Static member variables not behaving as expected - Python

Question

I defined my class foo as:

class Foo(object):
    # static member variables of the parent class Foo
    is_logged = False
    is_domain_set = False

    def __init__(self, username, password):
        # do something here.

    def login(self, login_url):
        # do the login ...
        if self.login_successful():
            self.is_logged = True
            self.is_domain_set = True
            # Output `True` as expected
            self.log.info("is_logged: %s" % self.is_logged)
            self.log.info("is_domain_set: %s" % self.is_domain_set)
            return

class Bar(Foo):
   # Foo is the parent class of Bar
   super(Bar, self).__init__(username, password)
   # Both are outputting `False` when it should be `True`
   self.log.info("is_logged: %s" % self.is_logged)
   self.log.info("is_domain_set: %s" % self.is_domain_set)

   if not self.is_logged:
      self.login(login_url)

Then they are used in another class, Baz:

class Baz(oject):

   def __init__(self, username, password, login):
       # -- not important for the problem.
       self.foo = Foo(username, password)
       # login for auth purpose
       if not self.foo.is_logged:
           self.foo.login(login_url)

       # Now use some of the objects of the `Bar` class, which
       # will do its job:
       self.bar = Bar(username, password)

The program flow is that simple. Foo is used for some basic operations like the login, while Bar is used to some other specific operations. However, I expect static variables to be shared between the base class (one per instance of the class), and it's child class. The intention is to have is_logged and is_domain_set to behave like a Singleton. What am I doing wrong?

The apparent behavior is that there's a static variable for Foo and one for Bar, even though Bar inherits from Foo. However, What am I trying to accomplish is different. I want Foo and Bar to share the same static variable. One option (bad) is to have a global variable, but I'm trying to do without this.

You want one global setting for is_logged_in? Can you explain your use case? This makes no sense to me. — Two-Bit Alchemist
– Two-Bit Alchemist, Commented Nov 2, 2015 at 19:10
@Two-BitAlchemist: I want them to share is_logged_in without using global variables. Makes sense? — cybertextron
– cybertextron, Commented Nov 2, 2015 at 19:12
So that only one user can be logged in at a time? I doubt that's the best way to do it if so. — Two-Bit Alchemist
– Two-Bit Alchemist, Commented Nov 2, 2015 at 19:14
@Two-BitAlchemist. No. If Foo already authenticated the is logged in, then Bar doesn't need to authenticate. I think the example I gave is clear about that. I only care about if it's logged or not. — cybertextron
– cybertextron, Commented Nov 2, 2015 at 19:16
I'm with Two-Bit Alchemist here; static variables by definition are shared across all instances, so if one user is logged in, they all are. — Daniel Roseman
– Daniel Roseman, Commented Nov 2, 2015 at 19:21

Daniel Roseman · Accepted Answer · 2015-11-02 19:12:58Z

2

You might have defined "static" variables (in Python they're usually known as class attributes) for is_logged and is_domain_set, but you immediately overwrite them with instance attributes when you assign to self.is_logged.

If you wanted to assign to the class variables only, you would need to do self.__class__.is_logged = True.

answered Nov 2, 2015 at 19:12

Daniel Roseman

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