length

length takes a LIST and returns how many items are in it, as an integer. That's it - the whole node. It's also one of the most useful in the pack, because half of list-related bugs come from not knowing the size of the thing you're iterating over. Every time you're about to index, slice, or guard a list, length is the number you want next to you.

How it works

It's a one-line wrapper:

return (len(list),)

len() counts the top-level items. Nested lists count as one item each - a list of three pairs has length 3, not 6. An empty list returns 0, never a negative number and never an error. The output is an INT named length, which feeds straight into comparisons (<=, >=, ==) from the same pack, or into if/else for size-based routing.

Where you'd use it

• Bounds checking. "Is this list big enough to safely get item at index 2?" Compare length against your index before you reach in.
• Batch sizes. Know how many frames, prompts, or seeds you're about to process - and warn or skip if it's zero.
• Empty-list guards. The pack's all returns True on an empty list and any returns False; when you want the fact of emptiness rather than a truthiness quirk, length == 0 is the unambiguous check.
• Progress logic. If you're consuming a list, length on the remainder tells you how much work is left.

Installing

It's in the "Basic data handling" pack by StableLlama. ComfyUI Manager: search "Basic data handling", install, restart. Or:

cd ComfyUI/custom_nodes
git clone https://github.com/StableLlama/ComfyUI-basic_data_handling

Restart ComfyUI. Zero dependencies, no models - pure Python wrappers, and one of the safest, most boring installs in the ecosystem.

Gotchas worth knowing

• It counts top-level items only - nesting is not flattened.
• Empty list → 0, always. Pair it with == 0 (not !) if you're writing the classic "skip when nothing to do" gate.
• It works on the pack's LIST type, not a native ComfyUI data list. If you're holding a data list, convert it to LIST first - the sizes aren't interchangeable concepts here.