And this is expected behavior. What is unexpected is when this behavior is afforded to new list or dictionary arguments in Python, just as it would be unexpected to say

    ruby> class A; end
     => nil 
    ruby> def foo(bar = A.new); return bar; end
     => nil 
    ruby> foo
     => #<A:0x00000101985060> 
    ruby> foo
     => #<A:0x0000010197a6b0>

and get back the same object each time `foo` is called in Ruby.

I've even seen a major Python library with this bug (I'm sorry, I don't recall which off-hand). It's really surprising behavior for new Python devs.