On Wed, Jan 5, 2011 at 10:07 AM, tinauser <tinau...@libero.it> wrote:
> Hallo list,
> here again I have a problem whose solution might be very obvious, but
> I really cannot see it:
> I have a class having as attribute a dictionary whose keys are names
> and values are instance of another class.
> This second class has in turn as an attribute a dictionary.
> I want a function of the first class that can change value of one of
> the second class instance's dictionary.
> however I cannot do it without modifying this attribute for ALL the
> instance of the second class contained in the first class' dictionary.
> What I'm doing wrong?
>
This is one of the biggest gotchas in Python. Default arguments are
only evaluated *once*, when the function/method is declared. Not every
time the function is called. Every instance of mistClass that didn't
specify a separate cDict gets the same object as its cDict. The
solution is to use a sentinel value (either None or a single object
and use an "is" comparison) and create a new dict in the constructor
if the default argument is still the sentinel.
> class mistClass():
> def __init__(self,name,cDict={}):
> print 'mistClass ',name,' Init'
> self._name=name
> self._cDict=cDict
>
should be changed to
sentinel = object()
class mistClass :
def __init__(self, name, cDict=sentinel) :
print 'mistClass ',name,' Init'
self._name=name
if cDict is not sentinel :
self._cDict=cDict
else :
self._cDict = {}
> class mistClassContainer():
> def __init__(self,name,dict_of_mistclass={}):
> print 'init mistClassContainer ',name
> self._name=name
> self._DOM=dict_of_mistclass
>
and do the same thing with this one.
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