(15) python中的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)