Python built-in function len

English document:

len(s)

Return the length (the number of items) of an object. The argument may be a sequence (such as a string, bytes, tuple, list, or range) or a collection ( such as a dictionary, set, or frozen set).

Instructions:

　1. Return the length of the object. The parameter can be a sequence (such as string, byte array, tuple, list and range object), or It is a collection (such as dictionary, set, immutable collection)

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

　2. If the parameter is of other types, it must implement the __len__ method and return an integer, otherwise an error will be reported.

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

>>> a = A('')
>>> len(a)
>>> a = A('Aim')
>>> len(a)
>>> 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 &#39;B&#39; has no len()
>>> class C:
    def __len__(self):
        return &#39;len&#39;

>>> c = C()
>>> len(c)
Traceback (most recent call last):
  File "<pyshell#71>", line 1, in <module>
    len(c)
TypeError: &#39;str&#39; object cannot be interpreted as an integer