None value in sys.modules no longer blocks import

[tiran]

BPO	31642
Nosy	@warsaw, @brettcannon, @ncoghlan, @tiran, @ericsnowcurrently, @serhiy-storchaka
PRs	bpo-31642: Restore blocking "from" import by setting None in sys.modules. #3834 [3.6] bpo-31642: Restore blocking "from" import by setting None in sys.modules. (GH-3834). #3923

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

Show more details

GitHub fields:

assignee = None
closed_at = <Date 2017-10-08.09:55:13.546>
created_at = <Date 2017-09-29.20:21:12.164>
labels = ['interpreter-core', 'type-bug', '3.7']
title = 'None value in sys.modules no longer blocks import'
updated_at = <Date 2017-10-08.09:55:13.493>
user = 'https://github.com/tiran'

bugs.python.org fields:

activity = <Date 2017-10-08.09:55:13.493>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2017-10-08.09:55:13.546>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2017-09-29.20:21:12.164>
creator = 'christian.heimes'
dependencies = []
files = []
hgrepos = []
issue_num = 31642
keywords = ['patch', '3.6regression']
message_count = 5.0
messages = ['303356', '303374', '303396', '303900', '303904']
nosy_count = 6.0
nosy_names = ['barry', 'brett.cannon', 'ncoghlan', 'christian.heimes', 'eric.snow', 'serhiy.storchaka']
pr_nums = ['3834', '3923']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31642'
versions = ['Python 3.6', 'Python 3.7']

[tiran]

Since Python 3.6, the blocking of imports is broken for 'from package import module' imports.

$ mkdir a
$ touch a/__init__.py a/b.py

>>> import sys
>>> sys.modules['a.b'] = None
>>> from a import b
>>> b is None
True
>>> import a.b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: import of 'a.b' halted; None in sys.modules

Tests with Python 2.7 to master:

$ python2.7 -c "from a import b; print(b)"
<module 'a.b' from 'a/b.py'>
$ python2.7 -c "import sys; sys.modules['a.b'] = None; from a import b"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: cannot import name b
$ python2.7 -c "import sys; sys.modules['a.b'] = None; from a import b"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: cannot import name b
$ python3.4 -c "import sys; sys.modules['a.b'] = None; from a import b"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: import of 'a.b' halted; None in sys.modules
$ python3.5 -c "import sys; sys.modules['a.b'] = None; from a import b"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: import of 'a.b' halted; None in sys.modules
$ python3.6 -c "import sys; sys.modules['a.b'] = None; from a import b"
$ ./python -c "import sys; sys.modules['a.b'] = None; from a import b"

[serhiy-storchaka]

This is a side effect of bpo-15767. The exception is raised in _find_and_load, but it is always silenced in _handle_fromlist.

[serhiy-storchaka]

PR 3834 provide possible solution of this issue.

Initially import errors were silenced for solving bpo-15715. An import error raised when import was blocked by setting None in sys.modules was not silenced because it has different error message. I don't know whether this was intentional. In bpo-15767 (5fdb8c897023) the error message test was replaced by the type test, and that exception become silenced (I suppose it was not intentional).

[serhiy-storchaka]

New changeset f07e2b6 by Serhiy Storchaka in branch 'master':
bpo-31642: Restore blocking "from" import by setting None in sys.modules. (bpo-3834)
f07e2b6

[serhiy-storchaka]

New changeset 6f059ab by Serhiy Storchaka in branch '3.6':
[3.6] bpo-31642: Restore blocking "from" import by setting None in sys.modules. (GH-3834). (bpo-3923)
6f059ab