Python内置函数len-Tutorial Python-php.cn

Python内置函数len

高洛峰

Lepaskan： 2016-11-05 14:19:15

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2082 orang telah melayarinya

英文文档：

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).

说明：

　1. 返回对象的长度，参数可以是序列(比如字符串、字节数组、元组、列表和range对象)，或者是集合(比如字典、集合、不可变集合)

>>> len(&#39;abcd&#39;) # 字符串
>>> len(bytes(&#39;abcd&#39;,&#39;utf-8&#39;)) # 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({&#39;a&#39;:1,&#39;b&#39;:2,&#39;c&#39;:3,&#39;d&#39;:4}) # 字典
>>> len({&#39;a&#39;,&#39;b&#39;,&#39;c&#39;,&#39;d&#39;}) # 集合
>>> len(frozenset(&#39;abcd&#39;)) #不可变集合

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　2. 如果参数为其它类型，则其必须实现__len__方法，并返回整数，否则报错。

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

>>> a = A(&#39;&#39;)
>>> len(a)
>>> a = A(&#39;Aim&#39;)
>>> 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

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