🔒 Global scope and UnboundLocalError

[primal gazelle]

I need help understanding why the first line in the f() raises an error. My understanding is that it should refer to the x defined outside the function.

[long palm]

@primal gazelle

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[primal gazelle]

What is the OUTPUT generated by this code?

x = 6

def f():
    print("Within f: x =", x)
    x = 12
    print("Within f: x =", x)

print("Before f: x =", x)
f()
print("After f: x =", x)

a. 
  an error message
b. 
  Before f: x = 6
  an error message
c. 
  Before f: x = 6
  Within f: x = 6
  Within f: x = 12
  After f: x = 12
d. 
  none of the above

[sour cedar]

it would if the # global x line wasn't commented out

[primal gazelle]

Sorry I added that comment when I was messing around with the question. Let me give the original question

[sour cedar]

But as for why it doesn't access the global value, it's because a function cannot access both local and global at the same time

[primal gazelle]

Even if the local variable is created after the line where it is trying to be accessed?

[sour cedar]

Yes. When a function is defined, it scans through it's code looking for any variable assignment. It knows ahead of time what local variables are expected to exist

[primal gazelle]

OH

[sour cedar]

So if the function sees x = 12 anywhere in the function (and the global keyword wasn't used), then it knows x must be a local version of x

so if you try and use print(x) before the assignment, it doesn't know to look for the global x

it assumes that you're trying to access a local x

[primal gazelle]

Thanks you for the explanation. What would the section in docs.python.org would this be explained more?

[sour cedar]

I'm not sure really

it falls under the topic of "scope" though

[primal gazelle]

Thank you !

[balmy void]

relevant literature here https://docs.python.org/3/reference/executionmodel.html#resolution-of-names

[primal gazelle]

Thank you !

[balmy void]

as usual (for anything except docs for specific functions) the python docs are boring to the point of effectively being useless, though

oh maybe not - the FAQ linked there is very useful

[long palm]

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