Concise idiom to initialize dictionaries

Gert-Jan den Besten gj.den.besten at hccnet.nl
Tue Nov 9 19:01:19 CET 2004


Frohnhofer, James wrote:
> My initial problem was to initialize a bunch of dictionaries at the start of a
> function.
> 
> I did not want to do
> def fn():
> 	a = {}
> 	b = {}
> 	c = {}
> 	. . .
> 	z = {}
> simply because it was ugly and wasted screen space.
> 
> First I tried:
> 
> 	for x in (a,b,c,d,e,f,g): x = {}
> 
> which didn't work (but frankly I didn't really expect it to.)
> Then I tried:
> 
> 	for x in ('a','b','c','d','e','f','g'): locals()[x]={}
> 
> which did what I wanted, in the interpreter.  When I put it inside a function,
> it doesn't seem to work.  If I print locals() from inside the function, I can
> see them, and they appear to be fine, but the first time I try to access one
> of them I get a "NameError: global name 'a' is not defined"
> 
> Now obviously I could easily avoid this problem by just initializing each
> dictionary, but is there something wrong about my understanding of locals,
> that my function isn't behaving the way I expect?



Any variable you assign within a function, is local to that function. 
You cannot refer to it from the enclosing function or module.

If you use a global declaration in the function, you can alter variables 
outside your function. (You can also pass variables using the function 
header.) e.g:


     x = 42

     def a_function():
         global x
         x = "another value"

     print x
     a_function()
     print x


But i don't think this is good programming practice.


Gert-Jan




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