Possible Duplicate:
“Least Astonishment” in Python: The Mutable Default Argument
I was just playing around with python and stumbled upon something that confuses me.
Why does t1 and t2 in the code below share the same decision dictionary? I have to write
t1=Decision({},txt1="Test1",txt2="Response Test1")
instead of
t1=Decision(txt1="Test1",txt2="Response Test1")
for them to have one each. Why?
I use Python 2.7 if that is of any importance.
class Decision(object):
def __init__(self,decisions={},txt1="",txt2=""):
self.decisions=decisions
self.txt1=txt1
self.txt2=txt2
def run(self):
if len(self.decisions)>0:
print self.txt1
print self.txt2
for i in self.decisions: print i + " - " + self.decisions[i][0]
ans=""
while True:
ans=raw_input("Enter answer: ")
if ans in self.decisions: break
if self.decisions[ans][1] is not None: self.decisions[ans][1].run()
t1=Decision(txt1="Test1",txt2="Response Test1")
t2=Decision(txt1="Test2",txt2="Response Test2")
t1.decisions["yes1"]=("Yes","t2")
t1.decisions["no1"]=("No","t3")
t2.decisions["yes2"]=("Yes","t3")
t2.decisions["no2"]=("No","t1")
print t1.decisions
<<<{'no2': ('No', 't1'), 'no1': ('No', 't3'), 'yes1': ('Yes', 't2'), 'yes2': ('Yes', 't3')}
print t2.decisions
<<<{'no2': ('No', 't1'), 'no1': ('No', 't3'), 'yes1': ('Yes', 't2'), 'yes2': ('Yes', 't3')}
By the way. I solved it by doing this:
class Decision(object):
def __init__(self,decisions=None,txt1="",txt2=""):
if decisions==None:self.decisions={}
else:self.decisions=decisions
...
But I would like to know why. Easier to remember not to do it when I now why I shouldn't.
