极客教程Python中的变量范围
Python语言中的变量是各种数据值和数据结构的存储单位。当一个变量被分配给任何Python对象时，它就指向那个对象，因为它们是内存位置中那个特定对象的引用，或者说是一个指针。Python编程语言不是 “静态类型”，与其他语言如C/C++/JAVA不同。变量在使用前不需要声明其类型或初始值。当一个变量最初被赋予一个值时，它被认为已经形成。
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
变量的范围
一个Python变量的范围指的是我们可以找到它的地方，如果有必要，可以访问它。
全局和局部变量
全局变量是指我们在任何函数之外声明和定义的变量，但不是专门针对任何函数的。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
假设一个变量被定义在该函数的局部范围内，并且与全局变量的名称相同。在这种情况下，它将只显示在该特定函数内提供给该变量的值，而不是在全局范围内分配的值。
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’被分配到一个字符串中，该字符串指出：”这是在全局范围内分配的。”。函数func()的print(a)语句将首先在其局部范围内搜索变量’a’。由于已经有一个变量’a’分配给了一个不同的字符串，该函数将打印这个变量的值，而不会进入全局范围。在这种情况下，该函数不会使用全局范围内的变量’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。它是否也会对全局变量’a’产生影响？下面的代码片断用来测试。
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全局关键字来使前面的程序发挥作用。只有在进行赋值或改变变量值时，我们才需要在函数内部使用Python global关键字。在显示和访问存在于全局范围内的变量时，不需要使用 global 关键字。为什么呢？因为在 func() 中对变量 ‘a’ 的赋值，Python “假定 “我们想要访问局部变量，这就是为什么最初的 print 命令会返回局部范围内的变量值。如果一个变量在函数中被修改或赋值，而没有被定义为全局变量，它就被认为是一个局部变量。下面的例子演示了如何使用 global 关键字来指示 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.
总结全局局部变量的范围。
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_()
非本地关键词
在Python中，嵌套函数是用Python非本地关键字来处理的。这个关键字的作用类似于全局关键字，只是在嵌套函数的情况下，它定义了一个变量，指的是外层包围函数中分配的变量，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