Terry Reedy <tjre...@udel.edu> writes: > On 11/13/2010 11:29 AM, Mark Wooding wrote: > > > Alas, Python is actually slightly confusing here, since the same > > notation `=' sometimes means assignment and sometimes means mutation. > > I disagree somewhat. An object is mutated by an internal assignment.
Some object types are primitive, provided by the runtime system; there are no `internal' variables to be assigned in these cases. (It is possible in principle in Python 3 to implement all of the compound data types using only assignment and closures.) > "ll[0] = 1" assigns 1 to the 0 slot of ll. > "o.a = 1" assigns 1 to the 'a' attribute of o. > This which might be implemented by assigning 1 to the 'a' slot of o.__dict__, > just as "a=1" might be implemented by assigning 1 to the 'a' slot of a > namespace dict. There's a qualitative difference here: simple assignment has semantics defined by the language and provided by the implementation, and can therefore be understood in isolation, using only lexically apparent information; whereas the complex assignments are implemented by invoking methods on the objects mentioned on the left hand side. The objects in question may choose not to implement some other semantics, so the runtime behaviour might not be determinable even in principle without actually executing the program. (Compound assignment has the same problem writ large.) > Assignment *always* binds an object to a target. No! Assignment /never/ binds. There is syntactic confusion here too, since Python interprets a simple assignment in a function body -- in the absence of a declaration such as `global' to the contrary -- as indicating that the variable in question should be bound to a fresh variable on entry to the function. But assignment itself doesn't perform binding. (This is a persistent error in the Python community; or, less charitably, the Python community gratuitously uses the word in a different sense from the wider programming-language-theory community. See Lisp literature passim, for example.) There's a two step mapping: names -> storage locations -> values. Binding affects the left hand part of the mapping; assignment affects the right hand part. -- [mdw] -- http://mail.python.org/mailman/listinfo/python-list
