Have importlib.resources.contents() return an interable instead of an iterator

[brettcannon]

BPO	33254
Nosy	@warsaw, @brettcannon, @miss-islington
PRs	bpo-33254: do not return an empty list when asking for the contents of a namespace package #6467 [3.7] bpo-33254: do not return an empty list when asking for the contents of a namespace package (GH-6467) #6664

^{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 2018-04-30.19:44:12.799>
created_at = <Date 2018-04-10.00:27:02.060>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'Have importlib.resources.contents() return an interable instead of an iterator'
updated_at = <Date 2018-04-30.19:44:12.799>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2018-04-30.19:44:12.799>
actor = 'brett.cannon'
assignee = 'brett.cannon'
closed = True
closed_date = <Date 2018-04-30.19:44:12.799>
closer = 'brett.cannon'
components = ['Library (Lib)']
creation = <Date 2018-04-10.00:27:02.060>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 33254
keywords = ['patch']
message_count = 5.0
messages = ['315155', '315156', '315170', '315959', '315961']
nosy_count = 3.0
nosy_names = ['barry', 'brett.cannon', 'miss-islington']
pr_nums = ['6467', '6664']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33254'
versions = ['Python 3.7', 'Python 3.8']

[brettcannon]

If you look at

cpython/Lib/importlib/resources.py

Line 247 in 0f914b5

return []

you will notice that the generator for importlib.resources.contents() return an empty list. While the intent was to have no values be returned by contents(), by virtue of being a generator that empty list is actually being returned as a value.

[warsaw]

Note that this is in the case where a user is asking for the contents of a namespace package (which by definition, can't have resources).

+1 on fixing this API wart.

Will you @brett.cannon will submit a PR? I'll do a quick review and then backport the change to the standalone library.

[brettcannon]

I had to stare at it for a while since I knew it was for namespace packages and I'm so used to them returning a magical value to signal that fact, but then realize we don't even support namespace packages so it wasn't magical at all. ;)

And yes, I'll do a PR and make you a reviewer probably this Friday.

[brettcannon]

New changeset 3ab9365 by Brett Cannon in branch 'master':
bpo-33254: do not return an empty list when asking for the contents of a namespace package (GH-6467)
3ab9365

[miss-islington]

New changeset 2e5fa38 by Miss Islington (bot) in branch '3.7':
bpo-33254: do not return an empty list when asking for the contents of a namespace package (GH-6467)
2e5fa38