Luigi - Unfulfilled %s at run time

I am trying to learn in a very simple way how luigi works. Just as a newbie I came up with this code

import luigi

class class1(luigi.Task):

  def requires(self):
     return class2()

  def output(self):
    return luigi.LocalTarget('class1.txt')

 def run(self):
    print 'IN class A'


class class2(luigi.Task): 

  def requires(self):
     return []

  def output(self):
     return luigi.LocalTarget('class2.txt')


if __name__ == '__main__':
  luigi.run()

Running this in command prompt gives error saying

raise RuntimeError('Unfulfilled %s at run time: %s' % (deps, ',', '.join(missing)))      

which is:

RuntimeError: Unfulfilled dependency at run time: class2__99914b932b  
   

This happens because you define an output for class2 but never create it.

Let's break it down...

When running

python file.py class2 --local-scheduler

luigi will ask:

• is the output of class2 already on disk? NO
• check dependencies of class2: NONE
• execute the run method (by default it's and empty method pass)
• run method didn't return errors, so job finishes successfully.

However, when running

python file.py class1 --local-scheduler

luigi will:

• is the output of class1 already on disk? NO
• check task dependencies: YES: class2
• pause to check status of class2
  • is the output of class2 on disk? NO
  • run class2 -> running -> done without errors
  • is the output of class2 on disk? NO -> raise error

luigi never runs a task unless all of its previous dependencies are met. (i.e. their output is on the file system)