Import sort: automatically detect third party packages with same base package as first party

[ivan94fi]

Hi, I have multiple python packages with the same first name, for example company.package1, company.package2, etc.
When in company.package1 Isort automatically flags company.package2 import as a third party, while Ruff detects every package starting with company as first party, changing the sort order.

If I use the following configuration:

[tool.ruff]
src = ["src"]

[tool.ruff.lint.isort]
known-third-party = ["company.package2"]

then company.package2 is correctly formatted as a third party package, but this is not scalable as I have many of these packages and I would need to configure every package differently and update every one with every new package created.

I also tried setting known-third-party = ["company.*"], but this makes all company imports as third party, even the first party ones.

Is there some option to configure Ruff the way I need? Thank you.

[hamedrabah]

This is the same-package heuristic rather than src discovery. By default, when Ruff analyzes a file inside company.package1, any import whose first package segment is company can be classified as first-party.

Disable that heuristic and let the full-path src lookup decide:

[tool.ruff]
src = ["src"]

[tool.ruff.lint.isort]
detect-same-package = false

For import company.package2, Ruff's source-root check requires the full company/package2 path to exist under this project's src. If only company/package1 exists, package2 then falls through to third-party, while package1 remains first-party.

This avoids maintaining known-third-party for every sibling namespace package. If multiple sibling packages are present in the same checkout, however, they really are all visible first-party sources to Ruff; in that case explicit sections or separate project configurations are necessary.