Python的变量作用域

合法

gcount = 0
 
def global_test():
    print (gcount)
global_test()

不合法，报错行：

tmp = gcount + 1

gcount = 0
 
def global_test():
    tmp = gcount + 1
    gcount = tmp
    print (gcount)
global_test()

不合法

gcount = 0
 
def global_test():
    gcount += 1
    print (gcount)
global_test()

合法

gcount = [0]
 
def global_test():
    gcount[0] += 1
    print (gcount)
global_test()

 

ChatGPT

gcount = 0
 
def global_test():
    gcount += 1
    print (gcount)
global_test()

This code will result in an error because the function tries to modify the global variable gcount without explicitly declaring it as global. Here's why:

Explanation

• In Python, variables inside a function are considered local by default.
• When you write gcount += 1, Python interprets it as:
  • Access gcount (assumes it's a local variable).
  • Increment it by 1.
  • Assign the new value back to gcount.

Since gcount is not declared as local or global, Python raises an UnboundLocalError, indicating that you're trying to use a variable before it has been assigned a value locally.

Corrected Code

To modify the global variable gcount inside the function, you need to explicitly declare it as global:

gcount = 0

def global_test():
    global gcount  # Declare gcount as global
    gcount += 1
    print(gcount)

global_test()

Output of Corrected Code

1

gcount = [0]
 
def global_test():
    gcount[0] += 1
    print (gcount)
global_test()

This code will work as expected because you are modifying the mutable object gcount (a list in this case) without needing to declare it as global. Here's why:

Explanation

• The variable gcount holds a reference to a list object [0].
• When you write gcount[0] += 1, you are modifying the first element of the list.
• Since you are not reassigning the gcount variable itself but altering the content of the list, the global keyword is not required.

Output

The output of this code will be:

[1]

Key Difference

• In the original code, you were trying to modify an immutable integer (gcount += 1), which requires reassigning the variable. For that, you need the global keyword.
• In the corrected code, you're modifying a mutable object (list), which does not require reassigning gcount, so no global declaration is necessary.

gcount = 0
 
def global_test():
    tmp = gcount + 1
    gcount = tmp
    print (gcount)
global_test()

This code will raise an error because you are trying to assign a value to gcount inside the function without declaring it as global. Here's a detailed explanation:

Explanation of Error

1. When the line tmp = gcount + 1 is executed, Python looks for the value of gcount.
   • Since gcount is a global variable, it is accessible, so tmp gets assigned 0 + 1 = 1.
2. On the next line, gcount = tmp, Python interprets gcount as a local variable because it is being assigned a value.
   • This means gcount is considered local to the function global_test.
   • However, the previous line (tmp = gcount + 1) tried to use gcount before it was defined locally.
3. This results in an UnboundLocalError because Python doesn't know about the global gcount unless you explicitly declare it as global.

Corrected Code

To fix this, you need to declare gcount as global to indicate that the function should use the global gcount:

gcount = 0

def global_test():
    global gcount  # Declare gcount as global
    tmp = gcount + 1
    gcount = tmp
    print(gcount)

global_test()

Output of Corrected Code

1

Alternatively, if you do not want to use the global keyword, you could work with a mutable object (e.g., a list):

gcount = [0]

def global_test():
    gcount[0] += 1  # Modify the first element of the list
    print(gcount[0])

global_test()

Output for List-Based Code

1