Confused about namespaces

[KvS]

Hi all,

to start with, excuse me, I'm still learning programming alltogether,
probably I'm making some fundamental mistake here...

I have the files settings.py, GUIclasses.py and main.py in the same
directory. In the file main.py are the statements:

import settings
from GUIclasses import *

class Toepassing(wx.A pp):
def OnInit(self):
window = KFrame(None, "Testerdete st", (1000,900))
self.SetTopWind ow(window)
window.Show(Tru e)
return True

app = Toepassing(None )
app.MainLoop()

In the file GUIclasses.py I have the statements:

import wx
import wx.lib.mixins.l istctrl as listmix

class KFrame(wx.Frame ):
def __init__(self, parent, title, Size):
...some code...
self.lst = settings.attrLi jst
....some more code......

Now if I run the main.py file I get the error:

File "G:\Programmere n\Codes contactenlijst\ GUIclasses.py", line 40,
in __init_
_
self.lst = settings.attrLi jst
NameError: name 'settings' is not defined

Why is this? Since "from GUIclasses import *" this KFrame is now at
"the lowest" namespace and should therefore be able to make use of any
variables living there, including "settings.* ", no?

Thanks in advance!

- Kees

[Chris Mellon]

Re: Confused about namespaces

On 18 Nov 2005 15:04:23 -0800, KvS <keesvanschaik@ gmail.com> wrote:[color=blue]
> Hi all,
>
> to start with, excuse me, I'm still learning programming alltogether,
> probably I'm making some fundamental mistake here...
>
> I have the files settings.py, GUIclasses.py and main.py in the same
> directory. In the file main.py are the statements:
>
> import settings
> from GUIclasses import *
>
> class Toepassing(wx.A pp):
> def OnInit(self):
> window = KFrame(None, "Testerdete st", (1000,900))
> self.SetTopWind ow(window)
> window.Show(Tru e)
> return True
>
> app = Toepassing(None )
> app.MainLoop()
>
> In the file GUIclasses.py I have the statements:
>
> import wx
> import wx.lib.mixins.l istctrl as listmix
>
> class KFrame(wx.Frame ):
> def __init__(self, parent, title, Size):
> ...some code...
> self.lst = settings.attrLi jst
> ....some more code......
>
> Now if I run the main.py file I get the error:
>
> File "G:\Programmere n\Codes contactenlijst\ GUIclasses.py", line 40,
> in __init_
> _
> self.lst = settings.attrLi jst
> NameError: name 'settings' is not defined
>
> Why is this? Since "from GUIclasses import *" this KFrame is now at
> "the lowest" namespace and should therefore be able to make use of any
> variables living there, including "settings.* ", no?
>[/color]

import is not like a C/C++ #include - the code in the imported module
is evaluated in it's own namespace, not in the namespace of the
importing file.

So no, this is expected behavior, and should be solved by importing
settings in GUIclasses.py as well.

You don't need to worry about multiple or redundent imports.

[color=blue]
> Thanks in advance!
>
> - Kees
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>[/color]

[Chris Mellon]

Re: Confused about namespaces

On 18 Nov 2005 15:04:23 -0800, KvS <keesvanschaik@ gmail.com> wrote:[color=blue]
> Hi all,
>
> to start with, excuse me, I'm still learning programming alltogether,
> probably I'm making some fundamental mistake here...
>
> I have the files settings.py, GUIclasses.py and main.py in the same
> directory. In the file main.py are the statements:
>
> import settings
> from GUIclasses import *
>
> class Toepassing(wx.A pp):
> def OnInit(self):
> window = KFrame(None, "Testerdete st", (1000,900))
> self.SetTopWind ow(window)
> window.Show(Tru e)
> return True
>
> app = Toepassing(None )
> app.MainLoop()
>
> In the file GUIclasses.py I have the statements:
>
> import wx
> import wx.lib.mixins.l istctrl as listmix
>
> class KFrame(wx.Frame ):
> def __init__(self, parent, title, Size):
> ...some code...
> self.lst = settings.attrLi jst
> ....some more code......
>
> Now if I run the main.py file I get the error:
>
> File "G:\Programmere n\Codes contactenlijst\ GUIclasses.py", line 40,
> in __init_
> _
> self.lst = settings.attrLi jst
> NameError: name 'settings' is not defined
>
> Why is this? Since "from GUIclasses import *" this KFrame is now at
> "the lowest" namespace and should therefore be able to make use of any
> variables living there, including "settings.* ", no?
>[/color]

import is not like a C/C++ #include - the code in the imported module
is evaluated in it's own namespace, not in the namespace of the
importing file.

So no, this is expected behavior, and should be solved by importing
settings in GUIclasses.py as well.

You don't need to worry about multiple or redundent imports.

[color=blue]
> Thanks in advance!
>
> - Kees
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>[/color]

[KvS]

Re: Confused about namespaces

Ok, makes sense but didn't seem "natural" to me, although it is an
obvious consequence of what you just pointed out, namely that modules
are evaluated in their own namespace, something to keep in mind... On
the other hand it does apparently work recursively "the other way
around" since I didn't explicitly import wx in main.py but only
indirect via importing GUIclasses in which wx is imported right?

Thanks. :).

[KvS]

Re: Confused about namespaces

Ok, makes sense but didn't seem "natural" to me, although it is an
obvious consequence of what you just pointed out, namely that modules
are evaluated in their own namespace, something to keep in mind... On
the other hand it does apparently work recursively "the other way
around" since I didn't explicitly import wx in main.py but only
indirect via importing GUIclasses in which wx is imported right?

Thanks. :).

[Chris Mellon]

Re: Confused about namespaces

On 18 Nov 2005 15:29:43 -0800, KvS <keesvanschaik@ gmail.com> wrote:[color=blue]
> Ok, makes sense but didn't seem "natural" to me, although it is an
> obvious consequence of what you just pointed out, namely that modules
> are evaluated in their own namespace, something to keep in mind... On
> the other hand it does apparently work recursively "the other way
> around" since I didn't explicitly import wx in main.py but only
> indirect via importing GUIclasses in which wx is imported right?
>[/color]

it worked because all the code that used stuff from the wx namespace
was in GUIclasses.py. If you tried to use create wx classes directly
in your toplevel file it would fail unless you did an "import wx"
first.

Now, as a slight consequence of the specific way you imported, it may
appear to do something different - when GUIClasses.py imported wx, an
entry in the modules namespace was created with the value "wx", and
the value being the wx module. When you do "from GUIClasses import *",
that imports all the symbols in GUIClasses namespace into your local
namespace, including the "wx" entry. This kind of cascading behavior
is one reason that "from foo import *" is frowned up, because the
namespace pollution can cause confusing bugs.
[color=blue]
> Thanks. :).
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>[/color]

[Chris Mellon]

Re: Confused about namespaces

On 18 Nov 2005 15:29:43 -0800, KvS <keesvanschaik@ gmail.com> wrote:[color=blue]
> Ok, makes sense but didn't seem "natural" to me, although it is an
> obvious consequence of what you just pointed out, namely that modules
> are evaluated in their own namespace, something to keep in mind... On
> the other hand it does apparently work recursively "the other way
> around" since I didn't explicitly import wx in main.py but only
> indirect via importing GUIclasses in which wx is imported right?
>[/color]

it worked because all the code that used stuff from the wx namespace
was in GUIclasses.py. If you tried to use create wx classes directly
in your toplevel file it would fail unless you did an "import wx"
first.

Now, as a slight consequence of the specific way you imported, it may
appear to do something different - when GUIClasses.py imported wx, an
entry in the modules namespace was created with the value "wx", and
the value being the wx module. When you do "from GUIClasses import *",
that imports all the symbols in GUIClasses namespace into your local
namespace, including the "wx" entry. This kind of cascading behavior
is one reason that "from foo import *" is frowned up, because the
namespace pollution can cause confusing bugs.
[color=blue]
> Thanks. :).
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>[/color]

[KvS]

Re: Confused about namespaces

Hmm. But actually I was doing this import from GUIclasses with exactly
this in mind, namely that it would make wx also available at top level.
I (in my naive understanding) see this as "natural" and actually
desirable, how could this cause confusing bugs? Do you mean multiple
"from ... import *"'s 'on top of each other' would cause foo1.foo2.attr
and foo1.attr both would become just attr and therefore ambiguous at
top level?

If you import foo in two different modules, does the interpreter then
create one instance of foo and create a reference in both modules to
the same foo rather than creating two different instances of foo?

[KvS]

Re: Confused about namespaces

Hmm. But actually I was doing this import from GUIclasses with exactly
this in mind, namely that it would make wx also available at top level.
I (in my naive understanding) see this as "natural" and actually
desirable, how could this cause confusing bugs? Do you mean multiple
"from ... import *"'s 'on top of each other' would cause foo1.foo2.attr
and foo1.attr both would become just attr and therefore ambiguous at
top level?

If you import foo in two different modules, does the interpreter then
create one instance of foo and create a reference in both modules to
the same foo rather than creating two different instances of foo?

[Chris Mellon]

Re: Confused about namespaces

On 18 Nov 2005 16:09:44 -0800, KvS <keesvanschaik@ gmail.com> wrote:[color=blue]
> Hmm. But actually I was doing this import from GUIclasses with exactly
> this in mind, namely that it would make wx also available at top level.[/color]

There's no reason not to just "import wx" if you want that.
[color=blue]
> I (in my naive understanding) see this as "natural" and actually
> desirable, how could this cause confusing bugs? Do you mean multiple
> "from ... import *"'s 'on top of each other' would cause foo1.foo2.attr
> and foo1.attr both would become just attr and therefore ambiguous at
> top level?[/color]

the second import will overwrite the first, making the first inaccessible
[color=blue]
>
> If you import foo in two different modules, does the interpreter then
> create one instance of foo and create a reference in both modules to
> the same foo rather than creating two different instances of foo?
>[/color]

No. It creates the foos within each module, but which foo you have
access to in the importing module is determined by the order of
import.
[color=blue]
> --
> http://mail.python.org/mailman/listinfo/python-list
>[/color]

[Chris Mellon]

Re: Confused about namespaces

On 18 Nov 2005 16:09:44 -0800, KvS <keesvanschaik@ gmail.com> wrote:[color=blue]
> Hmm. But actually I was doing this import from GUIclasses with exactly
> this in mind, namely that it would make wx also available at top level.[/color]

There's no reason not to just "import wx" if you want that.
[color=blue]
> I (in my naive understanding) see this as "natural" and actually
> desirable, how could this cause confusing bugs? Do you mean multiple
> "from ... import *"'s 'on top of each other' would cause foo1.foo2.attr
> and foo1.attr both would become just attr and therefore ambiguous at
> top level?[/color]

the second import will overwrite the first, making the first inaccessible
[color=blue]
>
> If you import foo in two different modules, does the interpreter then
> create one instance of foo and create a reference in both modules to
> the same foo rather than creating two different instances of foo?
>[/color]

No. It creates the foos within each module, but which foo you have
access to in the importing module is determined by the order of
import.
[color=blue]
> --
> http://mail.python.org/mailman/listinfo/python-list
>[/color]

[KvS]

Re: Confused about namespaces

> There's no reason not to just "import wx" if you want that.

Yes, that's clear. But if you would make some huge application that has
a large number of nested modules, each importing the former one, then
avoiding the use of "from ... import *" would mean that you have to use
long references like foo1.foo2.... to get to the lowest modules plus
that you'd have to check each module for imports outside this tree.

If you would use "from ... import *" (except at top level) you have to
be aware of overriding, but you can also use this to your advantage and
any foo1.attr reference would just work right, without further ado...
Or would in such a case a class hierarchy be the thing to use?
[color=blue]
> No. It creates the foos within each module, but which foo you have
> access to in the importing module is determined by the order of
> import.[/color]

Am I understanding correctly that if you have a module foo importing wx
and a module main importing both foo and wx there are actually two
instances of wx created, one referenced to by (at top level) foo.wx.*
and one wx.*? If this is indeed the case it isn't too good for the
performance doing this importing of wx multiple times right?

- Kees

[KvS]

Re: Confused about namespaces

> There's no reason not to just "import wx" if you want that.

Yes, that's clear. But if you would make some huge application that has
a large number of nested modules, each importing the former one, then
avoiding the use of "from ... import *" would mean that you have to use
long references like foo1.foo2.... to get to the lowest modules plus
that you'd have to check each module for imports outside this tree.

If you would use "from ... import *" (except at top level) you have to
be aware of overriding, but you can also use this to your advantage and
any foo1.attr reference would just work right, without further ado...
Or would in such a case a class hierarchy be the thing to use?
[color=blue]
> No. It creates the foos within each module, but which foo you have
> access to in the importing module is determined by the order of
> import.[/color]

Am I understanding correctly that if you have a module foo importing wx
and a module main importing both foo and wx there are actually two
instances of wx created, one referenced to by (at top level) foo.wx.*
and one wx.*? If this is indeed the case it isn't too good for the
performance doing this importing of wx multiple times right?

- Kees

[Diez B. Roggisch]

Re: Confused about namespaces

> Am I understanding correctly that if you have a module foo importing wx[color=blue]
> and a module main importing both foo and wx there are actually two
> instances of wx created, one referenced to by (at top level) foo.wx.*
> and one wx.*? If this is indeed the case it isn't too good for the
> performance doing this importing of wx multiple times right?
>[/color]

No, they aren't two instances. The first import will evaluate the
wx-module (there could be code in there executed upon importing it). All
subsequent imports of that module will only bind the already imported
module to the name.

So - the way to have one module be available in several other modules is
to import them in each of them - causing actually close to none
overhead, as the import statement is evaluated only once per file
(unless you put it in a loop or something, but this still only would
return the same reference.)

Regards,

Diez

[Diez B. Roggisch]

Re: Confused about namespaces

> Am I understanding correctly that if you have a module foo importing wx[color=blue]
> and a module main importing both foo and wx there are actually two
> instances of wx created, one referenced to by (at top level) foo.wx.*
> and one wx.*? If this is indeed the case it isn't too good for the
> performance doing this importing of wx multiple times right?
>[/color]

No, they aren't two instances. The first import will evaluate the
wx-module (there could be code in there executed upon importing it). All
subsequent imports of that module will only bind the already imported
module to the name.

So - the way to have one module be available in several other modules is
to import them in each of them - causing actually close to none
overhead, as the import statement is evaluated only once per file
(unless you put it in a loop or something, but this still only would
return the same reference.)

Regards,

Diez