于 2020 年 1 月 7 日更新
len()函数计算对象中的项目数。
其语法如下：
len(obj) -> length| 参数 | 描述 |
|---|---|
obj |
obj可以是字符串，列表，字典，元组等。 |
这是一个例子：
>>>
>>> len([1, 2, 3, 4, 5]) # length of list
5
>>>
>>> print(len({"spande", "club", "diamond", "heart"})) # length of set
4
>>>
>>> print(len(("alpha", "beta", "gamma"))) # length of tuple
3
>>>
>>> print(len({ "mango": 10, "apple": 40, "plum": 16 })) # length of dictionary
3试试看：
# length of list
print(len([1, 2, 3, 4, 5]))
# length of set
print(len({"spande", "club", "diamond", "heart"}))
# length of tuple
print(len(("alpha", "beta", "gamma")))
# length of dictionary
print(len({ "mango": 10, "apple": 40, "plum": 16 })) 具有讽刺意味的是，len()函数不适用于生成器。 尝试在生成器对象上调用len()将导致TypeError异常。
>>>
>>> def gen_func():
... for i in range(5):
... yield i
...
>>>
>>>
>>> len(gen_func())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'generator' has no len()
>>>试一试：
def gen_func():
for i in range(5):
yield i
print(len(gen_func())) 要在用户定义的对象上使用len()，您将必须实现__len__()方法。
>>>
>>> class Stack:
...
... def __init__(self):
... self._stack = []
...
... def push(self, item):
... self._stack.append(item)
...
... def pop(self):
... self._stack.pop()
...
... def __len__(self):
... return len(self._stack)
...
>>>
>>> s = Stack()
>>>
>>> len(s)
0
>>>
>>> s.push(2)
>>> s.push(5)
>>> s.push(9)
>>> s.push(12)
>>>
>>> len(s)
4
>>>试一试：
class Stack:
def __init__(self):
self._stack = []
def push(self, item):
self._stack.append(item)
def pop(self):
self._stack.pop()
def __len__(self):
return len(self._stack)
s = Stack()
print(len(s))
s.push(2)
s.push(5)
s.push(9)
s.push(12)
print(len(s))