the end of the loop, data is empty, which makes the condition false.

• When we break out of a while, we no longer have that guarantee. At first glance, we might incorrectly assume that the loop's condition is false after the loop ends. This makes break slightly dangerous.

• >

  data = [20, 3, 2, -5, 3, 2]
  
  while len(data) > 0:
    # Remove the first element in the list.
    item = data.pop(0)
    if item < 0:
      break
  
  len(data)

  Result:

  2

• The while's condition was len(data) > 0. But because of the break, data still had two elements after the loop ended.

• We just showed continue and break in while loops, but they also work in for loops. The details are exactly the same: continue skips to the next loop iteration, and break immediately ends the entire loop.