Detect missing typeshed packages

[tobiashenkel]

Original bug report - Pyrefly type inference doesn't work on the raw PyYAML library

Describe the Bug

When running a type check with pyrefy check test.py, I expect to see no error, but I get this:

Test script:

import yaml

def test_function() -> str:
    return yaml.safe_dump({'key': 'value'})

Pyrefly output:

ERROR Returned type `bytes | str | None` is not assignable to declared return type `str` [bad-return]
 --> test.py:5:12
  |
5 |     return yaml.safe_dump({'key': 'value'})
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
 INFO 1 error

Versions used:

• pyrefly: 0.36.2
• pyyaml: 6.0.3

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeSIEAtsbgE4AuABKalVADrpdeYxhMG8BgH0wAV3QBjBhFzoAFAEomAWgB8TOAzqIuTA0zowG4uuhZsohNGBgjM4mguAByANYxSrxE1cA3VChxGFcAXyUuEAAaEHFZKDgSckQQAGImAFUEiAZSJglpWXk4Hix%2BAvoqVFF0J2wYOgV8Xwh0BhUNLR09C0NjU3MCjhAAOXrG32B8MJGo2LJjMChSQgZcdgoMgAVSJZWtDBwCJil5SABzMxq5dEIuDIBlGBgmAAsGBmI4RAB6X8W-BWhHoF1%2BMHQv0wuCkcF%2BZ3Ql2uxUhlToTFQgWgqGwsFO5wgVzoN3kTFwxBRSS4ZAYb3kqn8jTgtyYAF4mCMAMyEACMACY5ugQGFYqgZBBGQAxaAwChoLB4IhkYVAA

(Only applicable for extension issues) IDE Information

No response

[stroxler]

Thanks for the issue!

The yaml library is not typed, so you likely want to install the types-PyYAML library as a dependency in order to get the types defined in typeshed

Ideally Pyrefly could do a better job letting you know when this is a likely problem.

cc @yangdanny97 @migeed-z @rchen152 since I think this is relevant to discussion last week around how to handle missing stubs - we could potentially bundle, but even just message with a clear description about what went wrong and what to do about it would probably go a long way.