15.Pythonic与python杂记

switcher ={
   0:'sunday',
   1:'monday',
   2:'thuesday'
}

day = 0
day_name=switcher.get(day,'Unknow')
print(day_name)  # sunday

def get_sunday():
    return 'sunday'

def get_monday():
    return 'monday'

def get_thuesday():
    return 'thuesday'
    
def get_default():
    return 'Unknow'

switcher ={
   0:get_sunday,
   1:get_monday,
   2:get_thuesday
}

day = 6
day_name=switcher.get(day,get_default)()
print(day_name)  # sunday

# 列表推到式
# 集合推到式
# map filter
# set
# dict
a = [1,2,3,4,5,6,7,8]
# b = [i*i for i in a]
b = [i**2 for i in a]
print(b)  # [1, 4, 9, 16, 25, 36, 49, 64]
c = [i**3 for i in a]
print(c)  # [1, 8, 27, 64, 125, 216, 343, 512]
d = [i**3 for i in a if i > 5]
print(d)  # [216, 343, 512]

e =(1,2,3,4,5,6,7,8)
f = [i**3 for i in a if i > 5]
print(f)  # [216, 343, 512]
g = {i**3 for i in a if i > 5}
print(g)  # {216, 512, 343}

students = {
    '喜小乐':18,
    '石敢当':20,
    '张三':15
}
# 字典
b = {key for key,value in students.items()}
print(b)  # {'石敢当', '喜小乐', '张三'}
b = {value for key,value in students.items()}
print(b)  # {18, 20, 15}
b = {value:key for key,value in students.items()}
print(b)  # {18: '喜小乐', 20: '石敢当', 15: '张三'}

# 元组
b = (key for key,value in students.items())
for x in b:
    print(x)
# 喜小乐
# 石敢当
# 张三

# None 空
# 空字符串 空的列表 0 False
a = ''
b = False
c =[]
print(a==None)      # False
print(b==None)      # False
print(c==None)      # False
print(a is None)    # False
print(type(None))   # <class 'NoneType'>

a = []
a =''
a = None   # 不存在
a = False  # 真假

判断为空的方式
if a:
if not a:

class Test():
    def __len__(self):
        return 0
        # return 8
        # return True

test = Test()
if test:
    print('S')  # True/8    
else:
    print('F')  # 0

print(len(Test()))   # 0
print(bool(Test()))  # False

class Test():
    def __bool__(self):
        return False

    def __len__(self):
        return True

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