Advanced variable (understand)

aims

Variable references
And immutable type variable
Local and global variables

01. Variable references
Variables and data are stored in the memory of
In Python function of parameter passing and return values are by reference passed

1.1 The concept of reference

In Python

Variables and data are stored separately
Data stored in a memory location
Variables stored in the address data in memory
Variables in the recorded data addresses , called references
Using the id () function can save the data to see where the variable memory address

Note: If the variable is defined, when assigned to a variable, essentially modified the data reference
variable is no longer a reference to the previous data
variable instead of a reference to the assignment of new data

Variable reference example 1.2

In Python, similar to the name of the variable paper notes attached to the data on

Definition of an integer variable a, and the value of 1

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a = 1

Will be assigned to a variable 2

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a = 2

B define an integer variable, and the value of a variable is assigned to b

Code	Icon
b = a

The variable b is attached to the second digital label 2

Pass parameters and return values of function 1.3

In Python, the function argument / return value are is by reference to the passed

def test(num):

    print("-" * 50)
    print("%d 在函数内的内存地址是 %x" % (num, id(num)))

    result = 100

    print("返回值 %d 在内存中的地址是 %x" % (result, id(result)))
    print("-" * 50)

    return  result

a = 10
print("调用函数前 内存地址是 %x" % id(a))

r = test(a)

print("调用函数后 实参内存地址是 %x" % id(a))
print("调用函数后 返回值内存地址是 %x" % id(r))

02. The variable type and immutable

Immutable type , the data memory can not be modified:
the digital type int, bool, float, complex, long (2.x)
string str
tuple tuple
Variable type , data in memory can be modified:
List list
dictionary dict

a = 1
a = "hello"
a = [1, 2, 3]
a = [3, 2, 1]

demo_list = [1, 2, 3]

print("定义列表后的内存地址 %d" % id(demo_list))

demo_list.append(999)
demo_list.pop(0)
demo_list.remove(2)
demo_list[0] = 10

print("修改数据后的内存地址 %d" % id(demo_list))

demo_dict = {"name": "name1"}

print("定义字典后的内存地址 %d" % id(demo_dict))

demo_dict["age"] = 18
demo_dict.pop("name")
demo_dict["name"] = "name2"

print("修改数据后的内存地址 %d" % id(demo_dict))

note:

The key can only be used dictionaries immutable types of data

The variable type of data changes, by a method to achieve

If you give a variable type of variable, the assignment of a new data, reference can modify
variables no longer on previous data reference
variable instead of a reference to the assignment of new data

Hash (hash)

Python is built into a function called names hash (o) of
receiving a hard type of data as a parameter
to return the result is an integer
It is a hashing algorithm , and its role is to extract the data pattern (fingerprint)
the same content to obtain the same result
different contents obtained different results
In Python, set the dictionary key-value pairs , it will first hash the key to decide how to save the data dictionary in memory, in order to facilitate the subsequent operation of the dictionary: add, delete, change,
keys on the key must be immutable data
value pairs value may be any type of data

03. Local and global variables

Local variables are the internal function of the variables defined only in the internal function
Global variables are defined outside the function variable (not a function defined in a), all functions inside can use this variable

Tip: In other development languages, most do not recommend the use of global variables - variable scope is too large, resulting in poor maintenance program!

3.1 Local variables

Local variables are the internal function of the variables defined only in the internal function
After the function is executed, the local variables inside a function, the system will be recovered
Different functions can be defined a local variable with the same name, but with each other does not affect

The role of local variables

Inside the function using, temporary hold internal functions require data

def demo1():

    num = 10

    print(num)

    num = 20

    print("修改后 %d" % num)

def demo2():

    num = 100

    print(num)

demo1()
demo2()

print("over")

Local variable life cycle

The so-called life cycle is variable from being created to the recovery system process
Local variables in a function execution will be created
After the function is executed local variable are systematically recycled
Local variables lifecycle within, can be used to store internal data using the temporary function

3.2 Global Variables

Global variables in a function outside the definition of variables, all internal functions can use this variable

# 定义一个全局变量
num = 10

def demo1():

    print(num)

def demo2():

    print(num)

demo1()
demo2()

print("over")

Function is executed, you will need to be addressed variables: Note:

First look inside the function if there is to specify the name of a local variable, if any, directly

If not, look for an external function if there is to specify the name of the global variable, if any, directly

If not, the program error!

1) function can not directly modify the global variable references

Global variables are defined outside the function variable (not a function defined in a), all functions inside can use this variable

Tip: In other development languages, most do not recommend the use of global variables - variable scope is too large, resulting in poor maintenance program!

Inside the function can obtain the corresponding reference data by a global variable
However, it is not allowed to directly modify the global variable references - the value of using a global variable modified assignment

num = 10

def demo1():

    print("demo1" + "-" * 50)

    # 只是定义了一个局部变量，不会修改到全局变量，只是变量名相同而已
    num = 100
    print(num)

def demo2():

    print("demo2" + "-" * 50)
    print(num)

demo1()
demo2()

print("over")

Note: Just inside the function defines a local variable of the same variable name it - can not directly modify global variables inside a function value

2) modify global variables inside a function value

If you need to modify the global variable in a function, you need to use global declared

num = 10

def demo1():

    print("demo1" + "-" * 50)

    # global 关键字，告诉 Python 解释器 num 是一个全局变量
    global num
    # 只是定义了一个局部变量，不会修改到全局变量，只是变量名相同而已
    num = 100
    print(num)

def demo2():

    print("demo2" + "-" * 50)
    print(num)

demo1()
demo2()

print("over")

3) global variable defined positions

To ensure that all functions are able to correctly use global variables, should global variables defined above other functions

a = 10

def demo():
    print("%d" % a)
    print("%d" % b)
    print("%d" % c)

b = 20
demo()
c = 30

note

Because the global variable c, after calling the function is only defined when the function is executed, the variable has not been defined, so the program will complain!

Code structure diagram is as follows

python advanced variable basis _

4) recommended that a global variable named

In order to avoid local and global variables confusion, when you define global variables, some companies there will be some development requirements, such as:
global variables before the name should be increased g_ or gl_ prefix
Tip: The specific format requirements, each company requires some differences may