lambda表达式+reduce

Posted 2020-11-21 liuxuanhe

func=lambda x:x+1
func(1)
Out[3]: 2

func=lambda x,y,z:x+1
func(1,2,3)
Out[5]: 2

lambda可以输入任意多个变量。

func=lambda x,y:lambda z:z+1
func(1)
Traceback (most recent call last):
  File "E:pythonlibsite-packagesIPythoncoreinteractiveshell.py", line 2963, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-8-9ed089a72395>", line 1, in <module>
    func(1)
TypeError: <lambda>() missing 1 required positional argument: ‘y‘
func(1,2)
Out[9]: <function __main__.<lambda>.<locals>.<lambda>(z)>
func(1,2)(1)
Out[10]: 2
func(2,3)(1)
Out[11]: 2

由错误可知，第一个括号对应最外层lambda，第一个括号写入两个，只是传参，实例化了一个lambda对象，第二次才会调用函数，并计算值。

from functools import reduce
print(reduce(lambda a,b: a+b ,[1,2,3,4]))
10

可以正常计算值，将传入的序列合并

from functools import reduce
print(reduce(lambda a: a+1 ,[1,2,3,4]))
Traceback (most recent call last):
  File "E:pythonlibsite-packagesIPythoncoreinteractiveshell.py", line 2963, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-16-246742737994>", line 1, in <module>
    print(reduce(lambda a: a+1 ,[1,2,3,4]))
TypeError: <lambda>() takes 1 positional argument but 2 were given
print(reduce(lambda a,b,c: a+b+c ,[1,2,3,4]))
Traceback (most recent call last):
  File "E:pythonlibsite-packagesIPythoncoreinteractiveshell.py", line 2963, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-17-1dfc623ed0ba>", line 1, in <module>
    print(reduce(lambda a,b,c: a+b+c ,[1,2,3,4]))
TypeError: <lambda>() missing 1 required positional argument: ‘c‘

reduce与lambda合用的那一层，只能传入两个参数，会合并传入的序列，所以传入两个值就够了

from functools import reduce

def add(x):
    x = x+1
    return x

def mut(x):
    x = 2 ** x
    return x

print(reduce(lambda f, g: lambda *a, **kw: g(f(*a, **kw)),[add,mut,add,mut])(1))
print(reduce(lambda f, g: lambda *a, **kw: f(g(*a, **kw)),[add,mut,add,mut])(1))

32
9

*a, **kw：表示传入的参数，个数，类型都任意，一种通配的表示方法

第一种是按传入的序列执行顺序操作，先加再幂再加再幂。

第二种是按传入的序列执行逆序操作，先幂再加再幂再加。

from functools import reduce
def test1(x,y,z):
    a=str(x)
    return a

def test2(x):
    a = x.split(‘ ‘)
    return a

def test3(x):
    b = []
    print(type(x[0]))
    for i in x:
        print(type(i))
        b.append(list(i))
        print(i)
    return b

print(reduce(lambda f, g: lambda x: (g(f(x))),[test1,test2,test3])([1,2,3],[234],1111))

Traceback (most recent call last):
  File "E:/1_python_code/code/python_1/test.py", line 220, in <module>
    print(reduce(lambda f, g: lambda x: (g(f(x))),[test1,test2,test3])([1,2,3],[234],1111))
TypeError: <lambda>() takes 1 positional argument but 3 were given

from functools import reduce
def test1(x,y,z):
    a=str(x)
    return a

def test2(x):
    a = x.split(‘ ‘)
    return a

def test3(x):
    b = []
    print(type(x[0]))
    for i in x:
        print(type(i))
        b.append(list(i))
        print(i)
    return b

print(reduce(lambda f, g: lambda x,y,z: (g(f(x,y,z))),[test1,test2,test3])([1,2,3],[234],11111))

<class ‘str‘>
<class ‘str‘>
[1,
<class ‘str‘>
2,
<class ‘str‘>
3]
[[‘[‘, ‘1‘, ‘,‘], [‘2‘, ‘,‘], [‘3‘, ‘]‘]]

内层lambda传入的参数，与test1匹配

from functools import reduce
def test1(x,y,z):
    print(‘test1‘)
    a=str(x)
    return a

def test2(x):
    print(‘test2‘)
    a = x.split(‘ ‘)
    return a

def test3(x):
    b = []
    print(‘test3‘)
    print(type(x[0]))
    for i in x:
        print(type(i))
        b.append(list(i))
        print(i)
    return b

print(reduce(lambda f, g: lambda x,y,z: (f(g(x,y,z))),[test1,test2,test3])([1,2,3],[234],11111))

Traceback (most recent call last):
  File "E:/1_python_code/code/python_1/test.py", line 223, in <module>
    print(reduce(lambda f, g: lambda x,y,z: (f(g(x,y,z))),[test1,test2,test3])([1,2,3],[234],11111))
  File "E:/1_python_code/code/python_1/test.py", line 223, in <lambda>
    print(reduce(lambda f, g: lambda x,y,z: (f(g(x,y,z))),[test1,test2,test3])([1,2,3],[234],11111))
TypeError: test3() takes 1 positional argument but 3 were given

Process finished with exit code 1


from functools import reduce
def test1(x,y,z):
    print(‘test1‘)
    a=str(x)
    return a

def test2(x):
    print(‘test2‘)
    a = x.split(‘ ‘)
    return a

def test3(x):
    b = []
    print(‘test3‘)
    print(type(x[0]))
    for i in x:
        print(type(i))
        b.append(list(i))
        print(i)
    return b

print(reduce(lambda f, g: lambda x: (f(g(x))),[test1,test2,test3])([1,2,3]))

test3
Traceback (most recent call last):
<class ‘int‘>
  File "E:/1_python_code/code/python_1/test.py", line 223, in <module>
<class ‘int‘>
    print(reduce(lambda f, g: lambda x: (f(g(x))),[test1,test2,test3])([1,2,3]))
  File "E:/1_python_code/code/python_1/test.py", line 223, in <lambda>
    print(reduce(lambda f, g: lambda x: (f(g(x))),[test1,test2,test3])([1,2,3]))
  File "E:/1_python_code/code/python_1/test.py", line 219, in test3
    b.append(list(i))
TypeError: ‘int‘ object is not iterable

Process finished with exit code 1

由两次的错误信息，及第二次错误的定位，更加肯定将函数内的f和g更换位置后，先执行了test3，是逆序执行。