Rspec cannot check logger info output

Question

I have an non-rails ruby application, that uses logger to handle log output to stdout.
I want to add specs for checking that output and found two ways to implement this and seems both ways don't work.

So both ways are in this spec.rb file:

require 'rspec' 
require 'logger'

describe 'Logger' do   
  it 'expect to output' do
    expect { Logger.new(STDOUT).info('Hello') }.to output.to_stdout    
  end

  it 'expect to recieve' do
    expect(Logger.new(STDOUT).info('Hello')).to receive(:info)   
  end 
end

I have output to terminal with log messages, but both verification are failed

jmanrubia · Accepted Answer · 2019-04-20 10:46:49Z

5

Replace STDOUT with $stdout. This works:

expect { Logger.new($stdout).info('Hello') }.to output.to_stdout

The reason is that Rspec internally modifies the global variable $stdout to capture its contents with an StringIO. When using the constant, this mechanism does not work because STDOUT keeps the original value of $stdout.

Notice that you will have a similar problem is you initialize the logger outside of the block. The logger object will store internally the original reference and it won't work:

logger = Logger.new($stdout)
expect { logger.info('Hello') }.to output.to_stdout # This won't work

As per the second question:

expect(Logger.new(STDOUT).info('Hello')).to receive(:info)

This is just a wrong usage of Rspec. It should be something like:

logger = Logger.new(STDOUT)
expect(logger).to receive(:info)   
logger.info("Hello")

edited Apr 20, 2019 at 10:46

answered Apr 20, 2019 at 10:31

jmanrubia

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Comments

Anthony · Accepted Answer · 2016-11-11 14:28:54Z

2

I typically just use doubles like this:

describe 'logging' do
  it 'handles output' do
    logger = double('logger')
    allow(Logger).to receive(:new).and_return(logger)

    run_some_code
    expect(logger).to receive(:info).with("Hello")
  end
end

answered Nov 11, 2016 at 14:28

Anthony

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4 Comments

ShockwaveNN Over a year ago

Looks like a pretty non-straightforward workaround, it's quite hard for me to understand how it works. I hope there is a better way

Anthony Over a year ago

Well you don't need to test that the logger class is actually doing it's job, you just want to ensure you're use of the logger class is outputting your specific message. This is the best way to do it IMO.

Wilson Silva Over a year ago

@Anthony, what if you subclass Logger and override info, like I did? Then you need to test it, otherwise your test coverage won't be 100%.

Anthony Over a year ago

@WilsonSilva then you'd need unit tests for your overrides not for the Logger parent class.

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