Python通过len函数返回对象长度

> len('abcd') # 字符串
4
> len(bytes('abcd','utf-8')) # 字节数组
4
> len((1,2,3,4)) # 元组
4
> len([1,2,3,4]) # 列表
4
> len(range(1,5)) # range对象
4
> len({'a':1,'b':2,'c':3,'d':4}) # 字典
4
> len({'a','b','c','d'}) # 集合
4
> len(frozenset('abcd')) #不可变集合
4

> class A:
  def __init__(self,name):
    self.name = name
  def __len__(self):
    return len(self.name)

> a = A('')
> len(a)
0
> a = A('Aim')
> len(a)
3
> class B:
  pass

> b = B()
> len(b)
Traceback (most recent call last):
 File "<pyshell#65>", line 1, in <module>
  len(b)
TypeError: object of type 'B' has no len()
> class C:
  def __len__(self):
    return 'len'

> c = C()
> len(c)
Traceback (most recent call last):
 File "<pyshell#71>", line 1, in <module>
  len(c)
TypeError: 'str' object cannot be interpreted as an integer