old_contents: def binary_search(arr, target):
      left = 0;
      right = len(arr) - 1
      while left <= right:
          mid = left + (right - left) // 2;
          if arr[mid] == target:
              return mid
          elif arr[mid] < target:
              left = mid + 1
          else:
              right = mid - 1
      return -1
  
  print(binary_search([1, 2, 3, 10], 1) == 0)
  print(binary_search([1, 2, 3, 10], 2) == 1)
  print(binary_search([1, 2, 3, 10], 3) == 2)
  print(binary_search([1, 2, 3, 10], 10) == 3)
  print(binary_search([1, 2, 3, 10], 9) == -1)
  print(binary_search([1, 2, 3, 10], 4) == -1)
  print(binary_search([1, 2, 3, 10], 0) == -1)
  print(binary_search([1, 2, 3, 10], 11) == -1)
  print(binary_search([5, 7, 8, 10], 3) == -1)
  
new_contents: def binary_search(arr, target):
      left = 0;
      right = len(arr) - 1
      while left <= right:
          mid = left + (right - left) // 2;
          if arr[mid] == target:
              return mid
          elif arr[mid] < target:
              left = mid + 1
          else:
              right = mid - 1
      return -1
  
  def bisect_left(arr, target):
      """Returns the leftmost position that `target` should
      go to such that the sequence remains sorted."""
      left = 0
      right = len(arr)
      while left < right:
          mid = (left + right) // 2
          if arr[mid] < target:
              left = mid + 1
          else:
              right = mid
      return left
  
  def bisect_right(arr, target):
      """Returns the rightmost position that `target` should
      go to such that the sequence remains sorted."""
      left = 0
      right = len(arr)
      while left < right:
          mid = (left + right) // 2
          if arr[mid] > target:
              right = mid
          else:
              left = mid + 1
      return left
  
  print(binary_search([1, 2, 3, 10], 1) == 0)
  print(binary_search([1, 2, 3, 10], 2) == 1)
  print(binary_search([1, 2, 3, 10], 3) == 2)
  print(binary_search([1, 2, 3, 10], 10) == 3)
  print(binary_search([1, 2, 3, 10], 9) == -1)
  print(binary_search([1, 2, 3, 10], 4) == -1)
  print(binary_search([1, 2, 3, 10], 0) == -1)
  print(binary_search([1, 2, 3, 10], 11) == -1)
  print(binary_search([5, 7, 8, 10], 3) == -1)
  
  print(bisect_left([1, 2, 3, 3, 10], 1) == 0)
  print(bisect_left([1, 2, 3, 3, 10], 2) == 1)
  print(bisect_left([1, 2, 3, 3, 10], 3) == 2) # First "3" is at index 2
  print(bisect_left([1, 2, 3, 3, 10], 10) == 4)
  
  # These return a valid index despite target not being in array.
  print(bisect_left([1, 2, 3, 3, 10], 9) == 4)
  print(bisect_left([1, 2, 3, 3, 10], 0) == 0) # Insert "0" at front
  print(bisect_left([1, 2, 3, 3, 10], 11) == 5) # Insert "5" at back
  
  print(bisect_right([1, 2, 3, 3, 10], 1) == 1)
  print(bisect_right([1, 2, 3, 3, 10], 2) == 2)
  print(bisect_right([1, 2, 3, 3, 10], 3) == 4) # Last "3" is at index 3, so insert new "3" at index 4
  print(bisect_right([1, 2, 3, 3, 10], 10) == 5)
  
  # These return a valid index despite target not being in array.
  print(bisect_right([1, 2, 3, 3, 10], 9) == 4)
  print(bisect_right([1, 2, 3, 3, 10], 0) == 0) # Insert "0" at front
  print(bisect_right([1, 2, 3, 3, 10], 11) == 5) # Insert "5" at back
  
  
subject: Add bisect_left and bisect_right (#63)