[Edu-sig] looking for explanations... globals dynamic dict

[Massimo Di Pierro]

In python variables assigned to dict and list are always references.

 >>> a = {}
 >>> b = a  # b and a point to the same dictionary
 >>> b['key'] = 'value'
 >>> print a['key']
value

primitive types like integers behave differently

 >>> a = 1
 >>> b = a  # b and a are two different 1s
 >>> b+=2
 >>> print a
1

You can copy the objects copied by a reference

 >>> a = {}
 >>> import copy
 >>> b = copy.deepcopy(a)  # b is a complete copy of a (also objects  
in keys and values are copied recursively)
 >>> b['key'] = 'value'
 >>> print a['key']
KeyError





On Mar 28, 2011, at 2:15 PM, John Zelle wrote:

> Hello,
>
> Isn't the issue here simply that globals does not return a copy of  
> the globals dictionary, it returns THE actual globals dictionary.  
> It's not some sort of callable, it is the globals dictionary,  
> period. There is no such thing as a static dictionary, they are  
> mutable, and this one changes every time you define a new variable.  
> For example, try:
>
> >>> g = globals()
> >>> g['k'] = 3
> >>> k
> 3
>
> So when you assign to new variables, this globals dictionary is  
> changing, and that leads to the iteration error. As long as you  
> iterate using existing variables, there is no problem.
>
> >>> g = globals()
> >>> k = None
> >>> v = None
> >>> for k,v in g.items():
> ...     print k,v
> ...
> g {'g': {...}, '__builtins__': <module '__builtin__' (built-in)>,  
> 'k': 'g', '__package__': None, 'v': {...}, '__name__': '__main__',  
> '__doc__': None}
> __builtins__ <module '__builtin__' (built-in)>
> k None
> __package__ None
> v None
> __name__ __main__
> __doc__ None
>
>
> --John
>
>
> From: edu-sig-bounces+john.zelle=wartburg.edu at python.org [edu-sig-bounces+john.zelle=wartburg.edu at python.org 
> ] on behalf of kirby urner [kirby.urner at gmail.com]
> Sent: Monday, March 28, 2011 1:19 PM
> To: edu-sig at python.org
> Subject: [Edu-sig] looking for explanations... globals dynamic dict
>
>
> One of my Python students alerted me to this state of affairs.
>
> I understand that what globals( ) returns will fluctuate as new  
> names come and go.
>
> What I less understand is why g isn't just a *snap shot* of what was  
> global at the time globals( ) ran.
>
> Now it's just a dictionary like any other (and yes, it contains  
> itself, as another global).
>
> So when I go to print its items, why should it care that the names k  
> and v have been added.
>
> Why isn't g just a static dictionary?
>
> I guess because it's tied to a callable that gets re-executed  
> whenever g is used.  g.items( ) is like globals( ).items( ) --  
> another call.
>
>
> >>> g = globals()
> >>> g
> {'__builtins__': <module 'builtins' (built-in)>, '__name__':  
> '__main__', '__doc__': None, 'g': {...}, '__package__': None}
> >>> len(g)
> 5
> >>> for k, v in g.items():  print(k, v)
>
> __builtins__ <module 'builtins' (built-in)>
> Traceback (most recent call last):
>   File "<pyshell#156>", line 1, in <module>
>     for k, v in g.items():  print(k, v)
> RuntimeError: dictionary changed size during iteration
> >>> g
> {'g': {...}, '__builtins__': <module 'builtins' (built-in)>, 'k':  
> '__builtins__', '__package__': None, 'v': <module 'builtins' (built- 
> in)>, '__name__': '__main__', '__doc__': None}
> >>> len(g)
> 7
> >>>
>
>
> Oh no, not again....
>
> >>> for t,v in g.items():  print(t,v)
>
> g {'g': {...}, '__builtins__': <module 'builtins' (built-in)>, 'k':  
> '__builtins__', '__package__': None, 't': 'g', 'v': {...},  
> '__name__': '__main__', '__doc__': None}
> Traceback (most recent call last):
>   File "<pyshell#160>", line 1, in <module>
>     for t,v in g.items():  print(t,v)
> RuntimeError: dictionary changed size during iteration
>
> This seems to solve the problem:
>
> >>> ================================ RESTART  
> ================================
> >>> g = dict(globals())
> >>> g
> {'__builtins__': <module 'builtins' (built-in)>, '__name__':  
> '__main__', '__doc__': None, '__package__': None}
> >>> for t,v in g.items():  print(t,v)
>
> __builtins__ <module 'builtins' (built-in)>
> __name__ __main__
> __doc__ None
> __package__ None
>
>
> This behavior still seems a little peculiar.
>
> >>> help(globals)
> Help on built-in function globals in module builtins:
>
> globals(...)
>     globals() -> dictionary
>
>     Return the dictionary containing the current scope's global  
> variables.
>
>
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