karatelabs/karate · error · RuntimeException

ka:data requires format 'varName:expression', got

Error message

ka:data requires format 'varName:expression', got: ${attrValue}

What it means

The ka:data processor parses its attribute value as 'varName:expression' (split on the first colon) and throws a plain RuntimeException when there is no colon or the colon is the first character (varName empty). The attribute value must name the client-side variable to populate and a server-side expression providing the initial data.

Solutions

1. Rewrite the attribute as ka:data="varName:expression" with a non-empty variable name before the first colon
2. Ensure the expression after the colon is a valid server-side expression resolvable in the template context
3. Avoid HTML-entity-encoding the colon; write it literally
4. Check rendered/template-manager output to confirm the attribute value reaches the processor intact

Example fix

<!-- before -->
<div ka:data="userData"></div>
<!-- after -->
<div ka:data="userData:session.user"></div>

if (attrValue == null || attrValue.indexOf(':') <= 0) throw new IllegalArgumentException("ka:data needs 'varName:expression', got: " + attrValue);

boolean valid = attrValue != null && attrValue.indexOf(':') > 0;

try { render(template, model); } catch (RuntimeException e) { if (e.getMessage() != null && e.getMessage().startsWith("ka:data requires format")) { log.error("bad ka:data attribute: {}", e.getMessage()); } throw e; }

When it happens

Trigger: Writing ka:data="something" without a colon, ka:data=":expr" with an empty variable name, or quoting/HTML-escaping that mangles the value so the colon is lost.

Common situations: Copy-pasting ka:data examples incompletely; forgetting the expression half when refactoring; HTML entity encoding (:) hiding the colon from the processor; dynamic templates building the attribute value wrongly.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• ka:dispatch supports a single `event @ trigger` delimiter…
• ka:dispatch trigger must be non-empty after `@`; got
• karate-markup does not support param lists in th:fragment…
• ka:dispatch requires an event name; got
• th:each requires `name : expression` form

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/de9a2625cc9f1e2b. Report an issue: GitHub.

Appendix: source

Thrown at karate-core/src/main/java/io/karatelabs/markup/KaDataProcessor.java:89

    protected void doProcess(ITemplateContext ctx, IModel model, IElementModelStructureHandler sh) {
        if (model.size() == 0) {
            return;
        }

        // Get the opening tag
        IProcessableElementTag openTag = (IProcessableElementTag) model.get(0);
        String elementName = openTag.getElementCompleteName().toLowerCase();

        // Get attribute value: "varName:serverExpression"
        String attrValue = openTag.getAttributeValue(getDialectPrefix(), DATA);
        if (attrValue == null || attrValue.isEmpty()) {
            return;
        }

        // Parse "varName:expression"
        int colonIndex = attrValue.indexOf(':');
        if (colonIndex <= 0) {
            throw new RuntimeException("ka:data requires format 'varName:expression', got: " + attrValue);
        }

        String varName = attrValue.substring(0, colonIndex).trim();
        String expression = attrValue.substring(colonIndex + 1).trim();

        // Evaluate the server expression to get initial data
        MarkupTemplateContext kec = (MarkupTemplateContext) ctx;
        Object initialData = kec.evalLocal(expression);
        String jsonData = initialData != null ? Json.stringifyStrict(initialData) : "{}";

        IModelFactory modelFactory = ctx.getModelFactory();

        // Build new attributes map (common for all elements)
        Map<String, String> newAttrs = new HashMap<>();
        for (var attr : openTag.getAllAttributes()) {
            String name = attr.getAttributeCompleteName();
            if (!name.equals(getDialectPrefix() + ":" + DATA)) {
                newAttrs.put(name, attr.getValue());

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