python-为什么可以在主作用域中声明的函数中使用变量而不是全局变量 [英] python - why is it possible to use a variable in a function that is declared in the main scope and is not global

问题描述

我最近注意到python所没有的某些东西，可以检查/访问在主作用域中声明的函数内部的变量，例如:

I recently noticed something that I was not expected with python, it is possible to check/access a variable inside a function that is declared in the main scope, e.g.:

def my_func(mval):
    print ("parameter:", mval)
    print ("myvar:", myvar)
    #myvar = "value" this is not allowed

print ("IN MAIN")
myvar = "123"
my_func(999)

输出:

IN MAIN
parameter: 999
myvar: 123

1. 为什么 my_func 可以访问 myvar ?然后为什么失败了试图改变价值?
2. 有没有办法防止这种行为，所以在这种情况下，它会说未定义变量 myvar 吗?

1. Why my_func can access the myvar? And then why it fails when trying to change the value?
2. Is there a way to prevent this behavior, so in this case it would say variable myvar is not defined?

我知道我们可以使用 global 来完成这项工作，这是有道理的，因为我们明确表示要访问的函数范围之外还有一个变量.

I know we could use global to make this work and it would make sense since we are explicitly saying there is a variable outside of the function scope that we want to access.

我发现这很棘手，因为它可能导致错误.

I find this tricky because it could lead to errors..

推荐答案

当您在 my_func 中显示 myvar 的内容时，使用您提供的确切代码作为参考，将检查全局变量表以查看它是否在执行中先前定义，这就是它起作用的原因.然后，当您尝试通过取消注释 myvar ="value" 在函数中为其分配值时，它将尝试将 myvar 定义为 my_func的局部变量，它隐藏了对 myvar 的全局实例的引用.

When you display the content of myvar in my_func with the exact code you gave as reference, it will check the global variable table to see if it was previously defined in the execution, which is why it works. Then, when you try to assign a value to it in the function by uncommenting myvar = "value", it tries to define myvar as a local variable of my_func, which shadows the reference to the global instance of myvar.

我不确定使用 global 关键字会引起什么问题，但是它应该可以正常工作.这是一个如何使用它的示例:

I'm not exactly sure how using the global keyword would cause any issue, but it should be working as intended. Here is an example of how you can use it:

def my_func(mval):
    global myvar
    print ("parameter:", mval)
    print ("myvar:", myvar)
    myvar = "value"

print ("IN MAIN")
myvar = "123"
my_func(999)
print("myvar after my_func:", myvar)

输出:

IN MAIN
parameter: 999
myvar: 123
myvar after my_func: value

如果您真的不想使用 global 关键字，则可以通过将 myvar 作为 my_func 并返回修改后的值以在主范围中重新分配它:

If you really don't want to use the global keyword, you can achieve the desired behavior by passing myvar as a parameter of my_func and return the modified value to reassign it in the main scope:

def my_func(mval, myvar):
    print ("parameter:", mval)
    print ("myvar:", myvar)
    myvar = "value"
    return myvar

print ("IN MAIN")
myvar = "123"
myvar = my_func(999, myvar)
print ("myvar after my_func:", myvar)

输出:

IN MAIN
parameter: 999
myvar: 123
myvar after my_func: value

如果已经构建了 my_func 返回一个值，请记住您可以在Python中返回多个值.因此，假设 my_func 将返回名为 returnvar 的变量，则上述代码可以写为:

If my_func was already built to return a value, remember that you can return multiple values in Python. Therefore, assuming my_func would return a variable called returnvar, the above code could be written as:

def my_func(mval, myvar):
    returnvar = mval + 1
    print ("parameter:", mval)
    print ("myvar:", myvar)
    myvar = "value"
    return myvar, returnvar

print ("IN MAIN")
myvar = "123"
myvar, returnvar = my_func(999, myvar)
print ("myvar after my_func:", myvar)
print ("returnvar:", returnvar)

输出:

IN MAIN
parameter: 999
myvar: 123
myvar after my_func: value
returnvar: 1000

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