Built-in python requirements plugin skips layer-only services (no functions defined)

Serverless Framework
1

The built-in serverless-python-requirements plugin in Serverless Framework v4 does not generate the PythonRequirementsLambdaLayer resource or PythonRequirementsLambdaLayerQualifiedArn
output for layer-only services (services with custom.pythonRequirements.layer configured but no functions: block).

Root Cause

The isFunctionRuntimePython guard in the built-in plugin was rewritten from the standalone version. When no functionObj is present in the hook arguments, the built-in version falls
through to a service-level check that iterates this.serverless.service.functions. For a layer-only service, this is empty, so it returns false — causing before and after hooks (including
layerRequirements()) to be skipped entirely.

Built-in (v4) — serverless/packages/serverless/lib/plugins/python/index.js at 83677fd76661a14d0447c5c38f2628e6598f1546 · serverless/serverless · GitHub

const isFunctionRuntimePython = (args) => {
const providerRuntime = this.serverless.service.provider.runtime
if (args[1]?.functionObj) {
const functionRuntime = args[1].functionObj.runtime
const runtime = functionRuntime || providerRuntime
return runtime ? runtime.startsWith(‘python’) : false
}
// Service-level hook: check if ANY function uses Python
const allFunctions = this.serverless.service.functions || {}
const hasPythonFunction = Object.values(allFunctions).some((func) => {
const runtime = func.runtime || providerRuntime
return runtime && runtime.startsWith(‘python’)
})
const hasPythonAgent = this.targetAgents.length > 0
return hasPythonFunction || hasPythonAgent
}

Standalone (pre-v4) — serverless-python-requirements/index.js at master · serverless/serverless-python-requirements · GitHub

const isFunctionRuntimePython = (args) => {
if (!args[1].functionObj || !args[1].functionObj.runtime) {
   return true;
}
   return args[1].functionObj.runtime.startsWith(‘python’);
};


The standalone version returned true when functionObj was absent, so layer-only services worked. The built-in version returns false.

Steps to Reproduce

  1. Create a service with custom.pythonRequirements.layer configured and no functions: block
  2. Deploy with Serverless Framework v4
  3. The PythonRequirementsLambdaLayer resource and PythonRequirementsLambdaLayerQualifiedArn output are not generated

Expected Behaviour

When custom.pythonRequirements.layer is configured, the plugin should process requirements and generate the layer resource/output regardless of whether functions are defined — matching
the standalone plugin’s behaviour.