def function_name (argument1, argument2, ...):
    <action>

# 一个简单的函数，用来打印 Hello world!
def hello():
    '''
    这个代码用来打印 Hello world
    '''
    print('Hello world!')
    print('Hello world again!')

hello()

Hello world!
Hello world again!

# 给 hello 函数增加参数
def hello(name):
    print('Hello' , name)

hello('Alice')
hello('Bob')

Hello Alice
Hello Bob

# 比较两个数，并返回较大的数：
def my_max(a,b): # 和 Python 内置的 max 函数区分开
    if a > b:
        return a
    else:
        return b
    
print(my_max(3,3.14))

3.14

def nop():
    pass

def my_print(str):
    print(str)

my_print() # 不加参数会报错，因为里面是必需参数

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[8], line 4
      1 def my_print(str):
      2     print(str)
----> 4 my_print()

TypeError: my_print() missing 1 required positional argument: 'str'

print('Hello',end='') # 使用 end 参数，在打印结束后打印空字符串
print('World')

HelloWorld

print('cats','dogs','mice')

print('cats','dogs','mice',sep=',') # 使用 sep 参数替换掉默认的分隔字符串

cats dogs mice
cats,dogs,mice

def info_print( name, age ):
   print ("名字: ", name)
   print ("年龄: ", age)
   return
 
# 两次打印结果一致，说明关键字参数使用时不需要指定顺序。
info_print( age=18, name="小明" )
info_print( name='小明', age=18)

名字:  小明
年龄:  18
名字:  小明
年龄:  18

def info_print( name, age = 30 ):
   print ("名字: ", name)
   print ("年龄: ", age)
   return
 
# 调用函数时没有传入 age 参数，因此使用默认值 30
info_print( name='小明')

名字:  小明
年龄:  30

def functionname([formal_args,] *var_args_tuple ):
   "函数_文档字符串"
   function_suite
   return [expression]

# 打印任何传入的参数
def info_print( arg1, *vartuple ):
   print ("输出: ")
   print (arg1)
   print (vartuple)
 
# 后三个参数都是未命名的变量参数
info_print( 70, 60, 50 ,'ASD')
# 也可以不向函数传递未命名的变量，此时的不定长参数就是一个空元组。
info_print( 10 )

输出: 
70
(60, 50, 'ASD')
输出: 
10
()

# 计算多个数的求和
def calc_sum(*numbers):
    sum = 0
    for n in numbers:
        sum = sum + n * n
    return sum

print(calc_sum(1,2,3))
print(calc_sum(1,5,9,12,17))

14
540

def functionname([formal_args,] **var_args_dict ):
   function_suite
   return [expression]

def info_print( arg1, **vardict ):
   print ("输出: ")
   print (arg1)
   print (vardict)
 
info_print(1, a=2,b=3)

输出: 
1
{'a': 2, 'b': 3}

# 传递不可变对象的例子
def change(a):
    print(id(a))   # 指向的是同一个对象
    a=10
    print(id(a))   # 一个新对象
 
a=1
print(id(a))
change(a)

140710768919336
140710768919336
140710768919624

# 传递可变对象的例子
def changeme( mylist ):
   "修改传入的列表"
   mylist.append([1,2,3,4])
   print ("函数内取值: ", mylist)
   return
 
# 调用changeme函数
mylist = [10,20,30]
changeme( mylist )
print ("函数外取值: ", mylist)

函数内取值:  [10, 20, 30, [1, 2, 3, 4]]
函数外取值:  [10, 20, 30, [1, 2, 3, 4]]

def add_end(L=[]):
    '''在传入的列表末尾添加一个 END'''
    L.append('END')
    return L

add_end([1, 2, 3]) # 正常调用

[1, 2, 3, 'END']

add_end() # 反复调用，会一直在空列表后添加 `END`

['END', 'END', 'END', 'END', 'END', 'END']

def add_end(L=None):
    if L is None:
        L = []
    L.append('END')
    return L

add_end() # 不管运行多少次，结果都不变。

['END']

def spam():
    eggs = 31337
spam()
print(eggs)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
Cell In[76], line 5
      3     eggs = 31337
      4 spam()
----> 5 print(eggs)

NameError: name 'eggs' is not defined

def spam():
    eggs = 99
    bacon()
    print(eggs)

def bacon():
    ham = 101
    eggs = 0

spam()

99

def spam():
    print(eggs)
eggs = 42
spam()
print(eggs)

42
42

def spam():
    eggs = 'spam local'
    print(eggs)

def bacon():
    eggs = 'bacon local'
    print(eggs) 
    spam()
    print(eggs)

eggs = 'global'
bacon()
print(eggs)

bacon local
spam local
bacon local
global

def spam():
    global eggs
    eggs = 'spam'

eggs = 'global'
spam()
print(eggs)

spam

def spam():
    print(eggs) # ERROR!
    eggs = 'spam local'

eggs = 'global'
spam()

---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
Cell In[7], line 6
      3     eggs = 'spam local'
      5 eggs = 'global'
----> 6 spam()

Cell In[7], line 2, in spam()
      1 def spam():
----> 2     print(eggs) # ERROR!
      3     eggs = 'spam local'

UnboundLocalError: cannot access local variable 'eggs' where it is not associated with a value

lambda [arg1 [,arg2,....,arg3]]:expression

#  没有参数
f = lambda: "Hello, world!"
print(f())

Hello, world!

# 将参数 + 10
plus_ten = lambda a : a + 10

print(plus_ten(5))

15

# 将两个参数相加
sum = lambda arg1, arg2: arg1 + arg2
 
print ("相加后的值为 : ", sum( 10, 20 ))
print ("相加后的值为 : ", sum( 20, 20 ))

相加后的值为 :  30
相加后的值为 :  40

def myfunc(n):
  return lambda a : a * n
 
mydoubler = myfunc(2) # 得到参数*2的匿名函数
mytripler = myfunc(3) # 得到参数*3的匿名函数
 
print(mydoubler(11))
print(mytripler(11))

22
33

# 阶乘，n! = n * (n-1) * ... *1
def fact(n):
    if n ==1:
        return 1
    return n * fact(n-1)

print(fact(1)) # 1! = 1
print(fact(5)) # 5! = 120

1
120

def spam(divideBy):
    return 42 / divideBy
print(spam(2))
print(spam(12))
print(spam(0))
print(spam(1))

21.0
3.5

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
Cell In[8], line 5
      3 print(spam(2))
      4 print(spam(12))
----> 5 print(spam(0))
      6 print(spam(1))

Cell In[8], line 2, in spam(divideBy)
      1 def spam(divideBy):
----> 2     return 42 / divideBy

ZeroDivisionError: division by zero

def spam(divideBy):
    try:
        return 42 / divideBy
    except ZeroDivisionError:
        print('Error: Invalid argument.')
        
print(spam(2))
print(spam(12))
print(spam(0))
print(spam(1))

21.0
3.5
Error: Invalid argument.
None
42.0

4. 函数¶

4.1 函数的定义¶

4.2 参数¶

4.2.1 必需参数¶

4.2.2 关键字参数¶

4.2.3 默认参数¶

4.2.4 不定长参数¶

4.2.5 参数传递¶

4.3 局部和全局作用域¶

4.3.1 局部和全局变量的关系¶

4.3.2 global 语句¶

4.4 匿名函数¶

4.5 递归函数¶

4.6 补充：异常处理¶

4.7 References¶