> For the complete documentation index, see [llms.txt](https://itswl.gitbook.io/python/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://itswl.gitbook.io/python/15python-zhong-de-none.md).

# 有关于None

**None 表示空 不同于** 空字符串 空的列表 0 False

**类型不同，值不同**

```
print(type(None)) <class 'NoneType'>None是None类
a=''
b=False
c=[]
print(a==None)  #False
print(b==None) #False
print(c==None) #False  #值不相等
```

**深入**

```
def fun():
    return None

a = fun()
if not a:
    print('S')
else:
    print('F')

if a is None:
    print('S')
else:
    print('F')
#S
#S
```

```
def fun():
    return None

a = []
if not a:
    print('S')
else:
    print('F')

if a is None:
    print('S')
else:
    print('F')
#S
#F
```

**类中默认非空**

```
class Test():
    pass

test = Test()
if test:
    print('S')
else:
    print('F')
#S
```

**类中为空的情况**

```
class Test():
    def __len__(self):
        return 0 #(只能为int类型)

test = Test()
if test:
    print('S')
else:
    print('F')
#F
```

```
class Test():
    def __len__(self):
        return 0 #(只能为int类型)

test = Test()
print(bool(None)) #False
print(bool({}))#False
print(bool([]))#False
print(bool(test))#False
```

**由bool决定True or False,与len无关** (即print 只有 bool call True,或者bool call False ))

```
class Test():

    def __bool__(self):
        print('bool called')
        return False#(/True)

    def __len__(self):
        print('len called')
        return True #(只能为int类型)

print(bool(Test()))
#bool called
#False#(/True)
```