Add inspect.locate and inspect.resolve

[merwok]

BPO	12915
Nosy	@terryjreedy, @vsajip, @ncoghlan, @ned-deily, @ezio-melotti, @merwok, @Trundle, @meadori, @durban, @ericsnowcurrently, @berkerpeksag, @1st1, @aixtools, @RJ722
PRs	bpo-12915: Add pkgutil.resolve_name #18310 bpo-12915: Improve Unicode support for package names and attributes. #18517 bpo-12915: Fix test to recognize when filesystem encoding cannot encode Unicode string #18720
Files	issue12915.diff issue12915_v2.diff

^{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 2020-03-03.10:12:12.716>
created_at = <Date 2011-09-06.16:30:32.929>
labels = ['type-feature', 'library', '3.9']
title = 'Add inspect.locate and inspect.resolve'
updated_at = <Date 2020-12-31.14:16:33.047>
user = 'https://github.com/merwok'

bugs.python.org fields:

activity = <Date 2020-12-31.14:16:33.047>
actor = 'RJ722'
assignee = 'none'
closed = True
closed_date = <Date 2020-03-03.10:12:12.716>
closer = 'vstinner'
components = ['Library (Lib)']
creation = <Date 2011-09-06.16:30:32.929>
creator = 'eric.araujo'
dependencies = []
files = ['28277', '28500']
hgrepos = []
issue_num = 12915
keywords = ['patch']
message_count = 32.0
messages = ['143626', '143796', '143911', '143960', '145941', '146020', '147597', '178625', '182938', '183109', '183115', '209878', '209893', '209895', '209902', '209903', '360664', '360681', '360710', '362000', '362007', '362022', '362023', '362132', '362161', '362881', '362987', '363023', '363044', '363250', '363251', '372552']
nosy_count = 15.0
nosy_names = ['terry.reedy', 'vinay.sajip', 'ncoghlan', 'ned.deily', 'ezio.melotti', 'eric.araujo', 'Trundle', 'meador.inge', 'daniel.urban', 'python-dev', 'eric.snow', 'berker.peksag', 'yselivanov', 'Michael.Felt', 'RJ722']
pr_nums = ['18310', '18517', '18720']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue12915'
versions = ['Python 3.9']

[merwok]

The need to resolve a dotted name to a Python object is spreading in the stdlib: pydoc has locate and resolve, packaging has util.resolve_name, unittest has something else, etc. For the benefit of stdlib maintainers as well as the community, I think such functionality should be exposed publicly by the inspect module.

[terryjreedy]

To be a little clearer, this is about dotted import names, not regular dotted names.
pydoc.locate(path, forceload=0):
"""Locate an object by name or dotted path, importing as necessary"""
pydoc.resolve(thing, forceload=0):
"""Given an object or a path to an object, get the object and its name."""

It is not completely clear to me how this is different from using __import__ but I will believe it is. It something like this is in at least 3 places, +1 on factoring it out to one place.

[merwok]

Sorry if I was unclear; the functions work with a dotted name as a string and resolve it to an object. __import__ works with module, whereas this kind of functions work with mod.name, pkg.mod.name.attr, etc., that is, they combine import and getattr.

[merwok]

In addition, error handling/reporting is not trivial to get right. We’ve had to fix the code in distutils2 and it’s still not quite right (bpo-12703).

I opened this report because I’d like to see all stdlib modules use the same functions and I’d prefer people to copy-paste the same robust code for backports.

[python-dev]

New changeset 1405df4a1535 by Éric Araujo in branch 'default':
Expand tests and fix bugs in packaging.util.resolve_name.
http://hg.python.org/cpython/rev/1405df4a1535

[vsajip]

The version in logging.config appears to be doing the same job, but is shorter:

def _resolve(name):
    """Resolve a dotted name to a global object."""
    name = name.split('.')
    used = name.pop(0)
    found = __import__(used)
    for n in name:
        used = used + '.' + n
        try:
            found = getattr(found, n)
        except AttributeError:
            __import__(used)
            found = getattr(found, n)
    return found

The line "used = used + '.' + n" could of course be improved.

[python-dev]

New changeset 5df1065ddb8b by Éric Araujo in branch 'default':
Expand tests and fix bugs in util.resolve_name.
http://hg.python.org/distutils2/rev/5df1065ddb8b

[berkerpeksag]

Attaching new patch to address Vinay Sajip's suggestions on Rietveld. Thank you, Vinay.

[ncoghlan]

Something to consider with these functions: it is probably desirable to also support the popular alternate notation which uses an explicit ":" to separate the module name from the reference within the module.

For example, setuptools entry points and nose test references both use that format.

The alternate notation should be fairly easy to both detect and handle through "name.partition(':')"

[ezio-melotti]

Can you provide a couple of examples of that notation?

[vsajip]

Can you provide a couple of examples of that notation?

entry_points = """
[console_scripts]
pybabel = babel.messages.frontend:main

[distutils.commands]
compile_catalog = babel.messages.frontend:compile_catalog
extract_messages = babel.messages.frontend:extract_messages
init_catalog = babel.messages.frontend:init_catalog
update_catalog = babel.messages.frontend:update_catalog

[distutils.setup_keywords]
message_extractors = babel.messages.frontend:check_message_extractors

[babel.checkers]
num_plurals = babel.messages.checkers:num_plurals
python_format = babel.messages.checkers:python_format

[babel.extractors]
ignore = babel.messages.extract:extract_nothing
python = babel.messages.extract:extract_python
javascript = babel.messages.extract:extract_javascript
"""

Source:

http://babel.edgewall.org/browser/trunk/setup.py#L38

[vsajip]

Another question to consider: is inspect the best place for this? I don't think it is, because

(a) It's not really an inspection facility
(b) Importing inspect to get this functionality would pull in lots
of stuff which wouldn't be used in the typical use case.

I think it makes more sense for it to be in importlib. Accordingly adding Brett to nosy, for his thoughts.

[brettcannon]

importlib.util.resolve_name() already exists for resolving an explicit relative import name to an absolute one, so closing this as out of date.