Why does Python let me use variables assigned in a nested block without the "global" keyword?

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?

A good discussion about this possible dupe question is [here](https://stackoverflow.com/q/3611760). — Jedi, Feb 01 '20 at 23:29
`userInput` is being defined in the global scope here; Python does not have block scope. — kaya3, Feb 01 '20 at 23:36
Note, for the future, that a question's title should *describe the question itself*; it shouldn't be about you, or about the general context of the question, but actually be a sentence that encapsulates the question in a nutshell, so people with the same underlying problem can find it and learn from its answers. — Charles Duffy, Feb 02 '20 at 00:16
(OdedBD was kind enough to suggest an edit with a better title, and I'm happy to have been able to approve it). — Charles Duffy, Feb 02 '20 at 00:18

Rafael Marques · Answer 1 · 2020-02-02T01:17:21.283

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

BTW -- I've simplified the code in the question to focus only on the OP's core question about variable scope. This makes the answer a bit less clear, unfortunate. — Charles Duffy, Feb 02 '20 at 00:22
I edited the post to be more specific with the post's code now — Rafael Marques, Feb 02 '20 at 01:18

score 0 · Answer 2 · edited Feb 02 '20 at 00:51

0

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

edited Feb 02 '20 at 00:51

John Kugelman

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answered Feb 01 '20 at 23:30

Oded BD

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5

actually, you don't need global to read the variable. You'll need it only if you want to change its value – Rafael Marques Feb 01 '20 at 23:46

score 0 · Answer 3 · edited Feb 02 '20 at 00:51

0

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.

edited Feb 02 '20 at 00:51

John Kugelman

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533
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answered Feb 02 '20 at 00:26

Why does Python let me use variables assigned in a nested block without the "global" keyword?

3 Answers3