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Python Games and login system _

01-try-except exception handling

02-continue And break The difference between

03- Human computer interaction explanation

04- String type and operation

05-datetime Kujing lecture  

06-pyinstaller- Convert code into program

Catalog

  Series column ：

One . Definition of function

  Two . The process of calling letters

3、 ... and .lambda function  

Four . Parameter passing of function ：

    1. Optional parameters ：

    2.  Variable quantity parameters ：

5、 ... and . The role of functions on variables   

6、 ... and . Recursion of a function                                                                                      

 1. Definition of function recursion ：                                                                                                        

 2. The use of recursion ： 

Xiaobian talks freely ：

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One . Definition of function

A function is a segment of 、 Reusable statement groups , Use the function name to represent and call the function through the function name . A function can also be regarded as a subroutine with a name , Execution can be called where needed , There is no need to write these statements repeatedly at every place of execution . Each time you use the function, you can provide different parameters as input , To realize the processing of different data ; After function execution , You can also feed back the corresponding processing results .
Functions can perform specific functions , Similar to black box , The use of functions does not need to understand the internal implementation principle of functions , As long as we know the input and output mode of the function . Strictly speaking , A function is a functional abstraction .
Some functions are written by users themselves , It's called a custom function ;Python The installation package also comes with some functions and methods , Include Python Built in functions ( Such as abs(、eval())、Python Functions in the standard library ( Such as math In the library sqrt()) etc. .
There are two main purposes of using functions : Reduce programming difficulty and code reuse . A function is a functional abstraction , It can be used to decompose a complex large problem into a series of simple small problems , Then continue to divide the small problem into smaller problems , When the problem is refined enough to be simple , We can divide and rule , Write a program for each small problem , And encapsulated by function , When all the small problems are solved , The big problem will be solved .                                                                                                                      Python Use def Reserved words define a function , The grammatical form is as follows :

def< Function name >(< parameter list >):
< The body of the function >
return < Return value list >
The function name can be any valid Python identifier ; The parameter list is the value passed to the function when it is called , There can be zero 、 One or more , When multiple parameters are passed, each parameter is separated by a comma , Keep parentheses when there are no arguments . The parameters in the parameter list in the function definition are formal parameters , Referred to as “ The formal parameter function body is the code executed each time the function is called , Consisting of one or more lines of statements . Use reserved words when you need to return a value return And return value list , Otherwise, the function can not return sentence , At the end of the function body ! Set to return control to the caller .    for example ： Here's a birthday song

t=input(" Please enter your name ：")
def happy():
    print("happy birthday to you")
def happy1(name):
    happy()
    happy()
    print("happy birthday dear {}".format(name))
happy1(t)

After definition , In this way, the function can be called multiple times  , Avoid code reuse . It also reduces the difficulty of programming .

  Two . The process of calling letters

To call a function, the program needs to execute the following 4 A step . 
(1) The caller pauses execution at the call
(2) Copy arguments to functions on call The parameter of . 
(3) Execute the function body statement .
(4) The return value is given at the end of the function call , The program returns to the pause before the call and continues to execute .

3、 ... and .lambda function  

The syntax is as follows :
< Function name > = lambda < parameter list >:< expression >
lambda Functions are the same as normal functions , Equivalent In the following form : 
def < Function name >(< parameter list >):
return < expression >
In short ,lambda The function is used to define simple Single 、 Functions that can be represented on one line , Returns a function type  

f=lambda x,y:x+y
type(f)
<class 'function'>
>>>f(10,20)
22

Four . Parameter passing of function ：

    1. Optional parameters ：

Because the parameters need to be input in order when calling the function , Optional parameters must be defined after non optional parameters .

def dup(str,times=3):

That is... With default values times Must be placed in a non optional parameter str Back .

    2.  Variable quantity parameters ：

Variable quantity parameter by adding （*） Realization . And can only appear at the end of the parameter list .

def t(x,*y):
  ....
>>>t(1,2,3,4)
# among 2,3,4 Is passed as a tuple to y

return Can return 0 individual ,1 individual , Multiple return values , When multiple values are returned , Save in tuple type .

5、 ... and . The role of functions on variables   

Variables include ： Global and local variables     1. Global variables ： It usually indicates variables other than the internal variables of the function , In the whole process .2. local variable ： Variables defined within a function , If you add a reserved word global, You can turn a local variable into a global variable , For list types , You can use the global list directly , Without the need for global To declare . If there is a real created and named ls A list of , Operating this list in this function does not change the global variable .

To sum up ,Python The effect of functions on variables Abide by the following principles . 
(1) A simple data type variable has the same name as a global variable or not , Variables are released only after the function is created and used inside the function and exits , If there is a global variable with the same name , Its value remains unchanged .
(2) Simple data type variables in use global After the reservation statement , Use as a global variable , After the function exits, the variable remains and the value is changed by the function .
(3) For global variables with combined data types , If there is no real variable with the same name created inside the function , The value of the global variable can be directly used and modified inside the function .
(4) If a combined data type variable is actually created inside the function , Whether there is a global variable with the same name or not, the function only operates on local variables , After the function exits, the local variable is released , The global variable value remains unchanged . 

6、 ... and . Recursion of a function                                                                                      

 1. Definition of function recursion ：

Call function in function definition middle note , Known as the recursive . Recursion has two important features ：（1） There are one or more base cases , The base case does not need to recurse again , It's just a definite expression .（2） Therefore, the recursive chain should end with one or more base examples .                                                                            

2. The use of recursion ： 

  Illustrate with examples ： Find the factorial of a number

def fact(n):
    if fact==0:
        return 1
    else:
        return n*fact(n-1)
num=eval(input(" Please enter an integer ："))
print(fact(abs(int(num))))

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