SystemError when compiling deeply nested for loops

[pppery]

BPO	27514
Nosy	@brettcannon, @birkenfeld, @ncoghlan, @benjaminp, @serhiy-storchaka, @1st1, @pppery, @ammaraskar
Files	nested_blocks.diff nested_blocks.diff2

^{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 2016-07-15.05:02:21.177>
created_at = <Date 2016-07-14.12:33:51.333>
labels = ['interpreter-core', 'type-bug']
title = 'SystemError when compiling deeply nested for loops'
updated_at = <Date 2016-07-15.05:02:21.167>
user = 'https://github.com/pppery'

bugs.python.org fields:

activity = <Date 2016-07-15.05:02:21.167>
actor = 'python-dev'
assignee = 'none'
closed = True
closed_date = <Date 2016-07-15.05:02:21.177>
closer = 'python-dev'
components = ['Interpreter Core']
creation = <Date 2016-07-14.12:33:51.333>
creator = 'ppperry'
dependencies = []
files = ['43725', '43726']
hgrepos = []
issue_num = 27514
keywords = ['patch']
message_count = 7.0
messages = ['270405', '270409', '270410', '270445', '270446', '270462', '270464']
nosy_count = 9.0
nosy_names = ['brett.cannon', 'georg.brandl', 'ncoghlan', 'benjamin.peterson', 'python-dev', 'serhiy.storchaka', 'yselivanov', 'ppperry', 'ammar2']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27514'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']

[pppery]

The following code:
for a in range(26):
for b in range(26):
for c in range(26):
for d in range(26):
for e in range(26):
for f in range(26):
for g in range(26):
for h in range(26):
for i in range(26):
for j in range(26):
for k in range(26):
for l in range(26):
for m in range(26):
for o in range(26):
for p in range(26):
for q in range(26):
for r in range(26):
for s in range(26):
for t in range(26):
for u in range(26):
for v in range(26):
for w in range(26):
pass
fails to compile with SystemError:too many statically nested blocks, which was described as a serious bug in bpo-27081.

[serhiy-storchaka]

There is a static limit for the number of statically nested blocks. Getting rid of this limit looks not easy.

But SystemError is not an exception that should be raised. SystemError is for errors that can't be occurred in normal case. It should only be caused by incorrect use of C API or hacking Python internals. I think SyntaxError is more appropriate in this case (as in case of passing more than 255 arguments to a function).

[ncoghlan]

+1 for Serhiy's recommendation. It's OK for the compiler to say "Don't do that", but the correct error is SyntaxError, even when it's a limitation later in the code generation pipeline (rather than failing to comply with the language grammar)

[ammaraskar]

This patch changes it to a SyntaxError instead of a SystemError.

[ammaraskar]

Amended patch with the comment fixed to refer to just the issue numbers according to advice from bitdancer.

[serhiy-storchaka]

LGTM.

[python-dev]

New changeset e6e7c8368c70 by Benjamin Peterson in branch '3.5':
make too many nested blocks be a SyntaxError instead of a SystemError (closes bpo-27514)
https://hg.python.org/cpython/rev/e6e7c8368c70

New changeset 345ec7455b75 by Benjamin Peterson in branch '2.7':
make too many nested blocks be a SyntaxError instead of a SystemError (closes bpo-27514)
https://hg.python.org/cpython/rev/345ec7455b75

New changeset d1da87d8b29c by Benjamin Peterson in branch 'default':
merge 3.5 (bpo-27514)
https://hg.python.org/cpython/rev/d1da87d8b29c