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?

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



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