RSpec: how to test file operations and file content - ruby

Question

In my app, I have the following code:
File.open "filename", "w" do |file|
file.write("text")
end
I want to test this code via RSpec. What are the best practices for doing this?

Answer 1

I would suggest using StringIO for this and making sure your SUT accepts a stream to write to instead of a filename. That way, different files or outputs can be used (more reusable), including the string IO (good for testing)
So in your test code (assuming your SUT instance is sutObject and the serializer is named writeStuffTo:
testIO = StringIO.new
sutObject.writeStuffTo testIO
testIO.string.should == "Hello, world!"
String IO behaves like an open file. So if the code already can work with a File object, it will work with StringIO.

Answer 2

For very simple i/o, you can just mock File. So, given:
def foo
File.open "filename", "w" do |file|
file.write("text")
end
end
then:
describe "foo" do
it "should create 'filename' and put 'text' in it" do
file = mock('file')
File.should_receive(:open).with("filename", "w").and_yield(file)
file.should_receive(:write).with("text")
foo
end
end
However, this approach falls flat in the presence of multiple reads/writes: simple refactorings which do not change the final state of the file can cause the test to break. In that case (and possibly in any case) you should prefer #Danny Staple's answer.

Answer 3

This is how to mock File (with rspec 3.4), so you could write to a buffer and check its content later:
it 'How to mock File.open for write with rspec 3.4' do
#buffer = StringIO.new()
#filename = "somefile.txt"
#content = "the content fo the file"
allow(File).to receive(:open).with(#filename,'w').and_yield( #buffer )
# call the function that writes to the file
File.open(#filename, 'w') {|f| f.write(#content)}
# reading the buffer and checking its content.
expect(#buffer.string).to eq(#content)
end

Answer 4

You can use fakefs.
It stubs filesystem and creates files in memory
You check with
File.exists? "filename"
if file was created.
You can also just read it with
File.open
and run expectation on its contents.

Answer 5

For someone like me who need to modify multiple files in multiple directories (e.g. generator for Rails), I use temp folder.
Dir.mktmpdir do |dir|
Dir.chdir(dir) do
# Generate a clean Rails folder
Rails::Generators::AppGenerator.start ['foo', '--skip-bundle']
File.open(File.join(dir, 'foo.txt'), 'w') {|f| f.write("write your stuff here") }
expect(File.exist?(File.join(dir, 'foo.txt'))).to eq(true)
end
end