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....@python.org [edu-sig-bounces+john.zelle=wartburg....@python.org
] on behalf of kirby urner [kirby.ur...@gmail.com]
Sent: Monday, March 28, 2011 1:19 PM
To: edu-sig@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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