Document rules for conditional imports

Conditional imports. in our case, optional 3rd party dependencies, are not covered by PEP-8 and gives us some flexibility. At the same time, this could cause ambiguity. How about agreeing on some consensus and document it where and how they should appear in the code?

Conditional imports in Optuna look something like the following.

import numpy 

import optuna

# The conditional import comes in a separate island after standard imports.
try:
    import cma
    _available = True
except ImportError:
    _available = False

Note that we additionally often assign a “available” variable depending on the condition. They frequently appear under visualization/* and integration/*.

[hvy]

For simple cases such as the one in the description, however, I’m wondering if the following isn’t cleaner. I.e. import at the PEP-8 suggested location, surround with try, except and allow a single assignment.

try:
    import cma
except ImportError:
    cma = None
import numpy 

import optuna

This does obviously not catch the error object but I believe that the reason is obvious in most of the cases for ImportErrors.

[hvy]

Yes, I think it’s fine to close this ticket. It’s non-trivial to establish proper guidelines and the merits aren’t that significant.