After one 2 months of python intensive development. I made this init
defaults error in my db classes constructors...
Shame on me :-[ Steven !
Your example shows me obvious.
Regards
Karim
On 01/26/2011 12:33 AM, Steven D'Aprano wrote:
Karim wrote:
Hello All,
Just to share on rageous bug I encounter where 2 lists which "should"
to be different (same id) because hold by different instances of the
same class are not in fact DIFFERENT, see below:
I think you are confused. If the two lists have the same ID, they
should be the same, not different.
>>> class Device():
... def __init__(self, parameters=[]):
... self.parameters = parameters
This creates a list ONCE, when the method is created, and then uses
that same list over and over again.
Consider this:
>>> import time
>>> now = time.ctime()
>>> now
'Wed Jan 26 10:05:31 2011'
>>> def test(t=now):
... print(t)
...
>>> test()
Wed Jan 26 10:05:31 2011
>>> time.sleep(60); test()
Wed Jan 26 10:05:31 2011
Are you surprised that time.ctime() only gets called *once*? I hope
not -- I would expect that you consider this example obvious and not
surprising.
How about this instead?
>>> def test2(t=time.ctime()):
... print(t)
...
>>> test2()
Wed Jan 26 10:09:10 2011
>>> time.sleep(60); test2()
Wed Jan 26 10:09:10 2011
I hope that this also will not be surprising. time.ctime() is called
once, when the function is created, and the result used as often as
needed.
It is no different when the default value is a list like []. The list
is created once, and used each time the function is called.
> When I discovered that I was puzzled because at the prompt:
>>> a = []
>>> b = []
>>> id(a)
140559202956496
>>> id(b)
140559202957000
But this test is completely different. You create TWO lists, not one.
A better test would be:
>>> a = []
>>> b = a
>>> id(a), id(b)
(3083146668, 3083146668)
I am not really understanding why my init in the class made it refers
to the same list object.
Because it only creates one list, when the method is defined, not each
time the method is called.
Only the *inside* of the method is executed each time the method is
called, not the method definition. If you want something to be
executed each time the method is called, you have to put it in the
body of the method:
>>> def test3(t=None):
... if t is None: t = time.ctime()
... print(t)
...
>>> test3()
Wed Jan 26 10:18:33 2011
>>> time.sleep(60); test3()
Wed Jan 26 10:19:37 2011
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