Defaulting Element Styling

[natankeddem]

Per discussion #1683 here is a POC for defaulting props. This will let clusters of elements or full pages to be set to the same props. The default_props method retains all the same arguments and functionality of the props method. Note that default_props modifies elements during __init__ therefore after instantiation elements will no longer be modified by changes initiated by default_props to the class attribute.

If this looks promising I can extend this to classes and/or style as well per the original discussion.

[falkoschindler]

Thanks for this PR, @natankeddem!
The approach looks very promising. I just made a few changes:

• We can define the _default_props as a static dictionary (as you did in an earlier commit). This avoids the need for hasattr etc. The crux might be to copy the dictionary when subclassing. Otherwise the default props of, e.g., a button would modify the default props of all other elements, because they share the same dictionary instance. (The advantage with copying the dictionary when subclassing is that elements inherit default props from a parent class, which is rather intuitive in my point of view.)
• I added a pytest to check various requirements, like what happens when subclassing buttons etc.
• I moved the demo into element_documentation. Even though it only contains buttons, it demonstrates a common feature for all UI elements.
• I simplified the demo to focus on the essential use case, and added additional information to the description.

Now let's add default classes and style as well. 😎

[natankeddem]

Changes look good, thank you. The copy in __init_subclass__ was interesting; you learn something new everyday! Looks like you want new PRs for class and style variants (no more commits to this PR)?

[falkoschindler]

@natankeddem Oh we can do it on this PR. Props, classes and style are so closely related (but don't overlap), they don't need to be split into separate PRs. And maybe we spot something that applies to props as well, e.g. a hint in the documentation about styling. So let's continue on this PR.

[falkoschindler]

Looks like we're ready to merge. 🙂
I only simplified the new methods a bit (we don't need the change detection anymore since we don't call update()), removed the ";" from the test classes and tweaked the demos a tiny bit.
Thanks again, @natankeddem, for you ongoing support!

[natankeddem]

Looks good, thank you for helping me through this!