• None
  • list
  • append

New users will often write code like:

seq = []
for i in range(10):
    seq = seq.append(i)

… And become confused when it crashes with AttributeError: 'NoneType' object has no attribute 'append'.
They assume that list.append returns a reference to the original list, but it actually returns None. The solution is to call .append without assigning the result to anything.

seq = []
for i in range(10):
    seq.append(i)