Where is the default parameter in Python function

Question

I think many people have seen the python\'s function which receives default parameters. For example:

def foo(a=[]):
    a.append(3)
    return a

Answer 1

I have found an interesting situation: in python 2.5.2 version, try the function 'foo()'

>>> foo()
[1]
>>> foo()
[1]
>>> foo()
[1]

Because the objects of the function called are different:

>>> id(foo())
4336826757314657360
>>> id(foo())
4336826757314657008
>>> id(foo())
4336826757314683160

In 2.7.2 version:

>>> foo()
[1]
>>> foo()
[1, 1]
>>> foo()
[1, 1, 1]

In this case, the object is the same each time calling the function:

>>> id(foo())
29250192
>>> id(foo())
29250192
>>> id(foo())
29250192

Is it a problem of different versions?