bpo-17792: more accurate error message for UnboundLocalError

[iritkatriel]

Currently:

>>> def g():
...    x = 42
...    del x
...    print(x)
...
>>> g()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in g
UnboundLocalError: local variable 'x' referenced before assignment
>>>

With this patch:

UnboundLocalError: local variable 'x' referenced before assignment or after deletion

https://bugs.python.org/issue17792

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[nanjekyejoannah]

Why do I think the error message is fine as is, mentioning the delete aspect doesn't sit so well with me, I mean the actual state is that the variable is unbound, which is standard enough. I don't know what @vstinner thinks of this?

[iritkatriel]

Why do I think the error message is fine as is, mentioning the delete aspect doesn't sit so well with me, I mean the actual state is that the variable is unbound, which is standard enough. I don't know what @vstinner thinks of this?

Well, the current message doesn't just say the variable is unbound. It says " local variable 'x' referenced before assignment", which is incorrect in my example.

[nanjekyejoannah]

Why do I think the error message is fine as is, mentioning the delete aspect doesn't sit so well with me, I mean the actual state is that the variable is unbound, which is standard enough. I don't know what @vstinner thinks of this?

Well, the current message doesn't just say the variable is unbound. It says " local variable 'x' referenced before assignment", which is incorrect in my example.

I see what you mean but my reasoning is, "before assignment" is kind of a synonym for unbound, which sort of makes sense to me, no?

[vstinner]

LGTM.

I'm used used to "using a variable" than "referencing a variable", but I'm fine with "local variable could have been referenced ...".

cc @pablogsal

[iritkatriel]

I see what you mean but my reasoning is, "before assignment" is kind of a synonym for unbound, which sort of makes sense to me, no?

We could do something like ‘using an unassigned variable’ if that sound better.

[vstinner]

English is not my first language, so I let the others decide. I approved the PR since it fix an issue and it's correct ;-)

[pablogsal]

I would prefer to use something generic that doesn't refer to any specific operation as there are other ways to trigger this that are not technically assignments or deletions. FOr example:

>>> def blech(x):
...    print(lel)
...    import lel
...
>>> blech(3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in blech
UnboundLocalError: local variable 'lel' referenced before assignment

[iritkatriel]

I agree, how about one of these?

UnboundLocalError: local variable 'lel' is not associated with a value

UnboundLocalError: reading undefined local variable 'lel'

[pablogsal]

I agree, how about one of these?

UnboundLocalError: local variable 'lel' is not associated with a value

UnboundLocalError: reading undefined local variable 'lel'

I personally think that these are still quite confusing. For example:

UnboundLocalError: reading undefined local variable 'lel'

I think people will still be confused with the message given that in:

x = 4
def foo():
    print(x)
    x = 3

they think the variable is defined.

UnboundLocalError: local variable 'lel' is not associated with a value

Same as before, people can still think that the variable is associated with a value later.

Maybe we could modify this slightly to:

UnboundLocalError: local variable 'lel' is not associated with a value at the time it was accessed

Maybe is too verbose....

[iritkatriel]

UnboundLocalError: local variable 'lel' is not associated with a value at the time it was accessed

Shorter:

UnboundLocalError: accessed local variable 'lel' where it is not associated with a value

[pablogsal]

UnboundLocalError: local variable 'lel' is not associated with a value at the time it was accessed

Shorter:

UnboundLocalError: accessed local variable 'lel' where it is not associated with a value

Looks great!

[iritkatriel]

I noticed another message just below for free variables - I think the same applied there -- see this unit test

cpython/Lib/test/test_scope.py

Line 270 in bb3e0c2

def testUnboundLocal_AfterDel(self):

[vstinner]

I suggest "cannot" :-)

%.200s is a ghost of the past, it can be removed.

[vstinner]

LGTM, but again, English is not my first language, so you should better rely on others opinion than mine :-D

[iritkatriel]

@pablogsal @nanjekyejoannah Are we all good here?

[pablogsal]

Are we all good here?

Ok from me 👍