Mark Bourne 在 2022年8月29日 星期一下午6:40:59 [UTC+8] 的信中寫道： > Jach Feng wrote: > > Chris Angelico 在 2022年8月29日 星期一下午1:58:58 [UTC+8] 的信中寫道： > >> On Mon, 29 Aug 2022 at 15:54, Jach Feng <jf...@ms4.hinet.net> wrote: > >>> > >>> Richard Damon 在 2022年8月29日 星期一上午10:47:08 [UTC+8] 的信中寫道： > >>>> On 8/27/22 7:42 AM, Mark Bourne wrote: > >>>>> Jach Feng wrote: > >>>>>> I have two files: test.py and test2.py > >>>>>> --test.py-- > >>>>>> x = 2 > >>>>>> def foo(): > >>>>>> print(x) > >>>>>> foo() > >>>>>> > >>>>>> x = 3 > >>>>>> foo() > >>>>>> > >>>>>> --test2.py-- > >>>>>> from test import * > >>>>>> x = 4 > >>>>>> foo() > >>>>>> > >>>>>> ----- > >>>>>> Run test.py under Winows8.1, I get the expected result: > >>>>>> e:\MyDocument>py test.py > >>>>>> 2 > >>>>>> 3 > >>>>>> > >>>>>> But when run test2.py, the result is not my expected 2,3,4:-( > >>>>>> e:\MyDocument>py test2.py > >>>>>> 2 > >>>>>> 3 > >>>>>> 3 > >>>>>> > >>>>>> What to do? > >>>>> > >>>>> `from test import *` does not link the names in `test2` to those in > >>>>> `test`. It just binds objects bound to names in `test` to the same > >>>>> names in `test2`. A bit like doing: > >>>>> > >>>>> import test > >>>>> x = test.x > >>>>> foo = test.foo > >>>>> del test > >>>>> > >>>>> Subsequently assigning a different object to `x` in one module does > >>>>> not affect the object assigned to `x` in the other module. So `x = 4` > >>>>> in `test2.py` does not affect the object assigned to `x` in `test.py` > >>>>> - that's still `3`. If you want to do that, you need to import `test` > >>>>> and assign to `test.x`, for example: > >>>>> > >>>>> import test > >>>>> test.x = 4 > >>>>> test.foo() > >>>>> > >>>> Yes, fundamental issue is that the statement > >>>> > >>>> from x import y > >>>> > >>>> makes a binding in this module to the object CURRECTLY bound to x.y to > >>>> the name y, but if x.y gets rebound, this module does not track the > >>>> changes. > >>>> > >>>> You can mutate the object x.y and see the changes, but not rebind it. > >>>> > >>>> If you need to see rebindings, you can't use the "from x import y" form, > >>>> or at a minimum do it as: > >>>> > >>>> > >>>> import x > >>>> > >>>> from x import y > >>>> > >>>> then later to get rebindings to x.y do a > >>>> > >>>> y = x.y > >>>> > >>>> to rebind to the current x.y object. > >>>> > >>>> -- > >>>> Richard Damon > >>> Yes, an extra "import x" will solve my problem too! Sometimes I am > >>> wondering why "from x import y" hides x? hum...can't figure out the > >>> reason:-) > >>> > >> "from x import y" doesn't hide x - it just grabs y. Python does what > >> you tell it to. :) > >> > >> ChrisA > > But I had heard people say that "from x import y" did import the whole x > > module into memory, just as "import x" did, not "grabs y" only. Is this > > correct? > `from x import y` does import the whole module x into memory, and adds > it to `sys.modules`. But it only binds the name `y` in the namespace of > module doing the import (and it binds it to the value of `x.y` at the > time the import is done - it doesn't magically keep them in sync if one > or the other is later reassigned). > > The point about the whole module being imported is that you don't save > any memory by using `from x import y` to avoid importing some very large > object `z` from `x`. Those other large objects might be needed by > functions which have been imported (e.g. your `foo` function still needs > `x` even if you haven't imported `x` - so it still needs to be loaded > into memory) or might be imported and used by other modules importing > `x`, so they still have to be loaded when any part of `x` is imported - > they just don't have to be bound to names in the importing module's > namespace. > > As Richard mentioned, if `x.y` is a mutable object (such as a list) you > can still mutate that object (e.g. add/remove items) and those changes > will be seen in both modules. That's because both are still bound to > the same object and you're mutating that existing object. If you assign > a new list to either, that won't be seen by the other. > > -- > Mark. When using dot notation to change variable, no matter if 'x.y' is a mutable or immutable object, the change will be seen in both modules except those early bindings.
--Jach -- https://mail.python.org/mailman/listinfo/python-list
