How to enable or disable panels with the click of a button

Question

When I click on the "new" button, some panels that were disabled appear. I read their code, but I didn't understand. How do I get certain panels to appear when I click a button?

Answer 1

It's a panels poll method

The panels are defined and sitting there polling away, waiting to "show".

The poll classmethod of a panel returns true only when the required context (or otherwise related) conditions are met

Poll method for custom panel

Hide Panel With Python ("A Duality Duplicate")

In example displayed in question :

(Speculating haven't checked (...lately), Turn on Developer Extras in settings, so can right click on 99% of UI, view source and see how its done... a common practice is to)

Have one master materials panel, that always polls. In this case the material slots, no panel header, has the option to add a material.

All the others seen are , subpanels ie Panels with their parent set to the master panel, or others in the space, that have poll requirements met.

In the properties space, there is a sort of "polling" option available by setting bl_context of the panel class to match that of the properties space context, matching the 'OBJECT', 'SCENE', 'RENDER' tabs.

Creating new tabs in properties panel

How do I add a panel to Object Properties?

These subpanels only poll when there is a context.material (In properties space there is a material context member, most likely context.material == context.object.active_material

When all the panels require the same poll it is common to see a mixin base class

class SomePanelBase:
    bl_context = 'MATERIAL'
    @classmethod
    def poll(cls, context):
        return context.material is not None

And for any defined panel

class MaterialFooPanel(SomePanelBase, bpy.types.Panel):
    ...

making any such defined panels only display if there is an active material.