David Perlman wrote:
OK, I thought I had this one fixed but it was weirder than
I thought. I think I understand what's going on, but I wanted to check
with the experts here.
I have the following class definition, which does not subclass
anything:
class oneStim:
def __init__(self, time, mods=[], dur=None, format='%1.2f'):
self.time=time
self.mods=mods
self.dur=dur
self.format=format
def __cmp__(self,other):
return cmp(self.time,other.time)
def __repr__(self):
timestr=self.format % self.time
if self.mods == []:
modstr=''
else:
modstr = '*' + ','.join(self.format % i for i in self.mods)
if self.dur == None:
durstr = ''
else:
durstr = ':' + (self.format % self.dur)
return timestr + modstr + durstr
def __len__(self):
return len(self.__repr__())
>>> a=oneStim(40)
>>> a
40.00
>>> a.mods.append(3)
>>> a
40.00*3.00
>>> a.dur=10
>>> a
40.00*3.00:10.00
>>> a.mods.append(1)
>>> a
40.00*3.00,1.00:10.00
So far so good, that's exactly what it's supposed to do. But now look:
>>> b=oneStim(50)
>>> b
50.00*3.00,1.00
The mods that were added to the first instance of oneStim also appear
in the second, newly created instance!
It appears that what is happening here is that the __init__() method is
being parsed by the interpreter once at initial run, and at that time
the statement "mods=[]" is being parsed, which means that the [] object
is being instantiated once there at the beginning. So every
instantiation of class oneStim ends up sharing a reference to the same
list object, instead of each one having its own.
I fixed this by changing it to "mods=None" and then setting it in the
body of the __init__ method. Works fine now.
My question is, is this just a quirky misbehavior, or is there a
principled reason why the code I have shown above only instantiates the
empty list in the arguments once?
RTFM: (Reference Manual, 7.6 Function definitions)
....
Default parameter values are evaluated when the function
definition is executed. This means that the _expression_ is
evaluated once, when the function is defined, and that that same
``pre-computed'' value is used for each call. This is especially
important to understand when a default parameter is a mutable object,
such as a list or a dictionary: if the function modifies the object
(e.g. by appending an item to a list), the default value is in effect
modified. This is generally not what was intended. A way around this is
to use None as the default, and explicitly test for it in
the body of the function, e.g.:
def whats_on_the_telly(penguin=None):
if penguin is None:
penguin = []
penguin.append("property of the zoo")
return penguin
Thanks for any insight. As I said, I got it to work fine now, so this
isn't critical, but I'm curious to understand why things work the way
they do. :)
--
-dave----------------------------------------------------------------
"Pseudo-colored pictures of a person's brain lighting up are
undoubtedly more persuasive than a pattern of squiggles produced by a
polygraph. That could be a big problem if the goal is to get to the
truth." -Dr. Steven Hyman, Harvard
_______________________________________________
Tutor maillist - Tutor@python.org
To unsubscribe or change subscription options:
http://mail.python.org/mailman/listinfo/tutor
--
Bob Gailer
Chapel Hill NC
919-636-4239
|
