Unhelpful UnboundLocalError due to del'ing of exception target

[warsaw]

BPO	17792
Nosy	@warsaw, @ncoghlan, @tiran, @benjaminp, @ezio-melotti, @bitdancer, @pmp-p, @ericsnowcurrently, @serhiy-storchaka, @phmc, @MojoVampire, @ilevkivskyi, @iritkatriel, @aaronwsmith
PRs	bpo-17792: more accurate error message for UnboundLocalError #24976
Files	issue17792.diff issue17792-2.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2013-04-18.23:55:22.492>
labels = ['expert-tkinter', 'type-crash', '3.11']
title = "Unhelpful UnboundLocalError due to del'ing of exception target"
updated_at = <Date 2021-10-02.03:18:05.344>
user = 'https://github.com/warsaw'

bugs.python.org fields:

activity = <Date 2021-10-02.03:18:05.344>
actor = 'sppraveenkumarinfo'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Tkinter']
creation = <Date 2013-04-18.23:55:22.492>
creator = 'barry'
dependencies = []
files = ['29936', '29989']
hgrepos = []
issue_num = 17792
keywords = ['patch']
message_count = 21.0
messages = ['187313', '187315', '187319', '187320', '187322', '187326', '187327', '187328', '187339', '187342', '187382', '187433', '187627', '187639', '187640', '187645', '389264', '394902', '394903', '402980', '402992']
nosy_count = 15.0
nosy_names = ['barry', 'ncoghlan', 'christian.heimes', 'benjamin.peterson', 'ezio.melotti', 'Arfrever', 'r.david.murray', 'pmpp', 'eric.snow', 'serhiy.storchaka', 'pconnell', 'josh.r', 'levkivskyi', 'iritkatriel', 'aaronwsmith']
pr_nums = ['24976']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue17792'
versions = ['Python 3.11']

[warsaw]

As described here:

http://www.wefearchange.org/2013/04/python-3-language-gotcha-and-short.html

the following code will produce an UnboundLocalError when the exception is triggered:

def bad():
    e = None
    try:
        do_something()
    except KeyError as e:
        print('ke')
    except ValueError as e:
        print('ve')
    print(e)

The reason is that the exception handling machinery del's e from the local namespace in order to break circular references caused by traceback. The ULE is pretty mysterious and certainly not helpful for figuring out what's going wrong (the blog post above describes how long it took me to find it ;).

Can we do better? What if instead of del'ing the target, we set it to None? We wouldn't get a ULE but the fact that print(e) would print None might be just as mysterious. Any other ideas?

(BTW, there's likely nothing to be done for Python < 3.4.)

[ezio-melotti]

Maybe we could raise a warning when the deleted name already exists in the local namespace?

(FWIW I knew about the fact that the name gets deleted, and still managed to get bitten by it a couple of times.)

[warsaw]

On Apr 19, 2013, at 12:01 AM, Ezio Melotti wrote:

Maybe we could raise a warning when the deleted name already exists in the
local namespace?

Ideally, I think a SyntaxError if you could detect a previously bound name in
the namespace being used as an exception target.

-Barry

[bitdancer]

And what if it weren't a print statement? An error is better than a "randomly" changed value, I think. I'm really not sure there is anything we can do here, beyond Ezio's warning suggestion.

[bitdancer]

This used to raise a SyntaxError. See bpo-4617.

[warsaw]

On Apr 19, 2013, at 12:37 AM, R. David Murray wrote:

And what if it weren't a print statement? An error is better than a
"randomly" changed value, I think. I'm really not sure there is anything we
can do here, beyond Ezio's warning suggestion.

Right. The point is, is it more helpful to have an unexpected value in the
local (e.g. None, which in fact may not be *totally* unexpected) or an
mystical exception? ;)

A warning would be fine. I just want to give programmers some help in trying
to figure out why their code is wrong.

[ncoghlan]

It seems to me that this kind of dubious-but-legal code is what http://docs.python.org/3/library/exceptions.html#SyntaxWarning is designed to handle.

Something like "SyntaxWarning: implicitly deleted exception variable referenced after end of except clause".

[bitdancer]

It occurs to me that one of the problems here is that the UnboundLocalError only occurs when the except clause is executed. Do you think it would be helpful and acceptable (in 3.4, as it is a behavior change) to have the 'as' variable *always* deleted at the end of the try/except block? Then you would always get the UnboundLocalError, instead of it mysteriously appearing only when the except was executed.

Granted it is a bit weird, but then so is the variable getting deleted in the first place.

[ncoghlan]

I don't think we want to mess with the control flow, as that would be even weirder and may interact strangely with else and finally clauses.

A SyntaxWarning would be unconditional (so it doesn't matter whether or not the exception is triggered at run time), happens at compile time (so developers will see it), but won't be triggered when loading from a cached bytecode file (so end users typically won't see it).

[ezio-melotti]

Attached a sketchy proof of concept. It only checks for locals, but it should probably check for globals too. Does the approach make sense?

[ericsnowcurrently]

FWIW this has come up before:

http://mail.python.org/pipermail/python-dev/2012-October/122504.html

and relatedly:

bpo-16429

[warsaw]

Ezio, the problem with your patch is that it also gives a warning on this code, which is totally safe:

def good():
    exc = None
    try:
        bar(int(sys.argv[1]))
    except KeyError as e:
        print('ke')
        exc = e
    except ValueError as e:
        print('ve')
        exc = e
    print(exc)

[ezio-melotti]

Attached a new patch that improves the following things:

• added some tests;
• the code in the previous message is now handled correctly;
• the patch now raises a proper SyntaxWarning;

There are still several problems though:

• it doesn't work for the global namespace. Is there a way to know if we are inside a function or not?
• the patch leaks, and I couldn't figure out where the leak is;
• I tried to include the name of the variable in the warning, but:
  • PyErr_WarnExplicit wants a const char* as msg and doesn't support formatting;
  • It doesn't seem possible to use other PyErr_Warn* functions here;
  • Using PyUnicode_FromFormat() works, but then I don't know how to convert the result to const char* (I used PyObject_REPR(), but that is probably wrong; PyUnicode_AS_DATA() might work but is deprecated; the PyUnicode_nBYTE_DATA() functions return a Py_UCSn*);
• more testcases are needed

While compiling I got this warning:
./setup.py:330: SyntaxWarning: "name 'why' is already defined but implicitly deleted after end of except clause"
except ImportError as why:

This comes from a code like:
try: foo()
except Exception as why: pass
try: bar()
except Exception as why: pass

and in this case there shouldn't be any warning. A possible way to fix this is to keep a "whitelist" of locals that are first defined as an except target, but then it will still break if there's a why = ... between the two try/except and it's starting to get too complicated...

[serhiy-storchaka]

• Using PyUnicode_FromFormat() works, but then I don't know how to convert the result to const char*

PyUnicode_AsUTF8()