Initializing field outside __init__

Question

I need a bit of help to understand how python initialising works. I have a class (Bar) with another class (Foo) as a field/variable. When I try to initialise this variable directly in Bar (not in the class __init__) all instances of Bar will point to the same Foo. But if I have an __init__ method, as in Bar2, each Bar2 instance will have a unique Foo instance. What is happening here?

class Foo():
    number = 0

class Bar():
    foo = Foo()

class Bar2():
    foo = None

    def __init__(self):
        self.foo = Foo()

first = Bar()
second = Bar()

print "Bar"
print first
print second
print first.foo
print second.foo

first = Bar2()
second = Bar2()

print "\nBar2"
print first
print second
print first.foo
print second.foo

The output will for example be:

Bar
<\__main__.Bar instance at 0x025B2AF8>
<\__main__.Bar instance at 0x025B2B20>
<\__main__.Foo instance at 0x004A3AA8>
<\__main__.Foo instance at 0x004A3AA8>

Bar2
<\__main__.Bar2 instance at 0x025B2B48>
<\__main__.Bar2 instance at 0x025B2AF8>
<\__main__.Foo instance at 0x025B2B70>
<\__main__.Foo instance at 0x025B2B98>

Using Bar both instances will refer to the same Foo instance. Why?

Edit: Corrected the error with printing first.foo twice for Bar. The resulting behaviour is still as seen in the output.

Answer 1

Bar.foo is a class variable. It is initialised once when the class is created.

(Note that your code als print first.foo twice, so no wonder the output is the same.)