Ruby: to_s vs inspect

puts and interpolation

The putsmethod implicitly calls the to_s method of the object passed as parameter (except for a String)

class Factory
  def initialize
    @workers  = { workers: [] }
    @machines = { machines: [] }
  end
  def add_worker(name)
    @workers[:workers] << { :name => name }
  end

  def add_machines(id)
    @machines[:machines] << { :id => id }
  end

  def to_s
    ws = @workers[:workers].map { |w| w[:name] }.join(', ')
    ms = @machines[:machines].map { |m| m[:id] }.join(', ')
    
    <<-EOF
    Worker Names[#{@workers[:workers].count}]: #{ws}
    Machine IDs[#{@machines[:machines].count}]: #{ms}
    EOF
  end
end
factory = Factory.new
factory.add_worker('Jim')
factory.add_worker('Larry')
factory.add_worker('Doug')
factory.add_machines('12QC-2323')
factory.add_machines('12QC-2324')
factory.add_machines('12QC-2325')

So, a call to puts with factory as parameter will implicitly call factory.to_s

> puts factory
    Worker Names[3]: Jim, Larry, Doug
    Machine IDs[3]: 12QC-2323, 12QC-2324, 12QC-2325

to_s is also implicitly called during String interpolation

> "#{factory}"
"    Worker Names[3]: Jim, Larry, Doug\n    Machine IDs[3]: 12QC-2323, 12QC-2324, 12QC-2325\n"

inspect

The inspect method returns a String that represents the calling object

[1,2,3].inspect # "[1,2,3]"

It’s also implicitly called by the p method

p [1,2,3] # "[1,2,3]"

Conclusion

to_s is usually used for logging purposes.

inspect is usually used for debugging purposes.

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