某些PEP是否为内置序列类型保证了len（...）的O（1）复杂度？

Question

Many questions about cost of len(...) are answered but I haven't found any link to Python documentation.

Is it by standard (documented in some PEP) or just way how it's currently implemented in most Python implementations?

Answer 1

Documentation about time complexity for certain built-in python objects is here .

The len() function in python just calls the __len__ method in your class. So if you built a custom class

class SlowLenList(object):

  def __init__(self, mylist):
    self.mylist = mylist

  def __len__(self):
    total = 1
    for item in self.mylist:
      total += 1
    return total

Then the complexity would be O(n) in this case. So it really depends on the object you are calling. I assume the built-in list and other objects are O(1) because they have an attribute on an instance that increments every time an item is added to the object.