it's not a scope issue. you are confusing variables and objects. a variable is a box that can hold an object
so x = 2 puts the object '2' in the box 'x'. following that with x = '3' changes the box 'x' to hold the object '3'. but lists are also boxes, different from variables. so x = [1,2,3] puts the a list object, that is a box that contains 1, 2 and 3, in 'x' then y = x puts THE SAME list object in box 'y'. then y[1] = 4 changes the second item in the list's box to 4 the print x gives "[1,4,3]" because you have changed contents of the list. in contrast x = 3 then y = x then y = 4 does not change x because you care changing variables, not list contents. to fix your code, you need to copy the list x = [1,2,3] y = list(x) # copy y[1] = 4 print x [1,2,3] surely this is ina faq? it comes up once a day... andrew gyro wrote: > Hi, > I was writing a Python script to perform some data analyses and was > surprised by some behavior I noted. A simple test program illustrating > the behavior is below. > I do not understand why the value of 'data' is being modified. I am > obviously missing something obvious, and would certainly appreciate an > explanation of why this is happening. > > Thank you. > > -gf > > ---------- > > #!/bin/env python > > class TestPop(object): > def round1(self,data1): > t = data1.pop(-1) > > def round2(self,data2): > t = data2.pop(-1) > > def tester(self): > data = range(10) > self.round1(data) > print data > self.round2(data) > print data > > if __name__ == '__main__': > tp = TestPop() > tp.tester() > -- > http://mail.python.org/mailman/listinfo/python-list > > -- http://mail.python.org/mailman/listinfo/python-list
