StreamField – TypeError: 'bool' object is not iterable

Question

Build: Wagtail CMS(1.13.1) with Django(1.11) in python3.6.

I am trying to create a pretty basic Streamfield block(CardBlock), but keep getting a type error. It is very similar to the examples in documentation, but I can not get it to work...

class CardBlock(StructBlock):
    image = ImageChooserBlock()
    heading = CharBlock(classname="full title")
    caption = RichTextBlock()
    class Meta:
        icon = 'image'

class HomePage(Page):
    intro = RichTextField(blank=True)
    showcase_title = RichTextField(blank=True)
    card = StreamField([('card', CardBlock())], default=True)

    content_panels = Page.content_panels + [
        FieldPanel('intro', classname="full"),
        MultiFieldPanel([
            FieldPanel('showcase_title'),
            StreamFieldPanel('card'),
        ]),
    ]

django is trying to "get_db_prep_value()". So, Wagtail tries to "get_prep_value()", for all children(streamchild instances) in value, as shown below. wagtail/wagtailcore/blocks/stream_block.py (line 257):

def get_prep_value(self, value):
    if value is None:
        # treat None as identical to an empty stream
        return []

    return [
        {
            'type': child.block.name,
            'value': child.block.get_prep_value(child.value),
            # assign a new ID on save if it didn't have one already
            'id': child.id or str(uuid.uuid4()),
        }
        for child in value  # child is a StreamChild instance
    ]

I am uncertain as to what this value is. What in my block classes needs to be changed to correct this value variable?

Edit1- Full Error:

$ python manage.py migrate
Operations to perform:
  Apply all migrations: admin, auth, blog, contenttypes, home, sessions, 
taggit, wagtailadmin, wagtailcore, wagtaildocs, wagtailembeds, wagtailforms, 
wagtailimages, wagtailredirects, wagtailsearch, wagtailusers
Running migrations:
  Applying home.0006_auto_20180220_1223...Traceback (most recent call last):
  File "manage.py", line 12, in <module>
    execute_from_command_line(sys.argv)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-              
packages/django/core/management/__init__.py", line 363, in 
execute_from_command_line
utility.execute()
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/core/management/__init__.py", line 355, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/core/management/base.py", line 330, in execute
  output = self.handle(*args, **options)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/core/management/commands/migrate.py", line 204, in handle
    fake_initial=fake_initial,
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/migrations/executor.py", line 115, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, 
fake_initial=fake_initial)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/migrations/executor.py", line 145, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, 
fake_initial=fake_initial)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/migrations/migration.py", line 129, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, 
project_state)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/migrations/operations/fields.py", line 86, in 
database_forwards
    field,
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/backends/sqlite3/schema.py", line 238, in add_field
    self._remake_table(model, create_field=field)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/backends/sqlite3/schema.py", line 113, in _remake_table
    self.effective_default(create_field)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/backends/base/schema.py", line 228, in effective_default
    default = field.get_db_prep_save(default, self.connection)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/models/fields/__init__.py", line 766, in get_db_prep_save
    prepared=False)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/django/db/models/fields/__init__.py", line 758, in get_db_prep_value
    value = self.get_prep_value(value)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/wagtail/wagtailcore/fields.py", line 109, in get_prep_value
    return json.dumps(self.stream_block.get_prep_value(value), 
cls=DjangoJSONEncoder)
  File "//anaconda/envs/WagtailCMS/lib/python3.6/site-
packages/wagtail/wagtailcore/blocks/stream_block.py", line 257, in 
get_prep_value
    for child in value  # child is a StreamChild instance
TypeError: 'bool' object is not iterable

Answer 1

The error is on the line:

card = StreamField([('card', CardBlock())], default=True)

The default parameter is used to specify a default/initial value for the field; in this case, you're setting that value to True, which doesn't make sense as a StreamField value. (A StreamField is a list of blocks, so it's trying and failing to loop over the value True to populate the stream...)

Perhaps you meant blank=False instead?