RSpec mocking an :each block - ruby

Question

I want to use RSpec mocks to provide canned input to a block.
Ruby:
class Parser
attr_accessor :extracted
def parse(fname)
File.open(fname).each do |line|
extracted = line if line =~ /^RCS file: (.*),v$/
end
end
end
RSpec:
describe Parser
before do
#parser = Parser.new
#lines = mock("lines")
#lines.stub!(:each)
File.stub!(:open).and_return(#lines)
end
it "should extract a filename into extracted" do
linetext = [ "RCS file: hello,v\n", "bla bla bla\n" ]
# HELP ME HERE ...
# the :each should be fed with 'linetext'
#lines.should_receive(:each)
#parser.should_receive('extracted=')
#parser.parse("somefile.txt")
end
end
It's a way to test that the internals of the block work correctly by passing fixtured data into it. But I can't figure out how to do the actual feeding with RSpec mocking mechanism.
update: looks like the problem was not with the linetext, but with the:
#parser.should_receive('extracted=')
it's not the way it's called, replacing it in the ruby code with self.extracted= helps a bit, but feels wrong somehow.

Answer 1

To flesh out the how 'and_yield' works: I don't think 'and_return' is really what you want here. That will set the return value of the File.open block, not the lines yielded to its block. To change the example slightly, say you have this:
Ruby
def parse(fname)
lines = []
File.open(fname){ |line| lines << line*2 }
end
Rspec
describe Parser do
it 'should yield each line' do
File.stub(:open).and_yield('first').and_yield('second')
parse('nofile.txt').should eq(['firstfirst','secondsecond'])
end
end
Will pass. If you replaced that line with an 'and_return' like
File.stub(:open).and_return(['first','second'])
It will fail because the block is being bypassed:
expected: ["firstfirst", "secondsecond"]
got: ["first", "second"]
So bottom line is use 'and_yield' to mock the input to 'each' type blocks. Use 'and_return' to mock the output of those blocks.

Answer 2

I don't have a computer with Ruby & RSpec available to check this, but I suspect you need to add a call to and_yields call [1] on the end of the should_receive(:each). However, you might find it simpler not to use mocks in this case e.g. you could return a StringIO instance containing linetext from the File.open stub.
[1] http://rspec.rubyforge.org/rspec/1.1.11/classes/Spec/Mocks/BaseExpectation.src/M000104.html

Answer 3

I would go with the idea of stubbing the File.open call
lines = "RCS file: hello,v\n", "bla bla bla\n"
File.stub!(:open).and_return(lines)
This should be good enough to test the code inside the loop.

Answer 4

This should do the trick:
describe Parser
before do
#parser = Parser.new
end
it "should extract a filename into extracted" do
linetext = [ "RCS file: hello,v\n", "bla bla bla\n" ]
File.should_receive(:open).with("somefile.txt").and_return(linetext)
#parser.parse("somefile.txt")
#parser.extracted.should == "hello"
end
end
There are some bugs in the Parser class (it won't pass the test), but that's how I'd write the test.