RSpec Stubbing: Return the parameter

Note: The stub method has been deprecated. Please see this answer for the modern way to do this.

stub! can accept a block. The block receives the parameters; the return value of the block is the return value of the stub:

class Interfaceenddescribe Interface do  it "should have a stub that returns its argument" do    interface = Interface.new    interface.stub!(:get_trace) do |arg|      arg    end    interface.get_trace(123).should eql 123  endend

The stub method has been deprecated in favor of expect.

expect(object).to receive(:get_trace).with(anything) do |value|   valueend

https://relishapp.com/rspec/rspec-mocks/v/3-2/docs/configuring-responses/block-implementation

You can use allow (stub) instead of expect (mock):

allow(object).to receive(:my_method_name) { |param1, param2| param1 }

With named parameters:

allow(object).to receive(:my_method_name) { |params| params[:my_named_param] }

Here is a real life example:

Let's assume we have a S3StorageService that uploads our files to S3 using the upload_file method. That method returns the S3 direct URL to our uploaded file.

def self.upload_file(file_type:, pathname:, metadata: {}) …

We want to stub that upload for many reasons (offline testing, performance improvements…):

allow(S3StorageService).to receive(:upload_file) { |params| params[:pathname] }

That stub only returns the file path.