nodejs/node · error · RuntimeError

at least one item has to be provided

Error message

at least one item has to be provided

What it means

Raised by jinja2.utils.Cycler.__init__ when constructed with no items. Cycler is the standalone helper used by the {% cycle %} statement and by user code to alternate values; with zero items there is nothing to return from .current or pop, so the constructor refuses with RuntimeError.

Source

Thrown at tools/inspector_protocol/jinja2/utils.py:579

    quote your attributes or HTML escape it in addition.
    """
    if dumper is None:
        dumper = json.dumps
    rv = dumper(obj, **kwargs) \
        .replace(u'<', u'\\u003c') \
        .replace(u'>', u'\\u003e') \
        .replace(u'&', u'\\u0026') \
        .replace(u"'", u'\\u0027')
    return Markup(rv)


@implements_iterator
class Cycler(object):
    """A cycle helper for templates."""

    def __init__(self, *items):
        if not items:
            raise RuntimeError('at least one item has to be provided')
        self.items = items
        self.reset()

    def reset(self):
        """Resets the cycle."""
        self.pos = 0

    @property
    def current(self):
        """Returns the current item."""
        return self.items[self.pos]

    def next(self):
        """Goes one item ahead and returns it."""
        rv = self.current
        self.pos = (self.pos + 1) % len(self.items)
        return rv

View on GitHub (pinned to 1b2de5e052)

Solutions

1. Supply at least one item: Cycler('a') or Cycler(*items or ['default']).
2. Validate the source list before construction: if not items: raise ValueError or fall back to a default.
3. In templates, prefer {% cycle 'default' %} with literal fallbacks when the data list may be empty.
4. Unit-test the helper with an empty input to fail fast at the right layer.

Example fix

// before
row_classes = config.get('row_classes', [])
c = Cycler(*row_classes)
// after
row_classes = config.get('row_classes') or ['default']
c = Cycler(*row_classes)

from jinja2.utils import Cycler
items = source_list or ['default']
assert items, 'Cycler requires at least one item'
c = Cycler(*items)

def has_cycler_items(items) -> bool:
    return len(items) > 0

# construction-time RuntimeError should be fixed at the call site,
# but if you must keep the process alive:
from jinja2.utils import Cycler
try:
    c = Cycler(*items)
except RuntimeError:
    c = Cycler('default')

When it happens

Trigger: Calling Cycler() with no positional arguments in Python code. Instantiating Cycler(*classes) where classes is an empty list. Constructing a Cycler from a context variable that was not populated (e.g. Cycler(*theme_row_classes) with theme_row_classes unset).

Common situations: Theme/configuration-driven cycling where the configured list is empty in some environments. Generic helper that builds Cyclers from arbitrary iterables without a length check. Copy-paste from an example that assumed non-empty input.

Related errors

• no items for cycling given
• Tried to call non recursive loop. Maybe you forgot the 'rec
• Template module attribute is unavailable in async mode
• Loop length for some iterators cannot be lazily calculated i
• extended multiple times

AI-assisted analysis of nodejs/node@1b2de5e052 (2026-08-13). Data as JSON: /api/errors/0c1a4d965de9a7f5. Report an issue: GitHub.