isort behavior depends on available libraries

[ppwwyyxx]

It appears that isort uses available libraries to determine whether a library is third-party or not. When bob is not installed, it produces:

import numpy

import bob

When bob is installed, it produces:

import bob
import numpy

This non-determinism is very undesired. A python project may have some optional dependencies that's only required by some of its files. Different developers of such project may not have the same dependencies installed and therefore get different outputs by isort. This makes it impossible to use isort as a mandatory linter or pre-commit hooks.

[ericbiewener]

Would the known_third_party option solve this?

[ppwwyyxx]

Thanks! This option solves the issue. So I'll need to add all optional dependencies to known_third_party.