Convert SyntaxWarning exception raised at code generation time to a SyntaxError

[serhiy-storchaka]

BPO	35029
Nosy	@serhiy-storchaka
PRs	bpo-35029: Replace the SyntaxWarning exception with a SyntaxError. #9999

^{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 2018-10-21.07:28:51.979>
created_at = <Date 2018-10-20.07:24:03.894>
labels = ['interpreter-core', 'type-feature', '3.8']
title = 'Convert SyntaxWarning exception raised at code generation time to a SyntaxError'
updated_at = <Date 2018-10-21.07:28:51.978>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2018-10-21.07:28:51.978>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2018-10-21.07:28:51.979>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2018-10-20.07:24:03.894>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 35029
keywords = ['patch']
message_count = 2.0
messages = ['328130', '328192']
nosy_count = 1.0
nosy_names = ['serhiy.storchaka']
pr_nums = ['9999']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35029'
versions = ['Python 3.8']

[serhiy-storchaka]

SyntaxError contains more useful information than SyntaxWarning, and SyntaxError is special cased in tracebacks for better reporting. When SyntaxWarning is raised as an exception, the error report doesn't contain information about the source. It is hard to find the source of a SyntaxWarning. Currently:

$ ./python syntaxwarning.py 
syntaxwarning.py:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?
  x = 1; assert (x, "msg")
$ ./python -We syntaxwarning.py 
SyntaxWarning: assertion is always true, perhaps remove parentheses?
$ ./python -m syntaxwarning
/home/serhiy/py/cpython3.7/syntaxwarning.py:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?
  x = 1; assert (x, "msg")
$ ./python -We -m syntaxwarning
Traceback (most recent call last):
  File "/home/serhiy/py/cpython3.7/Lib/runpy.py", line 183, in _run_module_as_main
    mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
  File "/home/serhiy/py/cpython3.7/Lib/runpy.py", line 153, in _get_module_details
    code = loader.get_code(mod_name)
  File "<frozen importlib._bootstrap_external>", line 860, in get_code
  File "<frozen importlib._bootstrap_external>", line 791, in source_to_code
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
SyntaxWarning: assertion is always true, perhaps remove parentheses?

The proposed PR replaces SyntaxWarning with a SyntaxError if the former was raised as an exception:

$ ./python -We syntaxwarning.py 
  File "syntaxwarning.py", line 1
    x = 1; assert (x, "msg")
           ^
SyntaxError: assertion is always true, perhaps remove parentheses?
$ ./python -We -m syntaxwarning
Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/runpy.py", line 183, in _run_module_as_main
    mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
  File "/home/serhiy/py/cpython/Lib/runpy.py", line 153, in _get_module_details
    code = loader.get_code(mod_name)
  File "<frozen importlib._bootstrap_external>", line 909, in get_code
  File "<frozen importlib._bootstrap_external>", line 839, in source_to_code
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/serhiy/py/cpython/syntaxwarning.py", line 1
    x = 1; assert (x, "msg")
           ^
SyntaxError: assertion is always true, perhaps remove parentheses?

Similar replacement is already performed for warnings raised for unrecognized escape sequences at parsing time (see bpo-32912).

[serhiy-storchaka]

New changeset d31e773 by Serhiy Storchaka in branch 'master':
bpo-35029: Replace the SyntaxWarning exception with a SyntaxError. (GH-9999)
d31e773