Why do directly imported variables behave differently than those attached to imported module?

Why do directly imported variables behave differently than those attached to imported module?

Post by Dun Pea » Thu, 05 May 2011 01:31:06


Hi!

Here's the demonstrating code:

# module foo.py
var = 0

def set():
global var
var = 1

Script using this module:

import foo
from foo import *

print var, foo.var
set()
print var, foo.var

Script output:

0 0
0 1

Apparently, the `var` we imported from `foo` never got set, but
`foo.var` on the imported `foo` - did. Why?

Thanks, D.
 
 
 

Why do directly imported variables behave differently than those attached to imported module?

Post by Daniel Klu » Thu, 05 May 2011 01:57:51


Because all names are references to some values, not other names (in
CPython, it means all names are PyObject*, and point directly to the
objects, not other pointers)

When you do `from foo import bar` it assigns globals()['bar'] of
current module to reference same value as `foo.bar`. Its now local
namespace name, not `foo` namespace, and therefore functions in `foo`
cannot modify this namespace.

Since ints are immutable, when you do `var = 1` you create new object
of type int, and re-assign `var` name to point to new object.

`foo.var`, on other hand, is a way to access `foo`'s own namespace, so
its exactly same name as globals()['var'] of `foo`.

--
With best regards,
Daniel Kluev

 
 
 

Why do directly imported variables behave differently than those attached to imported module?

Post by Chris Rebe » Thu, 05 May 2011 01:57:53


Because imports (and assignments generally) bind names to values, they
don't alias names to other names.

from foo import *

can be thought of as essentially doing:

import foo
set = foo.set
var = foo.var
del foo

So the new, separate name __main__.var gets the current value of
foo.var at import-time, which is the integer 0.
You then call foo.set(), which re-binds foo.var to a new value (i.e.
1) rather than mutating the existing value (which would be impossible
anyway since integers are immutable). This has absolutely no effect on
__main__.var, which is an entirely separate binding.

The behavior is comparable to that of function arguments. Values can
be mutated, but re-binding names has only local effect:
.... b = 1 # rebinds local name b
....
0
.... b.append(99) # mutates passed-in value
.... b = [42] # rebinds local name; has no outside effect
....
[7, 99]

Cheers,
Chris
http://www.yqcomputer.com/
 
 
 

Why do directly imported variables behave differently than those attached to imported module?

Post by Mel » Thu, 05 May 2011 02:00:28


They're different because -- they're different. `foo.var` is defined in the
namespace of the foo module. Introspectively, you would access it as
`foo.__dict__['var']` .

Plain `var` is in your script's namespace so you could access it as
`globals()['var']` . The values given to the vars are immutable integers,
so assignment works by rebinding. The two different bindings in
foo.__dict__ and globals() get bound to different integer objects.

Note too the possible use of `globals()['foo'].__dict__['var'] . (Hope
there are no typos in this post.)

Mel.
 
 
 

Why do directly imported variables behave differently than those attached to imported module?

Post by Terry Reed » Thu, 05 May 2011 02:13:12

Your problem is reveal in the subject line. As discussed in many other
threads, including a current one, Python does not have 'variables' in
the way that many understand the term. Python binds names to objects.
Binding statements (assignment, augmented assignment, import, def,
class, and others with 'as' clauses) all bind names to objects.

Suppose we both have address books. You copy all entries from my book to
yours. Afterwards, I update an entry in my book. That does not change
the now obsolete entry in your book.



Module foo's dict now has two entries for 'var' and 'set' bound to int 0
and a function. Note that 'set' is a builtin class name so not the best
choice.

Script creates module '__main__'


This binds 'foo' to module foo in __main_'s dict.


This copies foo's dict to __main__'s dict.


This changes foo's dict, rebinding 'var' to int 1. It does not change
__main__'s dict. How could it? In the same way, 'var = 2' would leave
foo's dict unchanged. __main__.var.


If instead of 'from foo import *', you had written 'from foo import var
as rav', then perhaps you would not have been so surprised that
__main__.rav and foo.var have independent fates.

You have discovered that using from x import * is a bad idea when module
x has global names that get rebound.

--
Terry Jan Reedy
 
 
 

Why do directly imported variables behave differently than those attached to imported module?

Post by harrismh77 » Thu, 05 May 2011 02:24:43


My two cents to add in addition to the correct accounts of [ Daniel,
Chris, Mel] is that you might have a misunderstanding of var, generally.

Python has no variables--- in the way you think of them in other
languages. Python only has references.

Var is not 'set' in the classical sense of the word...

var is a 'reference' (think C pointer, sort-of) to an object... '0'
and '1' are objects of type int and

var=0 means create a reference to object int('0') called var.

a=b=0 a is a reference to object '0'
b is a reference to object '0'

a does not reference b

This rest of the discussion has to do with name spaces:

See:

http://www.yqcomputer.com/
 
 
 

Why do directly imported variables behave differently than those attached to imported module?

Post by Dun Pea » Thu, 05 May 2011 03:51:03

OK, I understand now.

`from foo import var` means "create a module-global name `var` inside
the current module, and have it point at the object `foo.var` is
pointing at (following its evaluation)".

Naturally, regardless of whether `foo.var` ever changes, the global
`var` of the current module still points at the original object
`foo.var` was pointing to at the time of the `var = foo.var`
assignment.

Thanks, D.
 
 
 

Why do directly imported variables behave differently than those attached to imported module?

Post by Dun Pea » Thu, 05 May 2011 04:23:15

P.S. now I have to ask: is there a symbolic reference in Python, i.e.
a name foo that points to "whatever bar.baz is pointing at"?

Thanks, D.
 
 
 

Why do directly imported variables behave differently than those attached to imported module?

Post by Daniel Klu » Thu, 05 May 2011 04:55:04


Well, you could easily simulate that with proxy object,
class SymbolicReference(object):
def __init__(self, ns, name):
self.__ns = ns
self.__name = name

def __get(self):
return self.__ns[self.__name]

def __getattribute__(self, attr):
try:
return object.__getattribute__(self, attr)
except:
return self.__get().__getattribute__(attr)

def __repr__(self):
return self.__get().__repr__()

def __str__(self):
return self.__get().__str__()

1
10


--
With best regards,
Daniel Kluev
 
 
 

Why do directly imported variables behave differently than those attached to imported module?

Post by Duncan Boo » Thu, 05 May 2011 17:44:36


A function object has a reference to the dictionary for the module in which
it was defined; it needs this to access global variables. A class holds the
name of the module but has no reference to the actual module itself.

Yes, it is quite possible to delete a module object, but it won't generally
matter since any classes that are still referenced will prevent their
methods being deleted and any functions or methods that are still
accessible will keep the globals alive as long as required.

--
Duncan Booth http://www.yqcomputer.com/