How to make a variable's late binding crosses the module boundary?
Jach Feng
jfong at ms4.hinet.net
Mon Aug 29 21:02:53 EDT 2022
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... at 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
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