python全局变量_Python全局变量

小灰灰 2022-12-07 12:58 426阅读 0赞

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python全局变量

In this tutorial we will be discussing about Python global variables.

在本教程中，我们将讨论Python全局变量。

> A variable which is declared outside of any function i.e. in the global space is known as global variable.
> 
> 在任何函数外部(即在全局空间中)声明的变量称为全局变量。

Global variables are useful when multiple functions need to use the same data. In C, C++ we can use and modify global variables directly in local functions and if we declare a local variable with the same name of global variable then it shadows the global variable but this is not the case in Python.

当多个功能需要使用相同的数据时，全局变量很有用。 在C，C ++中，我们可以直接在局部函数中使用和修改全局变量，如果我们声明一个具有相同全局变量名称的局部变量，则它会掩盖全局变量，但在Python中并非如此。

Let’s see this with the help of one example :

让我们借助一个示例来了解这一点：

def foo():
        print(x)
        
    x = 5
    foo()
    print(x)

The output of the above program is:

上面程序的输出是：

*5* *5*

Here **x** is global variable and it can be accessed in function and the program works correctly but what if we try to change the variable value inside the function.

这里**x**是全局变量，可以在函数中访问它，程序可以正常运行，但是如果我们尝试在函数内部更改变量值，该怎么办。

def foo():
        x = x+2
        print(x)
        
    x = 5
    foo()
    print(x)

When we run the above code, we will get the below error:

当我们运行上面的代码时，我们将得到以下错误：

*UnboundLocalError: local variable ‘x’ referenced before assignment*

*UnboundLocalError：分配前已引用局部变量“ x”*

Here the problem is **x** in the function is now being treated as a **local variable**.

这里的问题是**函数**中的**x**现在被视为**局部变量** 。

In python if it is just reading the variable and the variable doesn’t exist locally then it will try to look into any containing scope (Global scope in our first example). But this is not the case when we are trying to change the variable value. Python assumes that any variable whose value is being changed in any function is local to the function unless explicitly told.

在python中，如果它只是读取变量而该变量在本地不存在，则它将尝试查看任何包含范围(在我们的第一个示例中为Global范围)。 但是，当我们尝试更改变量值时，情况并非如此。 Python假定，除非明确告知，否则在任何函数中其值都将被更改的任何变量都是该函数的局部变量。

To understand the above statement clearly let’s see one more example :

为了清楚地理解上述陈述，让我们再看一个示例：

def foo():
        x = 2   
        print(x)
        
    x = 5
    print(x)
    foo()
    print(x)

The output of the above code is:

上面代码的输出是：

*5* *2* *5*

If you are familiar with C or C++ you may expect:

如果您熟悉C或C ++，则可能会期望：

*5*  *2*  *2*

*5* *2* *2*

As your output but in Python the **x** declared inside the function is treated as local variable and has no relation with the **x** declared outside the function.

作为输出，但在Python中，在函数内部声明的**x**被视为局部变量，并且与在**函数**外部声明的**x**没有关系。

To use the **x** as global variable in **foo** we should use **global** keyword, **x** inside the function will then be treated as a global variable.

要在**foo中**使用**x**作为全局变量，我们应该使用**global**关键字，然后将函数内部的**x**视为全局变量。

Here is an example:

这是一个例子：

def foo():
        global x
        x = 2   
        print(x)
        
    x = 5
    print(x)
    foo()
    print(x)

The output here is:

输出是：

*5* *2* *2*

Using the global keyword we can make the function use the global copy of **x** and therefore any changes made to the function are reflected globally in **x**.

使用global关键字，我们可以使函数使用**x**的全局副本，因此对函数所做的任何更改都将全局反映在**x中** 。

**Why global keyword?**

**为什么要使用全局关键字？**

The global keyword is used because global variables are dangerous and are difficult to handle. Python wants to make sure that you really want to use global variable inside the function and therefore the global keyword is being used.

使用global关键字是因为全局变量很危险且难以处理。 Python希望确保您确实要在函数内部使用全局变量，因此正在使用global关键字。

Let’s take an example to cover all the above points.

让我们以一个示例来涵盖以上所有方面。

def hoo():
        print(x)
     
    def boo():
        x = 6
        print(x)
        
     
    def foo():
        global x
        x = 7
        print(x)
        
    x = 5
    hoo()  # print 5 
    boo()  # print 6
    hoo()  # print 5
    foo()  # print 7
    print(x) # print 7
    hoo()  # print 7
    boo()  # print 6

The output of the above program is:

上面程序的输出是：

*5* *6* *5* *7* *7* *7* *6*

Lets try to understand above code.

让我们尝试理解上面的代码。

*  First call to **hoo** prints the global variable as it is just reading the variable **x.**
    
    第一次调用**hoo会**打印全局变量，因为它只是在读取变量**x。**
 *  Calling **boo** will create a local copy of **x** inside the function and initialize it with 6 and therefore it will print 6. After the call to function is finished the local variable is discarded. This function will not affect global **x**.
    
    调用**boo**将在函数内部创建**x**的本地副本，并用6对其进行初始化，因此它将打印6。在对函数的调用完成之后，将丢弃本地变量。 此函数不会影响全局**x** 。
 *  As **x** is unaffected calling **hoo** again will print the global value of **x** i.e. 5
    
    由于**x**不受影响，再次调用**hoo**将输出**x**的全局值，即5
 *  **foo** is using the global keyword and therefore it will now use the global copy of **x** and change made will directly change global **x**.
    
    **foo**使用的是global关键字，因此它现在将使用**x**的全局副本，所做的更改将直接更改global **x** 。
 *  After the call to **foo** is executed value of global **x** is changed to 7 so **print(x)** will print 7.
    
    执行对**foo**的调用后，全局**x的**值更改为7，因此**print(x)**将输出7。
 *  **hoo** again will simply print the global value of **x** i.e. 7
    
    **hoo**将再次简单地打印**x**的全局值，即7
 *  Call to **boo** will create a local variable **x** with value 6 and will print it and global **x** will remain unchanged.
    
    调用**boo**将创建一个值为6的局部变量**x** ，并将其打印出来，而全局**x**将保持不变。

I hope now you got the idea about python global variables. Comment down below if you still have any queries.

我希望现在您对python全局变量有所了解。 如果您还有任何疑问，请在下面注释掉。

> 翻译自: [https://www.thecrazyprogrammer.com/2019/08/python-global-variables.html][https_www.thecrazyprogrammer.com_2019_08_python-global-variables.html]

python全局变量

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[https_www.thecrazyprogrammer.com_2019_08_python-global-variables.html]: https://www.thecrazyprogrammer.com/2019/08/python-global-variables.html