add kiwisolver not install warning for import error

[homosapien-lcy]

Now, kiwisolver is listed as optional but in some demo as mentioned in issue #1018, it can raise import error. This PR gives a specific error when it's not installed and the related tests are run. Closes #1018

[dpinte]

As discussed during the standup, let's move from using print to warnings.warn

[mdickinson]

Just to make sure I understand: the problem being solved here is that while kiwisolver is optional for Enable itself, some of the Enable examples use it. Is that correct?

If so, I don't think we want any sort of warning for general Enable use (e.g., when importing enable.api). The warning should be limited to the examples that need it.

[homosapien-lcy]

Just to make sure I understand: the problem being solved here is that while kiwisolver is optional for Enable itself, some of the Enable examples use it. Is that correct?

If so, I don't think we want any sort of warning for general Enable use (e.g., when importing enable.api). The warning should be limited to the examples that need it.

Thanks for the comments. There are about 10 tests that are using this package, it seems inefficient if we try to add individual warnings to each of them. Is there a way we can add warning for all the imports (like wrapping imports with a decorator)?

[corranwebster]

There are about 10 tests that are using this package.

For tests that require kiwisolver you want to skip the tests unless it is installed, not warn. But also it should possibly be in the "test" optional dependencies.

Edit to add: which it looks is exactly what the tests are doing.

[mdickinson]

There are about 10 tests that are using this package

I think the tests are fine as they are - we're already skipping the tests for ConstraintsContainer if kiwisolver is not installed. I don't see any need to change anything there.

For the examples, I see two examples that use the ConstraintsContainer: contraints_demo and image_draw. The goal here would be to get a better error message for someone who runs one of those examples without having kiwisolver installed.

One other possibility, available since Python 3.7 (see PEP 562), would be to add a module-level __getattr__ to the enable.api module so that when someone tries to explicitly use ConstraintsContainer from enable.api, they get a clear message that kiwisolver is required. (The key point here is that there's currently no warning for the majority of Enable users who don't have kiwisolver installed and don't care about the ConstraintsContainer. We don't want to introduce such a warning, since such a warning would be unnecessary noise.)

[dpinte]

Thanks @mdickinson and @corranwebster. Agreed on not needing any noise with the warnings for general enable use.

@homosapien-lcy as @mdickinson suggests, let's just get the two examples updated with a better error message.

[homosapien-lcy]

Thanks @mdickinson and @corranwebster. Agreed on not needing any noise with the warnings for general enable use.

@homosapien-lcy as @mdickinson suggests, let's just get the two examples updated with a better error message.

OK, got it fixed

[mdickinson]

LGTM