Importing unstubbed submodules of stubbed modules with silent imports gives inconsistent errors

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

Start by reproducing the three commands in the issue with mypy's silent-imports handling, then trace how import x.y differs from from x import y when x has a stub and y does not. Done means the inconsistent behavior is resolved and the incomplete-stub case produces the intended diagnostic consistently.

Written by the indexing model from the issue text.

Description

bug needs discussion priority-1-normal topic-usability

In this example, the boto package has a stub, but the boto.foo module does not:

$ python3 -m mypy --py2 --silent-imports -c 'import boto.foo'
# no error
$ python3 -m mypy --py2 --silent-imports -c 'from boto import foo'
<string>:1: error: Module has no attribute 'foo'
$ python3 -m mypy --py2 --silent-imports -c 'import boto.foo; boto.foo'
<string>:1: error: "module" has no attribute "foo"
# in real code, this error is on line where boto.foo is used

As you can see, the behavior here is inconsistent. import boto.foo gives no error, but attempting to use boto.foo later in the program does give an error.

I think the correct behavior here is for import x.y with --silent-imports to be an error when x has a stub but y does not, because this feels conceptually similar to other cases of incomplete stubs. Otherwise, you'd be in danger of importing nonexistent submodules from packages you have full stubs for, with no way to lock that down. The only downside to this approach is it means you have to use a type ignore when the stubs are incomplete, but that's already the case most of the time.

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