Concise idiom to initialize dictionaries

Bengt Richter bokr at oz.net
Wed Nov 10 09:06:21 CET 2004


On Tue, 9 Nov 2004 16:40:48 -0000, "Frohnhofer, James" <james.frohnhofer at csfb.com> 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?
>
Others explained why locals()[x] worked interactively, and not in a function,
but if you just want a one-liner, you could just unpack a listcomp:

 >>> a,b,c = [{} for i in xrange(3)]
 >>> a,b,c
 ({}, {}, {})

If you have single-letter names, you can avoid the count by stepping through the letters:

 >>> x,y,z = [{} for dont_care in 'xyz']
 >>> x,y,z
 ({}, {}, {})

Or if you have a long target list and you can just type it and copy/paste it like:

 >>> fee,fie,fo,fum,bim,bah = [{} for ignore in 'fee,fie,fo,fum,bim,bah'.split(',')]
 >>> fee,fie,fo,fum,bim,bah
 ({}, {}, {}, {}, {}, {})
 >>> map(id, (fee,fie,fo,fum,bim,bah))
 [9440400, 9440976, 9438816, 9441120, 9440256, 9440544]


The dicts are separate, as you can see:

 >>> id(a),id(b),id(c)
 (9153392, 9153248, 9439248)
 >>> a,b,c = [{i:chr(i+ord('0'))} for i in xrange(3)]
 >>> a,b,c
 ({0: '0'}, {1: '1'}, {2: '2'})

Regards,
Bengt Richter



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