scoping questions

Noam Raphael noamr at correctme.users.sourcephorge.net
Wed Jun 2 20:52:45 CEST 2004


David Stockwell wrote:
> Hi,
> 
> Another of my crazy questions.  I'm just in the process of learning so 
> bear with me if you can.   I actually ran it....  with two test cases
> 
> TEST CASE 1:
> Say I have the following defined:
> --- beginning of code snippet ----
> 
> def me(aFile):
>   """
>     Note I am testing scoping
>   """
>   aFile = 'hello world'
>   print aFile
> 
> 
> 
> aFile = open('/tmp/test','r')
> me(aFile)
> data = aFile.read()
> print data
> 
> ------ end of code snippet ----
> my test file has a sentence 'This is a test of the /tmp/test file'
> 
> When I run it I observed this output:
> hello world
> This is a test of the /tmp/test file
> 
> Now what this means to me and this is where I need your help:
> 
> When I call the 'me' function, its passing a reference to my original 
> aFile variable.
> Inside the me function, I'm guessing it is now  a new reference to the 
> same original aFile contents.  When I assign it to a simple string, it 
> simply changes the local reference to point to that string.  Since its a 
> copy of the reference, it doesn't affect the caller's value.
> 
> In essence if i understand correctly
> 
> At the global scope I have
> 
> variable aFile that points to an instance of a 'file'
> 
> Inside the me function scope I have
> a parameter named aFile that is a local copy of the original reference 
> of what global::aFile was pointing to.
> Essentially local::aFile is pointing to a file object
> 
> At this point I have two references to the file object.
> 
> When I assign the new value to aFile (local) it simply does the 
> assignment.  The global reference is untouched.
> 
> I would sort of expect this behavior as I am not returning anything
> 
> If test #1 is true, then test case 2 boggles me a bit
> 
> TEST CASE 2:
> -------
> def me(aFile):
>   """
>     Note I am testing scoping
>   """
>   aFile = 'hello world'
>   print aFile
> 
> 
> def me2(dog):
>   """
>     Note I am testing scoping
>   """
>   print "Inside me2"
>   dog.close()
> 
> 
> aFile = open('/tmp/test','r')
> me(aFile)
> data = aFile.read()
> print "test1", data
> aFile.close()
> aFile = open('/tmp/test','r')
> 
> me2(aFile)
> print "test2", aFile.read()
> =====
> 
> It bombs out on the last line because aFile was closed in the function 
> 'me2'.
> 
> Perhaps the way to explain it is that Inside me2 when my copy of the 
> original reference is made, I have a global and local variable both 
> referencing the same 'object'.   I am able to do operations in the local 
> me2 scope and have them effect the behavior of the global scope.
> 
> Its just a bit confusing because python is apparently smart enough to 
> realize that my action on the local reference hasn't redefined the 
> capability of the global so it has allowed this to occur on the actual 
> global file referenced object. (as opposed to just a local copy).  And I 
> didn't return anything....
> 
> 
> I'm just confused a bit here.... Perhaps someone can clarify
> 
> Thanks
> 
> David
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> 
Hello,
I'm not sure I've understood entirely, but I'll try to explain: When you 
  have a reference to an object, you can do anything you like with it - 
close it if it's a file, for example. When you write, if the function 
'me', aFile = 'hello world', you simply discard your reference to the 
file, and create a new reference, called 'aFile' too, to the string 
'hello world' you've created. You do all this in the local namespace, so 
it doesn't affect the global namespace.
Maybe this is the point that should be clarified: There's only one pool 
of objects. What's global and local are the namespaces - references to 
those objects.

I hope this helped,
Noam Raphael



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