Description

This PR implements #13094, proposed by @laymonage.

It adds support for specifying Meta.preview_value and Meta.default as callables (with no arguments), following the convention used by Django model field defaults. This allows for dynamic values (e.g. database lookups, timestamps) without requiring subclassing or eager evaluation.

This also aligns with some of the use cases discussed in #7992.

Demo of the use case mentioned by #7992.

With the demo block code below, we can dynamically generate the datetime without overriding get_preview_value():

class BlockQuote(StructBlock):
    text = TextBlock()
    attribute_name = CharBlock(blank=True, required=False, label="e.g. Mary Berry")

    class Meta:
        icon = "openquote"
        template = "blocks/blockquote.html"
        preview_value = staticmethod(lambda: {
            "text": f"This preview was generated at {datetime.now().strftime('%H:%M:%S')}",
            "attribute_name": "Preview system"
        })
        description = "test block showing"

Test cases

Added for test_preview_value_falls_back_to_default and test_preview_value