ruby - How can I appropriately mock out a method that returns yield? -

it's common in ruby methods take blocks this:

class file   def open(path, mode)     perform_some_setup     yield   ensure     do_some_teardown   end end 

it's idiomatic method this:

def frobnicate   file.open('/path/to/something', 'r') |f|     f.grep(/foo/).first   end end 

i want write spec doesn't hit filesystem, ensures pulls right word out of file, like:

describe 'frobnicate'   'returns first line containing substring foo'     file.expects(:open).yields(stringio.new(<<eof))       not line       foo bar baz       not line either     eof     expect(frobnicate).to match(/foo bar baz/)     end end 

the problem here that, mocking out call file.open, i've removed return value, means frobnicate return nil. if add file.returns('foo bar baz') chain, however, i'd end test doesn't hit of code i'm interested in; contents of block in frobnicate , test still pass.

how might appropriately test frobnicate method without hitting filesystem? i'm not particularly attached particular testing framework, if answer "use awesome gem that'll you" i'm ok that.

it seems need mock call file little differently. getting syntax errors running code as-is, i'm not sure version of rspec you're on, if you're on 3.x job:

frobnicate_spec.rb

gem 'rspec', '~> 3.4.0' require 'rspec/autorun'  rspec.configure |config|   config.mock_with :rspec end  def frobnicate   file.open('/path/to/something', 'r') |f|     f.grep(/foo/).first   end end  rspec.describe 'frobnicate'   'returns first line containing substring foo'     allow(file).to receive(:open).and_call_original     allow(file).to receive(:open).and_yield stringio.new <<-eof       not line       foo bar baz       not line either     eof     expect(frobnicate).to match(/foo bar baz/)   end end 

invoke ruby frobnicate_spec.rb can use specified rspec version.

source: rspec mocks expecting messages , yielding responses