importlib: '.submodule' is not a relative name (no leading dot)

[vadmium]

BPO	25771
Nosy	@brettcannon, @vadmium

^{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 = 'https://github.com/brettcannon'
closed_at = <Date 2015-12-04.23:20:54.862>
created_at = <Date 2015-12-01.05:31:34.060>
labels = ['easy', 'type-bug']
title = "importlib: '.submodule' is not a relative name (no leading dot)"
updated_at = <Date 2015-12-05.00:19:14.124>
user = 'https://github.com/vadmium'

bugs.python.org fields:

activity = <Date 2015-12-05.00:19:14.124>
actor = 'martin.panter'
assignee = 'brett.cannon'
closed = True
closed_date = <Date 2015-12-04.23:20:54.862>
closer = 'brett.cannon'
components = []
creation = <Date 2015-12-01.05:31:34.060>
creator = 'martin.panter'
dependencies = []
files = []
hgrepos = []
issue_num = 25771
keywords = ['easy']
message_count = 5.0
messages = ['255642', '255654', '255895', '255896', '255909']
nosy_count = 3.0
nosy_names = ['brett.cannon', 'python-dev', 'martin.panter']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25771'
versions = ['Python 3.6']

[vadmium]

>>> import importlib.util
>>> importlib.util.resolve_name(".submodule", None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/proj/python/cpython/Lib/importlib/util.py", line 26, in resolve_name
    '(no leading dot)'.format(name))
ValueError: '.submodule' is not a relative name (no leading dot)

This message sounds like nonsense. Perhaps the it should say something like:

'.submodule' should be an absolute name (no leading dot)

or:

relative import of '.submodule' not allowed outside of a package

[brettcannon]

So both proposed messages are correct depending on what you want to accomplish; it all depends on whether the leading dot was the mistake or the missing package was.

And the message does make some sense if you read it more like "'.submodule' is not a relative name (drop the leading dot)". I do agree, though, it's hard to read as written.

[python-dev]

New changeset b3a0765671d6 by Brett Cannon in branch 'default':
Issue bpo-25771: Tweak ValueError message when package isn't specified
https://hg.python.org/cpython/rev/b3a0765671d6

[brettcannon]

Fixed in default (left 3.5 alone since it technically isn't a bug fix but a clarification; fine if someone else wants to handle the backport).

[vadmium]

Thanks, the fix is fine and there is no big need to backport it. For the record, I only came across this playing with runpy. Old message:

$ python3 -m .submodule
/sbin/python3: Error while finding spec for '.submodule' (<class 'ValueError'>: '.submodule' is not a relative name (no leading dot))

New message:

$ ./python -m .submodule
/media/disk/home/proj/python/cpython/python: Error while finding spec for '.submodule' (<class 'ValueError'>: no package specified for '.submodule' (required for relative module names))