title: How to get all instances of a certain class in python?
body: <p>Someone asked a similar one [question]:<a href="https://stackoverflow.com/questions/328851/printing-all-instances-of-a-class">Printing all instances of a class</a>. 
  While I am less concerned about printing them, I'd rather to know how many instances are currently "live". 
  The reason for this instance capture is more like a setting up a scheduled job, every hour check these "live" unprocessed instances and enrich the data. After that, either a flag in this instance is set or just delete this instance. 
  Torsten Marek 's answer in [question]:<a href="https://stackoverflow.com/questions/328851/printing-all-instances-of-a-class">Printing all instances of a class</a> using weakrefs need a call to the base class constructor for every class of this type, is it possible to automate this? Or we can get all instances with some other methods?</p>