Incomplete traceback with exec on SyntaxError

[stefanseefeld]

BPO	30496
Nosy	@brettcannon, @stefanseefeld, @ncoghlan, @benjaminp, @serhiy-storchaka, @1st1
Files	pyerror.tgz

^{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 = <Date 2017-05-29.18:46:42.063>
created_at = <Date 2017-05-28.13:28:54.786>
labels = ['interpreter-core', 'invalid', 'type-bug', '3.7']
title = 'Incomplete traceback with `exec` on SyntaxError'
updated_at = <Date 2017-05-29.18:46:42.062>
user = 'https://github.com/stefanseefeld'

bugs.python.org fields:

activity = <Date 2017-05-29.18:46:42.062>
actor = 'stefan'
assignee = 'none'
closed = True
closed_date = <Date 2017-05-29.18:46:42.063>
closer = 'stefan'
components = ['Interpreter Core']
creation = <Date 2017-05-28.13:28:54.786>
creator = 'stefan'
dependencies = []
files = ['46908']
hgrepos = []
issue_num = 30496
keywords = []
message_count = 5.0
messages = ['294645', '294691', '294699', '294707', '294709']
nosy_count = 6.0
nosy_names = ['brett.cannon', 'stefan', 'ncoghlan', 'benjamin.peterson', 'serhiy.storchaka', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30496'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7']

[stefanseefeld]

The following code is supposed to catch and report errors encountered during the execution of a (python) script:

import traceback
import sys

try:
    env = {}
    with open('script') as f:
        exec(f.read(), env)
except:
    type_, value_, tb = sys.exc_info()
    print (traceback.print_tb(tb))

However, depending on the nature of the error, the traceback may contain the location of the error within the executed script file, or it may only report the above exec(f.read(), env) line.

The attached tarball contains both the above as well as a 'script' that exhibit the problem.

Is this a bug or am I missing something ? Are there ways to work around this, i.e. determine the correct (inner) location of the error ?

(I'm observing this with both Python 2.7 and Python 3.5)

[stefanseefeld]

Some further experiements:

Replacing the exec(f.read(), env) line by

code = compile(f.read(), 'script', 'exec')
exec(code, env)

exhibits the same behaviour. If I remove the try...except, the correct
(full) traceback is printed out. So it looks like the issue is with the traceback propagation through exception handlers when the error happens during parsing.

[stefanseefeld]

Answering my own question:

It appears I can get the location of a syntax error by inspecting the
raised SyntaxError, which solves my specific use-case.
The bug remains, though: The traceback is incomplete if it stems from a syntax error.

[serhiy-storchaka]

The traceback contain the location of the code that raised an exception when executed. In case of NameError this is a line in your script "x=u". In case of SyntaxError the code that failed is not in your script (it still is not executed), but in a compiler implicitly called by exec(). The line with exec() is correctly reported.

The behavior looks correct to me.

[stefanseefeld]

OK, fair enough, that makes sense. As I said, in my last message, I was mainly simply trying to figure out the exact location of the error in the executed script, which I got from inspecting the SyntaxError.
So if all of this is expected behaviour, I think we can close this issue.

Many thanks for following up.