cube-js/cube · error · UserError

Unsupported Python vfpdef children node: ${node.constructor.

Error message

Unsupported Python vfpdef children node: ${node.constructor.name}: ${node.getText()}

What it means

Thrown by PythonParser.transpileToJs() when a VfpdefContext (a function-parameter definition inside a lambda/def parameter list) has no NAME token, so no identifier can be created for the parameter. The visitor throws a UserError naming the context and its text.

Source

Thrown at packages/cubejs-schema-compiler/src/parser/PythonParser.ts:226

          }
        } else if (node instanceof TrailerContext) {
          const name = node.NAME();
          const argsList = node.callArguments();
          if (argsList) {
            // trailer with callArguments have a single child: CallArgumentsContext
            // which was already processed (see other if branch)
            return children[0];
          } else if (name) {
            return { identifier: t.identifier(name.getText()) };
          } else {
            throw new UserError(`Unsupported Python Trailer children node: ${node.constructor.name}: ${node.getText()}`);
          }
        } else if (node instanceof VfpdefContext) {
          const name = node.NAME();
          if (name) {
            return t.identifier(name.getText());
          } else {
            throw new UserError(`Unsupported Python vfpdef children node: ${node.constructor.name}: ${node.getText()}`);
          }
        } else if (node instanceof VarargslistContext) {
          return { args: children };
        } else if (node instanceof LambdefContext) {
          return t.arrowFunctionExpression(children[0].args, children[1]);
        } else if (node instanceof Not_testContext) {
          if (node.getChildCount() === 1) {
            return children[0];
          }
          return t.unaryExpression('!', children[0]);
        } else if (node instanceof And_testContext) {
          if (children.length === 1) {
            return children[0];
          }
          return children.reduce((left, right) => t.logicalExpression('&&', left, right));
        } else if (node instanceof Or_testContext) {
          if (children.length === 1) {
            return children[0];

View on GitHub (pinned to 7d981676b3)

Solutions

Fix syntax errors in lambda/function parameter lists in the data model file (run the file through Python to validate first)
Rewrite parameter definitions as plain identifiers without type annotations
Simplify the lambda to a single plain parameter, e.g. lambda x: ...
Report as a bug if valid Python triggers it, including the snippet from the error text

Example fix

// before (cubes.py)
convert = lambda x: x.upper  # malformed/empty param from typo: lambda : .upper
// after
convert = lambda x: x.upper()

Defensive patterns

Strategy: validation

Validate before calling

// Validate lambdas/params with real Python first
const { execSync } = require('child_process');
try { execSync(`python3 -c "compile(open('model.py').read(),'model.py','exec')"`); }
catch { throw new Error('Model file has Python syntax errors (check lambda parameter lists)'); }

Try / catch

try {
  parsePythonAndTranspileToJs(source);
} catch (e) {
  if (e instanceof UserError && e.message.includes('Unsupported Python vfpdef')) {
    throw new Error(`Malformed lambda parameter: ${e.message}`);
  }
  throw e;
}

Prevention

Ensure every lambda has well-formed, plain parameters (lambda x: ...)
Run python3 -m py_compile on model files before deploying
Avoid type-annotated or starred parameters in model lambdas
Double-check for stray colons/commas in parameter lists

When it happens

Trigger: parsePythonAndTranspileToJs visits a VfpdefContext where node.NAME() is null — typically from ANTLR error recovery on malformed parameter lists, or parameter forms (typed/starred params) not represented as a plain NAME in the grammar path taken.

Common situations: Usually follows a syntax error in a lambda or function definition in the data model (e.g. lambda with malformed arguments) where recovery produced an empty vfpdef node; rarely hit by valid Python.

Related errors

AI-assisted analysis of cube-js/cube@7d981676b3 (2026-09-02). Data as JSON: /api/errors/a6c772125bd6c3c6. Report an issue: GitHub.