Dear Sage team,
i guess the following is more a python- than a Sage-problem, and also
i have a work-around. But perhaps you can help me to learn something.
I have the following code in a module MyTest.so:
class T:
def __init__(self,D={}):
self.Data = D
def __repr__(self):
return str(self.Data)
def __setitem__(self,I,v):
self.Data[I] = v
Now, creating two instances of T and changing one of them changes the
other, too:
sage: from MyTest import T
sage: a=T()
sage: a[(4,3)]=2
sage: b=T()
sage: b
{(4, 3): 2}
sage: a[(3,4)]=5
sage: b
{(3, 4): 5, (4, 3): 2}
But why? Both in a and b, the Data-attribute should be initialized as
{}. Moreover, a.Data and b.Data should be different objects, because
"D={}" should create a *new* empty dictionary each time. So, i don't
see what goes wrong.
The work-around is:
def __init__(self,D={}):
self.Data = copy(D)
However, i don't understand why copy() is needed here.
Thanks in advance for an explanation
Simon
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