Counterintuitive Python behavior

[dominikush at yahoo.com]

Hello,

one thing I like very much about Python is that statements
work like you would expect them to work. Take for example
the use of dict.values() for dictionaries: If you store the
result of dict.values(), and change the dictionary after-
wards, the previously stored result remains untouched.

>>> dict = {'a':1,'b':2}
>>> list = dict.values()
>>> list
[1, 2]
>>> dict['a']=3
>>> list
[1, 2]
>>> dict
{'a': 3, 'b': 2}

However, if a dictionary has lists as value entries, I get
a counterintuitive behavior (which, just recently, broke my
code): If you change the dict, the list you previously
created via dict.values() gets automagically updated. A nice
feature, but nothing I would have expected!

>>> dict = {'a':[1],'b':[2]}
>>> list = dict.values()
>>> list
[[1], [2]]
>>> dict['a'].append(3)
>>> dict
{'a': [1, 3], 'b': [2]}
>>> list
[[1, 3], [2]]

Looks like that in the first case a copy is returned while
in the latter case list references are returned. Ok, but
according to Python's philosophy I shouldn't mind if I work
with lists in the dictionary or anything else. If the
behavior depends on the knowledge of the type of values I
put into a dictionary, I find that somehow counterintuitive.

Who is wrong here: my intuition or Python (2.2)? If it's
my intuition, how can I train my thinking about Python's
execution model, so that my intuition get's better ;-)

Best regards,

Dominikus