I am baffled by the Python behaviour below. Why the attributes of second and third instances (b, c), i, are the class attribute i but a behaves differently?
In [47]: class Foo:
...: i=0
...:
In [48]: a = Foo()
In [49]: a.i = 1
In [50]: a.i
Out[50]: 1
In [51]: Foo.i
Out[51]: 0
In [52]: b = Foo()
In [53]: b.i
Out[53]: 0
In [54]: Foo.i is b.i
Out[54]: True
In [55]: Foo.i is a.i
Out[55]: False
In [56]: c = Foo()
In [57]: Foo.i is c.i
Out[57]: True
Here's what happens. When you do:
a.i = 1
You create an instance variable with a name that shadows the class attribute. The class attribute is still there, though:
>>> class Foo:
... i = 0
...
>>> a = Foo()
>>> Foo.i
0
>>> a.i = 69
>>> a.i
69
>>> a.__class__.i
0
>>> del a.i # deletes the instance attribute, resolving lookup on class
>>> a.i
0
To see what lives in the instance's namespace, check out the instance dict:
>>> a = Foo()
>>> a.__dict__
{}
>>> a.i = 1
>>> a.__dict__
{'i': 1}
a.i = 1 does NOT change Foo.i as you intended. It rather assigns an instance member of a. It is very clear if you see a.__dict__ and b.__dict__.
In [11]: a.__dict__
Out[11]: {'i': 1}
In [13]: b.__dict__
Out[13]: {}
If you want to really change class member variable, you should use Foo.i = 1. That will impact all Foo instances. Again, it is very clear on Foo.__dict__.
In [17]: Foo.__dict__
Out[17]:
mappingproxy({'__dict__': <attribute '__dict__' of 'Foo' objects>,
'__doc__': None,
'__module__': '__main__',
'__weakref__': <attribute '__weakref__' of 'Foo' objects>,
'i': 0})
int is int... Python caches small integers. Perhaps it is better to use a dummy class for such demonstrations, although, in this case I don't think it will affect your demonstration.