updating locals() and globals() (WAS: How do I do this? (eval() on the left hand side))

Bengt Richter bokr at oz.net
Thu Dec 9 21:24:25 CET 2004


On Wed, 8 Dec 2004 20:22:52 -0500, "Terry Reedy" <tjreedy at udel.edu> wrote:

>To respond to and summarize several posts in this discussion:
>
>Within a function, where the local namespace is distinct from the global 
>(module) namespace, CPython usually implements the local namespace 
>internally as a fixed-length array.  When this is true, locals() is a 
>*copy* of the local namespace and not the namespace itself.  Once that dict 
>is created, the history of how it was created is immediately forgotten, 
>just as with any other ordinary Python dict.
>
>That dict can be bound to a name or other target and modified like any 
>other dict, and there could be reasons to do so.  However, modifying it has 
>no more effect on the local namespace than modifying any other local dict.
>
It doesn't appear to be _quite_ ordinary though (note that print d['x'] inside
does print 3 the first time and 5 after the d['x']=5 assignment, but is not
returned in d when d returns in the results):

I think I'd rather locals() totally refuse updates than allow retrievable updates
in a way that leaves me wondering what kind of object it is and what happens to it
when it is exported from a function (some kind of closure stuff involved in converting
a proxy to a dict? (will speculation)) ;-/

 >>> def f():
 ...     x = 3
 ...     d = locals()
 ...     D = dict(locals())
 ...     print 'x', x
 ...     print "d['x']",d['x']
 ...     d['x'] = 5
 ...     print 'x', x
 ...     print "d['x']",d['x']
 ...     print "D['x']", D['x']
 ...     D['x'] = 7
 ...     print "d['x']",d['x']
 ...     print "D['x']", D['x']
 ...     return d, D, locals()
 ...
 >>> d,D,L = f()
 x 3
 d['x'] 3
 x 3
 d['x'] 5
 D['x'] 3
 d['x'] 5
 D['x'] 7
 >>> d
 {'x': 3, 'd': {...}, 'D': {'x': 7, 'd': {...}}}
 >>> D
 {'x': 7, 'd': {'x': 3, 'd': {...}, 'D': {...}}}
 >>> L
 {'x': 3, 'd': {...}, 'D': {'x': 7, 'd': {...}}}
 >>> L['d']['x'] # not 5
 3
 >>> L['D']['x'] # is 7
 7
 >>> d['x']
 3
 >>> D['x']
 7
 >>>
 
Regards,
Bengt Richter



More information about the Python-list mailing list