On Tue, 30 Aug 2022 at 02:38, Jach Feng <jf...@ms4.hinet.net> 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?
>
In order to do any sort of import, Python has to run the whole module.
But after that, something gets set in your module so that you can get
access to it.
import x
# is kinda like
go_and_run("x")
x = fetch_module("x")
from x import y
# is kinda like
go_and_run("x")
y = fetch_module("x").y
Either way, the whole module gets run, but then there's an assignment
into your module that depends on what you're importing.
ChrisA
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