Question

Okay, it took me a little while to narrow down this problem, but it appears python is doing this one purpose. Can someone explain why this is happening and what I can do to fix this?

File: library/testModule.py

class testClass:

    myvars = dict()

    def __getattr__(self, k):
        if self.myvars.has_key(k):
            return self.myvars[k]

    def __setattr__(self, k, v):
        self.myvars[k] = v

    def __str__(self):
        l = []
        for k, v in self.myvars.iteritems():
            l.append(str(k) + ":" + str(v))

        return " - ".join(l)

test.py

from library import testModule

#I get the same result if I instantiate both classes one after another
c1 = testClass()
c1.foo = "hello"
c2 = testClass()

print("c1: " + str(c1) + "\n")
print("c2: " + str(c2) + "\n")

Output:

c1: foo:hello
c2: foo:hello

My best guess is that because library has an "__init__.py" file, the whole module is loaded like a class object and it’s now become part of a lasting object.. is this the case?

Answer 1

myvars is a property of the class, not the instance. This means that when you insert an attribute into myvars from the instance c1, the attribute gets associated with the class testClass, not the instance c1 specifically. Since c2 is an instance of the same class, it also has the same attribute.

You could get the behavior you want by writing this:

class testClass:
    def __init__(self):
        self.myvars = dict()

    def __getattr__(self, k):
        if self.myvars.has_key(k):
            return self.myvars[k]

    def __setattr__(self, k, v):
        self.myvars[k] = v

    def __str__(self):
        l = []
        for k, v in self.myvars.iteritems():
            l.append(str(k) + ":" + str(v))
        return " - ".join(l)