Unbound variable in python

Question

I am trying to make a toy singleton in python to learn the ins and outs of the language and am running into a problem with how python works. I declare the class like this

class ErrorLogger:
  # Singleton that provides logging to a file  
  instance = None

  def getInstance():
    # Our singleton "constructor"
    if instance is None :
      print "foo"

when I call it with

log = ErrorLogger.getInstance()

I get

 File "/home/paul/projects/peachpit/src/ErrorLogger.py", line 7, in getInstance
    if instance is None :
 UnboundLocalError: local variable 'instance' referenced before assignment

What is going on here, shouldn't instance be statically assigned Null? What would be the right way to do this?

You can use @staticmethod or @clsmethod, both of which are described very well here: stackoverflow.com/questions/136097/… — SimonT
– SimonT, Commented Apr 22, 2014 at 4:00

Trein · Accepted Answer · 2014-04-22 03:56:56Z

5

You have to call it with ErrorLogger prefix as it is a static variable.

class ErrorLogger:
  # Singleton that provides logging to a file  
  instance = None

  @staticmethod
  def getInstance():
    # Our singleton "constructor"
    if ErrorLogger.instance is None :
      print "foo"

answered Apr 22, 2014 at 3:56

Trein

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2 Comments

fr1tz Over a year ago

You can also use @classmethod, the differences are subtle but still there.

Blckknght Over a year ago

An advantage of @classmethod is that you'd get the class as an argument to the method, so you could use it to grab the singleton directly: @classmethod def getInstance(cls): if cls.instanece is None: ...

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