Chris Angelico於 2019年2月27日星期三 UTC+8上午9時25分11秒寫道： > On Wed, Feb 27, 2019 at 12:21 PM <jf...@ms4.hinet.net> wrote: > > > > Gregory Ewing at 2019/2/27 AM 5:26:49 wrote: > > > Thomas Jollans wrote: > > > > I imagine there's a justification for the difference in behaviour to do > > > > with the fact that the body of a class is only ever executed once, while > > > > the body of a function is executed multiple times. > > > > > > I suspect there isn't any deep reason for it, rather it's just > > > something that fell out of the implementation, in particular > > > the decision to optimise local variable access in functions > > > but not other scopes. > > > > > > When compiling a function, the compiler needs to know which > > > variables are local so that it can allocate slots for them in > > > the stack frame. But when executing a class body, the locals > > > are kept in a dict and are looked up dynamically. > > > > If the compiler can do any decision on the variable's name, when it goes to > > line of print, how it handle it? > > > > x = 0 > > def f(): > > print(x) > > > > def g(): > > print(x) > > x = 1 > > print(y) > > > > Not sure what you mean by "decision", but the definition is that since > x is assigned to in g(), it is local to g(). It's not local to f(), so > the global is visible.
So, may I say that the Python compiler is a multi-pass one? --Jach > > (Incidentally, "print" is handled exactly the same way. Since neither > function assigns to print, it's not local, so the built-in is found.) > > ChrisA -- https://mail.python.org/mailman/listinfo/python-list
