Python 中的变量作用域

2024 年 8 月 29 日 | 阅读 6 分钟

Python 语言中的变量充当各种数据值和数据结构的存储单元。当一个变量被赋值给任何 Python 对象时，它会指向该对象，因为它们是指向内存位置中特定对象的引用或指针。Python 编程语言不像 C/C++/JAVA 等其他语言那样是“静态类型的”。在显式声明变量的类型或初始值之前，不需要声明。当变量被赋予初始值时，即认为它已被创建。

代码

# Python program to show how variables are assigned to the data values
   
# Assigning a variable name to an integer
integer = 45                     
   
# Assigning a variable name to a floating point number
floating = 1456.8           
   
# Assigning a variable name to a string
string = "John"             
   
print("This value is stored in the variable 'integer':- ", integer)
print("This value is stored in the variable 'floating':- ", floating)
print("This value is stored in the variable 'string':- ", string)

输出

This value is stored in the variable 'integer':-  45
This value is stored in the variable 'floating':-  1456.8
This value is stored in the variable 'string':-  John

变量作用域

Python 变量的作用域是指我们可以找到它，并在必要时访问它的区域。

全局变量和局部变量

全局变量是在任何函数外部声明和定义的变量，不特定于任何函数。程序的任何部分都可以使用它们。

代码

# Python program to show how any function can use global variables

# Declaring a global variable
a = 2

# Accessing the global variable in the main function
print("This is in the main function:- ", a)

# Defining a function that uses the global variable
def f1():
    print("This is in function 'f1()':- ", a)

# Defining another function that uses the global variable
def f2():
    print("This is in function 'f2()':- ", a)

# Calling the functions
f1()
f2()

输出

This is in the main function:- 
2
This is in function 'f1()':- 
2
This is in function 'f2()':- 
2

假设一个变量在函数的局部作用域内定义，并且它与全局变量具有相同的名称。在这种情况下，它将仅显示在该特定函数内为变量提供的值，而不是全局作用域中赋给它的值。

代码

# Python program to show function accesses the variable value that is in the function's scope

# Declaring a global variable
a = "This is assigned in the global scope."

# Defining a function that has a variable having the name same as the global variable
def func():
    a = "This is defined inside the function."
    print(a)
   
# Calling the function
func()

输出

This is defined inside the function.

在定义函数 func() 之前，变量 'a' 被赋值为字符串“This is assigned in the global scope.”。函数 func() 的 print(a) 语句将首先在其局部作用域中搜索变量 'a'。由于已经有一个变量 'a' 被赋值为不同的字符串，该函数将打印该变量的值，而不会查找全局作用域。在这种情况下，函数将不会使用全局作用域中的变量 'a' 的值。

下一个问题是，在函数 'func()' 中修改变量 'a' 的值会有什么结果？这是否也会影响全局变量 'a'？下面的代码片段用于测试这一点

代码

# Python program to show function accesses the variable value that is in the function's scope

# Declaring a global variable
a = "This is assigned in the global scope."

# Defining a function that has a variable having the name same as the global variable
def func():
    a = "This is defined inside the function."
    print("Accessed in the local scope of function:- ", a)
   
# Calling the function
func()

# Accessing the variable a in the global scope after changing its value inside the function
print("Accessed in the global scope:- ", a)

输出

Accessed in the local scope of function:-  This is defined inside the function.
Accessed in the global scope:-  This is assigned in the global scope.

我们必须使用 Python 的 global 关键字才能使前面的程序起作用。只有在赋值或更改变量值时，我们才需要在函数内部使用 Python 的 global 关键字。显示和访问全局作用域中存在的变量不需要 global 关键字。为什么？由于在 func() 内部对变量 'a' 进行了赋值，Python“假定”我们想要访问局部变量，这就是为什么第一个 print 命令返回局部作用域中变量值的原因。如果在函数内部修改或赋值了一个变量，而没有将其定义为全局变量，那么它将被视为局部变量。下面的示例演示了如何使用 global 关键字指示 Python 解释器我们希望修改全局变量

代码

# Python program to show function accesses the variable value that is in the function's scope

# Declaring a global variable
a = "This is assigned in the global scope."

# Defining a function that uses the global keyword before assigning a new value to the global variable
def func():
    global a
    a = "This is defined inside the function."
   
print("Accessed in the local scope of function:- ", a)
   
# Calling the function
func()

# Accessing the variable a in the global scope after changing its value inside the function
print("Accessed in the global scope:- ", a)

输出

Accessed in the local scope of function:-  This is defined inside the function.
Accessed in the global scope:-  This is defined inside the function.

总结全局局部变量作用域。

代码

# Python program to summarise the global and local variable scope.

# Defining a variable in the global scope
a = 'in the global scope'
   
# This function will print the global variable because there does not exist any local variable 'a'
def func():
    print('Inside func(): ', a)
   
# This function will print the value assigned to the local variable 'a'
# Because we are not using the global keyword
def local():    
    a = 'inside function local()'
    print('Inside local(): ', a)
   
# This function will modify the global variable because we are using the global keyword
def global_():    
    global a
    a = 'changed inside function global_()'
    print('Inside global_() : ', a)
   
# Calling the functions
# Value of 'a' in global scope after each function call
print('global: ', a)
func()
print('global: ', a)
local()
print('global: ', a)
global_()
print('global: ', a)

输出

global:  in the global scope
Inside func():  in the global scope
global:  in the global scope
Inside local(): 
inside function local()
global:  in the global scope
Inside global_() : 
changed inside function global_()
global:  changed inside function global_()

Nonlocal 关键字

在 Python 中，嵌套函数使用 Python 的 nonlocal 关键字进行处理。这个关键字的作用类似于 global 关键字，不同之处在于，在嵌套函数的情况下，它定义了一个变量来引用在外部封闭函数中赋给的变量，而不是全局变量。

代码

# Python program to show how the nonlocal keyword works

print ("Using the nonlocal keyword before changing a:")

# Assigning 'a' in the global scope
a = 0

# Defining a function
def outer():

    # Giving new value to 'a' in the local scope of the 'outer' function
    # Didn't use global keyword here
    a = 3
    def inner():

        # Defining the variable scope as the local scope of the 'outer' function
        nonlocal a 

        # Changing the value of 'a' in the local scope of the 'inner' function
        a = 14

        # Printing the value of 'a' in the local scope of the 'inner' function
        print("Inside 'inner' function:- ", a)
    
    # Calling the inner function
    inner()

    # Priting the value of 'a' inside the function
    print("Inside the 'outer' function:- ", a)

# Calling the main function
outer()

# Printing the value of 'a' in the global scope
print("In the global scope:- ", a)

# Performing the same steps as above without using the nonlocal keyword
print ("\nNot using the nonlocal keyword before changing a:")

# Assigning 'a' in the global scope
a = 0

def outer():
    a = 3

    def inner():

        # Not using the nonlocal keyword before assigning a new value to a
        a = 14
        print("Inside 'inner' function:- ", a)

    inner()
    print("Inside the 'outer' function:- ", a)
   
# Calling the main function
outer()

print("In the global scope:- ", a)

输出

Using the nonlocal keyword before changing a:
Inside 'inner' function:- 
14
Inside the 'outer' function:-  14
In the global scope:- 
0

Not using the nonlocal keyword before changing a:
Inside 'inner' function:- 
14
Inside the 'outer' function:-  3
In the global scope:- 
0

下一主题Python 中的字母

Python 中的变量作用域

变量作用域

全局变量和局部变量

Nonlocal 关键字

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