Scope of Python Class Variable?

Question

Are class variables essentially global variables?

Consider this example, in bar.py:

class Bar(object):
    b = {}

then in foo.py:

def foo1():
    import bar
    bar.Bar.b["key"] = "foo1"


def foo2():
    import bar
    print bar.Bar.b.get("key", "foo2")

foo1()
foo2()

The "key" value is persistently "foo1."

For my purposes this seems like a global variable. But why isn't the class variable being reset or deleted between the calls to foo1 and foo2?

Answer 1

It's a class attribute. In your case, the dict is bound to the class Bar . Everytime you import bar , you get the same class bar.Bar (after all, it is in the bar 's global namespace), and consequentially, you get the same dict bar.Bar.b .

If you want each Bar instance to have a different dict, you need to make an instance:

class Bar(object):
    def __init__(self):
        self.b = {}

And then:

def foo1():
    import bar
    bar.Bar().b["key"] = "foo1"


def foo2():
    import bar
    print bar.Bar().b.get("key", "foo2")

foo1()
foo2()

---

As far as why the module bar isn't being garbage collected, when you import something, python stores a reference to that module (see sys.modules ). The next time you import that module, python picks out the proper item from sys.modules and gives you a reference to it. There are a few reasons for this.

• Efficiency -- Why re-evaluate code you've already evaluated?
• Sane behavior

Imagine a module with a special value:

# mod.py
CONSTANT = object()

now you want to use that in foo.py and bar.py .

# bar.py
import mod
def bar():
    return mod.CONSTANT

# foo.py
import mod
import bar
if bar.bar() is mod.CONSTANT:
    do_something()

If the import wasn't cached, then mod.CONSTANT could be different in foo and in bar which would be very surprising.