isalpha

The sibling of isalnum, one notch stricter: only letters pass. If you're validating a name - a subject, a style, a label - where digits just don't make sense, this is the check.

What it does

isalpha wraps str.isalpha(). It returns True only if every character in the string is alphabetic - a letter - and the string has at least one character. The moment a digit, space, or punctuation shows up, it's False.

One string input, one BOOLEAN output. That's the node.

Why you'd reach for it

Name validation, mostly. "Is this string a plausible subject name?" - if it contains digits or symbols, it probably isn't, and you can route it elsewhere. It's the natural gate before a string becomes a key, a filename component, or a comparison target. Like the rest of the is-* family, the output is a boolean that feeds cleanly into the pack's control-flow nodes for branching.

The realistic split in this family: isalpha for "letters only," isalnum for "letters and/or digits," isascii for "everything's plain ASCII." Pick by what your downstream actually tolerates.

Installing

Part of Basic data handling by StableLlama. In ComfyUI Manager search "Basic data handling" → Install → restart. Or:

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

Restart. Zero runtime dependencies - pure stdlib - so the install is trivial and can't break anything else.

Where people get burned

The same two quirks as isalnum. Empty string → False (the "at least one character" rule), and Unicode letters count, so "Café" returns True. If you were expecting ASCII-only behavior, remember this is checking letters, not ASCII letters - that's what isascii is for. And spaces always fail, which catches people who check a whole sentence.