Mock File#open and #read

mock_open.rb

require 'ostruct'

class SomeClass
  def run( filename )
    File.open filename, 'r' do | f |
      puts f.read
    end 
  end
end

RSpec.describe SomeClass do
  it 'mocks open...' do
    allow( File ).to receive( :open ).with( 'dummy', 'r' ).and_yield(
      OpenStruct.new( { :read => 'Mock content of file...' } )
    )
    
    SomeClass.new.run 'dummy'
    
  end
end