为什么 Python 让我使用在没有“global”关键字的嵌套块中分配的变量？

Question

Why does this program work even if I do not define the userInput variable in the global scope?

intInput = True
while intInput == True:
    try:
        userInput = int(input())
        intInput = False
    except ValueError: 
        print('You must enter an integer.')

print(userInput)  # shouldn't this fail, since userInput was defined in a block?

Answer 1

Your code works because python don't have block scopes . You can learn more about it on this thread: Block scope in Python

If you define your variable inside a while statement, it's the same thing as defining your variable at the global scope (in your case).

intInput = True
while intInput == True:
    try:
        # userInput here is global, because it's inside a while statement only
        userInput = int(input())
        intInput = False
    except ValueError: 
        print('You must enter an integer.')

print(userInput)

Basically, the variable declared inside a "block scope" , will have its scope from where it is being declared.

For example, if your code was inside a function, then the userInput variable would have the function's scope, and then, your code would generate an error:

intInput = True

def do_your_thing():
    while intInput == True:
        try:
            # userInput being declared here will have the function's scope
            userInput = int(input())
            intInput = False

        except ValueError: 
            print('You must enter an integer.')

# you are trying to access userInput outside its scope
print(userInput)

Now, if you try to run this code, you'll see the error:

Traceback (most recent call last):
  File "teste.py", line 28, in <module>
    print(userInput)
NameError: name 'userInput' is not defined

Answer 2

In Python you don't need to define the variables in the global scope. Everywhere you put var_name = value you have a new variable.

If you want to declare a global variable and then use it in function or class scope you should use the global keyword like this:

test = 123
def func()
   global a
   print(a)  # 123

Answer 3

The code you have is ok and you did define it globally since you put in in a loop instead of a function. If you defined it in a function then it would give you an error.