15、函数


15.1 基本函数

1）例如读取文件函数 read_file
##函数结构：

###################################################测试_START###################################################
#声明函数def 函数名称(参数):
def read_file(file_path):
#with 上下文管理器打开文件，函数执行完自动关闭，不需手动close
    with open(file_path,'r',encoding='utf-8') as f:
#f.read()读文件内容，赋值给res
        res = f.read()
#return 返回值
    return res

#调用read_file函数，变量当前路径 1.txt 文件
f1 = read_file('1.txt')
print(f1)

###################################################测试_END###################################################




2）多个参数，(*args,**kwatgs)
*args       #tuple，任意多个无名参数
**kwargs    #dict

###################################################测试_START###################################################
print('================================*args=======================')
###tuple，
def func_1(*args):
    print(f'value:{args},type:{type(args)}')

func_1(1,2,3)

print('================================**kwargs=======================')
def func_2(**kwargs):
    print(f'value:{kwargs},type:{type(kwargs)}')
#如果函数传参**kwargs，那么必须传入字典格式
func_2(a=123,b=234,c=345)

print('================================*args+**kwargs=======================')
def func_3(*args,**kwargs):
    print(f'value:{args},type:{type(args)}')
    print(f'value:{kwargs},type:{type(kwargs)}')

func_3(1,2,3,a=123,b=234,c=345)

###################################################测试_END###################################################






3）缺省参数介绍
def func_3(x=3):
    print(x)

#比如直接调用func_3，会默认传参x=3，输出为3
func_3()


###################################################测试_START###################################################
In [26]:  def test_print(name,age='未知',s_class='未知',gender=True):
    ...:      gender_text = '男'
    ...:      if not gender:
    ...:          gender_text = '女'
    ...:      print(f'姓名：{name} ，性别：{gender_text}，年龄：{age}，班级：{s_class}。')
    ...:

In [27]: test_print(name='wzw',age=12,s_class=6)
姓名：wzw ，性别：男，年龄：12，班级：6。

In [28]: test_print(name='wcf')
姓名：wcf ，性别：男，年龄：未知，班级：未知。

In [30]: test_print(name='xxx',gender=False)
姓名：xxx ，性别：女，年龄：未知，班级：未知。

###################################################测试_END###################################################


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15.2 高阶函数
1）函数2调用函数1：
In [1]: def func_1(a,b):
   ...:     return a + b
   ...:

In [2]: def func(a,b,func_1):
   ...:     return(func_1(a,b) * func_1(a,b))
   ...:

In [3]: func(1,2,func_1)
Out[3]: 9


###注意使用内置高阶函数时，函数的参数代入的函数fx，不需要使用fx(a,b)或fx()
2）map(函数，可迭代对象)  作用：将函数一次作用到每个元素上，并将结果以可迭代对象返回
In [15]: def fx(x):
    ...:     return x * x
    ...:
    ...: l = [1,2,3,4,5,6,7,8,9]
    ...:
    ...: res = map(fx,l)

In [16]:

In [16]: list(res)
Out[16]: [1, 4, 9, 16, 25, 36, 49, 64, 81]

3)上述map函数的实现逻辑，用for循环来简单理解：
In [17]: def fx(x):
    ...:     return x * x
    ...:

In [18]: l = [1,2,3,4,5,6,7,8,9]

In [19]: res = []

In [20]: for i in l:
    ...:     res.append(fx(i))
    ...:

In [21]: list(res)
Out[21]: [1, 4, 9, 16, 25, 36, 49, 64, 81]


4）reduce函数，需导入内置函数
#reduce(函数，可迭代对象)  将可迭代对象，代入函数计算结果；将结果与可迭代对象下一个元素，继续代入函数计算

In [32]: from functools import reduce

In [33]: def fx(a,b):
    ...:     return a + b
    ...:

In [34]: l = [1,2,3,4,5,6,7,8,9]

In [35]: res = reduce(fx,l)

In [36]: res
Out[36]: 45

5）数字char -> int
#原理，比如'12345'，(((1*10+2)*10+3)*10+4)*10+5
In [6]: def str2int(s):                                
   ...:     def f(a,b):                    
   ...:         return a*10 + b            
   ...:     def char2int(c):               
   ...:         return int(c)              
############################这里利用map函数，将数字字符串s（可迭代对象），拆分为list(map(char2int,s)) = [1,2,3,4,5]
   ...:     num = reduce(f,map(char2int,s))
   ...:     return num                     
   ...:                                    
                                           
In [7]: num = str2int('12345')             
                                           
In [8]: num                                
Out[8]: 12345                              
                                           
In [9]: type(num)                          
Out[9]: int  




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15.3 匿名函数

In [7]: lambda x : x * x
Out[7]: <function __main__.<lambda>(x)>

In [8]: (lambda x: x * x)(3)
Out[8]: 9

#等价
In [5]: def func(x):
   ...:     return x * x
   ...:

In [6]: func(3)
Out[6]: 9




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15.4 偏函数
###类似“缺省参数”功能，但由于有些函数并不是自己定义（编写、使用）的函数，所以可以为函数增加定义属性
In [12]: from functools import partial

In [13]: int_2 = partial(int,base=2)

In [14]: int_2('100')
Out[14]: 4

In [15]: int_2('110')
Out[15]: 6

附：
int('100', base=10) # 十进制,结果为 100
int('100', base=2)  # 二进制,结果为 4 


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15.5 递归函数
1）基础函数：阶乘
In [6]: def func(a):
   ...:     if a == 1:
   ...:         return 1
   ...:
   ...:     res = (a * func(a-1))
   ...:     return res
   ...:

In [7]: func(1)
Out[7]: 1

In [8]: func(5)
Out[8]: 120



2）尾递归：将结果先计算出来，防止堆栈
In [8]: def func_1(next_num,summ_num):
   ...:     if next_num == 1:
   ...:         return summ_num
   ...:     return func_1(next_num - 1, next_num * summ_num)
   ...:

In [9]: func(3)
Out[9]: 6

In [10]:

In [10]: func(5)
Out[10]: 120