class OrderAdmin(admin.ModelAdmin):
    readonly_fields = [
        "shop",
        "date",
        "order_id",
        "extra_data_pretty", #just a random name
        "extra_data", # actually a field from the model
    ]

    # This does nothing?
    def extra_data(self, obj):
        return mark_safe(
            "<pre>"
            + escape(pprint.PrettyPrinter(indent=2).pformat(obj.extra_data))
            + "</pre>"
        )
    # This works as intended
    def extra_data_pretty(self, obj):
        return mark_safe(
            "<pre>"
            + escape(pprint.PrettyPrinter(indent=2).pformat(obj.extra_data))
            + "</pre>"
        )