英文:
exec function is not executing and is not raising error (python)
问题
我是一名初学者程序员，我试图理解这三个函数之间的区别。原来我的问题有点不同，但我简化了它，不想占用你很多时间。今天我实际上学会了exec函数，无论我如何尝试使用该函数的不同变体，我都无法理解这两个版本v1和v2之间的区别。我想使用v1版本，但它不会更改变量p5。如果有人能帮我澄清，我会很高兴。
注意：原始问题有p1，p2，p3，...，p9，并且我从用户输入中获取索引。因此，格式化字符串很重要。
p5 = 5
index_of_p = 5
def change_p5_v1():
exec(f"global p{index_of_p}")
exec(f"p{index_of_p} = 3")
print(p5) # 输出 5
def change_p5_v2():
exec("global p5")
exec("p5 = 3")
print(p5) # 输出 5
def change_p5_v3():
global p5
p5 = 3
print(p5) # 输出 3
change_p5_v1()
change_p5_v2()
change_p5_v3()
英文:
I'm a beginner programmer and I am trying to understand the difference between these three functions. Originally my problem is a bit different but I made it this easy for not to take your time a lot. I actually learned exec function today and no matter how much I try different variations with the function, I can't understand the difference between these v1 and v2. I want to use the v1 version but it doesn't change the variable p5. If anyone can clarify me I would be so glad.
NOTE: Original problem have p1,p2,p3,...,p9 and I get the index from the user input. So it is important to formatting string.
p5 = 5
index_of_p = 5
def change_p5_v1():
exec(f"global p{index_of_p}")
exec(f"p{index_of_p} = 3")
print(p5) # Output 5
def change_p5_v2():
exec("global p5")
exec("p5 = 3")
print(p5) # Output 5
def change_p5_v3():
global p5
p5 = 3
print(p5) # Output 3
change_p5_v1()
change_p5_v2()
change_p5_v3()
答案1
得分: 2
你可以在你的 exec 调用中指定要更新的命名空间：
def change_p5_v1():
exec(f"p{index_of_p} = 3", globals())
print(p5) # 输出 3
英文:
You can specify the namespace you want to update in your exec call:
def change_p5_v1():
exec(f"p{index_of_p} = 3", globals())
print(p5) # Output 3
答案2
得分: 1
exec() 依次运行是完全独立的；第一个 exec("global ...") 不会影响下一个 exec()。
无论如何，你永远不应该使用 eval() 或 exec()，除非你有一些_非常_具体的需求，并且作为初学者，嗯，你没有。
你似乎正在使用 p{index_of_p} 基本上是一个字典，或者“变量变量” - 由于你没有分配给 p，所以你也不需要 global。
p = {0: 0, 1: 10}
index = 0
def change_p():
p[index] += 1
print(p)
change_p()
print(p)
change_p()
print(p)
index = 1
change_p()
print(p)
英文:
exec()s run one after the other are entirely separate; the first exec("global ...") doesn't affect the next exec().
Anyway, you should never use eval() or exec() unless you have some very specific need, and as a beginner, you, well, don't.
What you seem to be doing with p{index_of_p} is basically a dict, or
"variable variables" - and since you're not assigning into p, you don't need global either.
p = {0: 0, 1: 10}
index = 0
def change_p():
p[index] += 1
print(p)
change_p()
print(p)
change_p()
print(p)
index = 1
change_p()
print(p)
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