On Wed, 04 Jan 2006 10:54:17 -0800, KraftDiner wrote: > I was under the assumption that everything in python was a refrence... > > so if I code this: > lst = [1,2,3] > for i in lst: > if i==2: > i = 4 > print lst > > I though the contents of lst would be modified.. (After reading that > 'everything' is a refrence.)
See, this confusion is precisely why I get the urge to slap people who describe Python as "call by reference". It isn't. It isn't "call by value" either -- Python never copies objects unless you explicitly tell it to. It is "call by object" -- you pass around *objects*. Internally, this is quite fast, because the entire object doesn't need to be moved, only pointers to objects, but you don't get the behaviour of either call by reference or call by value. In the above code, i is a name bound one at a time to the ints [1,2,3]. When you re-assign i to 4, that doesn't change the object 2 into the object 4, because ints are immutable. Only the name i is rebound to a new object 4. That doesn't change objects like lst which include 2 inside them. See this for more detail: http://effbot.org/zone/call-by-object.htm -- Steven. -- http://mail.python.org/mailman/listinfo/python-list
