This comes up on the list about once a week on this list. See: http://www.nexedi.org/sections/education/python/tips_and_tricks/python_and_mutable_n/view
-Larry Bates [EMAIL PROTECTED] wrote: > Hi All: > > Here's a piece of Python code and it's output. The output that Python > shows is not as per my expectation. Hope someone can explain to me this > behaviour: > > [code] > class MyClass: > def __init__(self, myarr=[]): > self.myarr = myarr > > myobj1 = MyClass() > myobj2 = MyClass() > > myobj1.myarr += [1,2,3] > > myobj2.myarr += [4,5,6] > > print myobj1.myarr > print myobj2.myarr > [/code] > > The output is: > [1, 2, 3, 4, 5, 6] > [1, 2, 3, 4, 5, 6] > > Why do myobj1.myarr and myobj2.myarr point to the same list? The > default value to __init__ for the myarr argument is [], so I expect > that every time an object of MyClass is created, a new empty list is > created and assigned to myarr, but it seems that the same empty list > object is assigned to myarr on every invocation of MyClass.__init__ > > It this behaviour by design? If so, what is the reason, as the > behaviour I expect seems pretty logical. > > Thanks. > > Vaibhav > -- http://mail.python.org/mailman/listinfo/python-list
