>>>>> Antoon Pardon <[EMAIL PROTECTED]> (AP) wrote: >AP> When someone gets confused over the difference between rebinding or >AP> mutating a variable on an intermediate scope, the explanation he >AP> mostly seems to get boils down to: one is rebinding, the other is >AP> mutation, this is a fundametal difference in python.
>AP> My impression is that they seem to say that the fundamental difference >AP> between mutation and rebinding implies the specific behaviour python >AP> has now. IMO this explanation is incomplete. The python developers >AP> could have chosen that a line like 'c.a = ...' would have resulted >AP> in c being included in the local scope. Then rebinding and mutation >AP> would still be fundamentally different from each other but the specific >AP> confusion over why 'k[0] = ...' worked as expeced but 'k = ...' didn't, >AP> will disappear. That seems nonsense to me. If they had chosen that 'c.a = ...' would imply that c would become a local variable, what would the value of c have to be then, if there was no prior direct assignment to c? Would it become a new binding like in 'c = ...'? From what class should it become an instance? Or would it become a copy of the value that 'c' has in an outer scope? I don't know any programming language where an assignment to c.a does create a new c, rather than modifying the existing value of c. That would have been a very strange language design. Similarly for 'k[0] = ...'. What would happen with the other elements of k? There are no situations in Python where an assignment to c.a or c[0] suddenly lets spring c into existence. You always need an already existing binding for c for this to be valid. And it always uses that binding, and doesn't move or copy it to a different block. Some of the confusing originates from the fact that assignment in Python is subtly different from assignment in other programming languages. In most languages variables denote memory locations or collections of memory locations. An assignment then means changing the contents of those memory locations. In python variables are bound to values, and assignment means (re)binding the name to a possibly different value. With an example: most other languages have boxes with the name of the variable on them. An assignment changes the contents of the box. In Python you have values (objects) and an assignment means sticking a label with the name of the variable on the object. Often the difference is unnoticable, but there are subtle cases where this really makes a difference. When the lefthandside of an assignment is an object attribute, subscription or slice then the assignment is syntactic sugar for a mutation operation implemented by the object, which usually changes the value of the object but could do something completely different. (Finally, the lefthandside can also be a [nested] tuple or list, in which case it is a collection of parallel assignments. And oh yes, there are also other binding operations e.g. a function or class definition.) -- Piet van Oostrum <[EMAIL PROTECTED]> URL: http://www.cs.uu.nl/~piet [PGP 8DAE142BE17999C4] Private email: [EMAIL PROTECTED] -- http://mail.python.org/mailman/listinfo/python-list
