Concise idiom to initialize dictionaries

[Frohnhofer, James]

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?


[color=blue]
> -----Original Message-----
> From: python-list-bounces+james.f rohnhofer=csfb. com@python.org
> [mailto:python-list-bounces+james.f rohnhofer=csfb. com@python.org]On
> Behalf Of Dennis Lee Bieber
> Sent: Tuesday, November 09, 2004 10:31 AM
> To: python-list@python.org
> Subject: Re: Determining combination of bits
>
>
> On Mon, 8 Nov 2004 21:18:36 -0800, "news.west.cox. net"
> <sean.berry2@co x.net> declaimed the following in comp.lang.pytho n:
>[color=green][color=darkred]
> > > Note: 2^1 = 2, so your dictionary is already in error...
> > >[/color]
> >
> > The dictionary was filled with arbitrary values, not
> > { x : 2^x } values like you might have thought.[/color]
>
> Well, you had stated "powers of two"... If all you wanted is a
> bit mapping you could probably drop the dictionary and just use a list
> of the values, indexed by the bit position, and my first attempt
> logic...
>[color=green]
> >
> > It is actually more like {1:123, 2:664, 4:323, 8:990, 16:221... etc}
> >
> >[/color]
>
> CheckBoxes = [ "FirstChoic e",
> "SecondChoi ce",
> "ThirdChoic e",
> "FourthChoi ce",
> "FifthChoic e",
> "SixthChoic e" ]
>
>
> for num in [22, 25, 9]:
> bit = 0
> while num:
> if num & 1:
> print CheckBoxes[bit],
> bit = bit + 1
> num = num >> 1
> print
>
> SecondChoice ThirdChoice FifthChoice
> FirstChoice FourthChoice FifthChoice
> FirstChoice FourthChoice
>
> where "num" is the sum of the checkbox index values (or whatever
> selection mechanism is used), assuming /they/ were set up in 2^(n+1)
> scheme (n = bit position, starting with 0)...
>
> --[color=green]
> > =============== =============== =============== =============== == <
> > wlfraed@ix.netc om.com | Wulfraed Dennis Lee Bieber KD6MOG <
> > wulfraed@dm.net | Bestiaria Support Staff <
> > =============== =============== =============== =============== == <
> > Home Page: <http://www.dm.net/~wulfraed/> <
> > Overflow Page: <http://wlfraed.home.ne tcom.com/> <[/color]
> --
> http://mail.python.org/mailman/listinfo/python-list
>[/color]

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[Larry Bates]

Re: Concise idiom to initialize dictionaries

It is almost certain that you should use either a list of dictionaries
or a dictionary containing other dictionaries for this. Creation of
26 distinct dictionaries is almost never a good solution as it makes
it nearly impossible to iterate over them and would make code to
manipulate them unable to be generalized easily.

Example:

#
# To create a list of dictionaries
#
list_of_dicts=[]
for i in range(26):
list_of_dicts.a ppend({})

#
# Now you can reference each dictionary as:
#
# list_of_dicts[0], list_of_dicts[1], ...
#

or

import string
dict_of_dicts={ }
for letter in string.ascii_lo wercase:
dict_of_dicts[letter]={}

#
# Now you can reference each dictionary as:
#
# dict_of_dicts['a'], dict_of_dicts['b'], ...

Larry Bates


Frohnhofer, James wrote:[color=blue]
> 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?
>
>
>
>[color=green]
>>-----Original Message-----
>>From: python-list-bounces+james.f rohnhofer=csfb. com@python.org
>>[mailto:python-list-bounces+james.f rohnhofer=csfb. com@python.org]On
>>Behalf Of Dennis Lee Bieber
>>Sent: Tuesday, November 09, 2004 10:31 AM
>>To: python-list@python.org
>>Subject: Re: Determining combination of bits
>>
>>
>>On Mon, 8 Nov 2004 21:18:36 -0800, "news.west.cox. net"
>><sean.berry2@ cox.net> declaimed the following in comp.lang.pytho n:
>>
>>[color=darkred]
>>>>Note: 2^1 = 2, so your dictionary is already in error...
>>>>
>>>
>>>The dictionary was filled with arbitrary values, not
>>>{ x : 2^x } values like you might have thought.[/color]
>>
>> Well, you had stated "powers of two"... If all you wanted is a
>>bit mapping you could probably drop the dictionary and just use a list
>>of the values, indexed by the bit position, and my first attempt
>>logic...
>>
>>[color=darkred]
>>>It is actually more like {1:123, 2:664, 4:323, 8:990, 16:221... etc}
>>>
>>>[/color]
>>
>>CheckBoxes = [ "FirstChoic e",
>> "SecondChoi ce",
>> "ThirdChoic e",
>> "FourthChoi ce",
>> "FifthChoic e",
>> "SixthChoic e" ]
>>
>>
>>for num in [22, 25, 9]:
>> bit = 0
>> while num:
>> if num & 1:
>> print CheckBoxes[bit],
>> bit = bit + 1
>> num = num >> 1
>> print
>>
>>SecondChoic e ThirdChoice FifthChoice
>>FirstChoice FourthChoice FifthChoice
>>FirstChoice FourthChoice
>>
>>where "num" is the sum of the checkbox index values (or whatever
>>selection mechanism is used), assuming /they/ were set up in 2^(n+1)
>>scheme (n = bit position, starting with 0)...
>>
>>--[color=darkred]
>> > =============== =============== =============== =============== == <
>> > wlfraed@ix.netc om.com | Wulfraed Dennis Lee Bieber KD6MOG <
>> > wulfraed@dm.net | Bestiaria Support Staff <
>> > =============== =============== =============== =============== == <
>> > Home Page: <http://www.dm.net/~wulfraed/> <
>> > Overflow Page: <http://wlfraed.home.ne tcom.com/> <[/color]
>>--
>>http://mail.python.org/mailman/listinfo/python-list
>>[/color]
>
>
> =============== =============== =============== =============== =============== ===
> This message is for the sole use of the intended recipient. If you received
> this message in error please delete it and notify us. If this message was
> misdirected, CSFB does not waive any confidentiality or privilege. CSFB
> retains and monitors electronic communications sent through its network.
> Instructions transmitted over this system are not binding on CSFB until they
> are confirmed by us. Message transmission is not guaranteed to be secure.
> =============== =============== =============== =============== =============== ===
>[/color]

[Raymond Hettinger]

Re: Concise idiom to initialize dictionaries

[Frohnhofer, James][color=blue]
> 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.[/color]

Use exec().
[color=blue][color=green][color=darkred]
>>> for c in 'abcdefghijklmn opqrstuvwxyz':[/color][/color][/color]
exec c + ' = {}'


Of course, as others have pointed out, the whole idea is likely misguided and
you would be better served with a list of unnamed dictionaries.


Raymond Hettinger

[Peter Otten]

Re: Concise idiom to initialize dictionaries

Frohnhofer, James wrote:
[color=blue]
> 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.[/color]

Here is a bunch of dictionaries that spring into existence by what is
believed to be magic :-)
[color=blue][color=green][color=darkred]
>>> class AllDicts:[/color][/color][/color]
.... def __getattr__(sel f, name):
.... d = {}
.... setattr(self, name, d)
.... return d
.... def __repr__(self):
.... items = self.__dict__.i tems()
.... items.sort()
.... return "\n".join(map(" %s -> %r".__mod__, items))
....[color=blue][color=green][color=darkred]
>>> ad = AllDicts()
>>> ad.a[1] = 99
>>> ad.b[2] = 42
>>> ad.b[3] = 11
>>> ad[/color][/color][/color]
a -> {1: 99}
b -> {2: 42, 3: 11}

Peter

[Bengt Richter]

Re: Concise idiom to initialize dictionaries

On Tue, 9 Nov 2004 16:40:48 -0000, "Frohnhofer , James" <james.frohnhof er@csfb.com> wrote:
[color=blue]
>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?
>[/color]
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:
[color=blue][color=green][color=darkred]
>>> a,b,c = [{} for i in xrange(3)]
>>> a,b,c[/color][/color][/color]
({}, {}, {})

If you have single-letter names, you can avoid the count by stepping through the letters:
[color=blue][color=green][color=darkred]
>>> x,y,z = [{} for dont_care in 'xyz']
>>> x,y,z[/color][/color][/color]
({}, {}, {})

Or if you have a long target list and you can just type it and copy/paste it like:
[color=blue][color=green][color=darkred]
>>> fee,fie,fo,fum, bim,bah = [{} for ignore in 'fee,fie,fo,fum ,bim,bah'.split (',')]
>>> fee,fie,fo,fum, bim,bah[/color][/color][/color]
({}, {}, {}, {}, {}, {})[color=blue][color=green][color=darkred]
>>> map(id, (fee,fie,fo,fum ,bim,bah))[/color][/color][/color]
[9440400, 9440976, 9438816, 9441120, 9440256, 9440544]


The dicts are separate, as you can see:
[color=blue][color=green][color=darkred]
>>> id(a),id(b),id( c)[/color][/color][/color]
(9153392, 9153248, 9439248)[color=blue][color=green][color=darkred]
>>> a,b,c = [{i:chr(i+ord('0 '))} for i in xrange(3)]
>>> a,b,c[/color][/color][/color]
({0: '0'}, {1: '1'}, {2: '2'})

Regards,
Bengt Richter

[bruno modulix]

Re: Concise idiom to initialize dictionaries

Larry Bates a écrit :[color=blue]
> It is almost certain that you should use either a list of dictionaries
> or a dictionary containing other dictionaries for this. Creation of
> 26 distinct dictionaries is almost never a good solution as it makes
> it nearly impossible to iterate over them and would make code to
> manipulate them unable to be generalized easily.
>
> Example:
>
> #
> # To create a list of dictionaries
> #
> list_of_dicts=[]
> for i in range(26):
> list_of_dicts.a ppend({})
>[/color]

or just
list_of_dicts = [{} for i in range(26)]

or if you want a dict of dicts :
dod = dict([(i, {}) for i in range(26)])

[Caleb Hattingh]

Re: Concise idiom to initialize dictionaries

Peter, respect :)

For interest sake, how would such a thing look with new-style classes? My
(likely misinformed) impression is that __getattr__ for example, doesn't
behave in quite the same way?

thx
Caleb


On Tue, 09 Nov 2004 20:12:15 +0100, Peter Otten <__peter__@web. de> wrote:
[color=blue]
>
> Here is a bunch of dictionaries that spring into existence by what is
> believed to be magic :-)
>[color=green][color=darkred]
>>>> class AllDicts:[/color][/color]
> ... def __getattr__(sel f, name):
> ... d = {}
> ... setattr(self, name, d)
> ... return d
> .. def __repr__(self):
> ... items = self.__dict__.i tems()
> ... items.sort()
> ... return "\n".join(map(" %s -> %r".__mod__, items))
> ...[color=green][color=darkred]
>>>> ad = AllDicts()
>>>> ad.a[1] = 99
>>>> ad.b[2] = 42
>>>> ad.b[3] = 11
>>>> ad[/color][/color]
> a -> {1: 99}
> b -> {2: 42, 3: 11}
>
> Peter
>[/color]

[Steven Bethard]

Re: Concise idiom to initialize dictionaries

Caleb Hattingh <caleb1 <at> telkomsa.net> writes:[color=blue]
>
> For interest sake, how would such a thing look with new-style classes? My
> (likely misinformed) impression is that __getattr__ for example, doesn't
> behave in quite the same way?[/color]

Just the same[1] =)
[color=blue][color=green][color=darkred]
>>> class AllDicts(object ):[/color][/color][/color]
.... def __getattr__(sel f, name):
.... d = {}
.... setattr(self, name, d)
.... return d
.... def __repr__(self):
.... items = self.__dict__.i tems()
.... items.sort()
.... return '\n'.join(['%s -> %r' % item for item in items])
....[color=blue][color=green][color=darkred]
>>> ad = AllDicts()
>>> ad.a[1] = 99
>>> ad.b[2] = 42
>>> ad.b[3] = 11
>>> ad[/color][/color][/color]
a -> {1: 99}
b -> {2: 42, 3: 11}[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

I believe that __getattr__ works just the same (but to check for yourself, see
http://docs.python.org/ref/attribute-access.html). I think what you're thinking
of is __getattribute_ _ which new-style classes offer *in addition* to
__getattr__. While __getattr__ is called only if an attribute is not found,
__getattribute_ _ is called unconditionally for every attribute access.

Steve

[1] modulo my preference for list comprehensions/generator expressions instead
of map

[Caleb Hattingh]

Re: Concise idiom to initialize dictionaries

Steve,

Not only did you answer my silly question, but also the question I
actually wanted to ask ...__getattribu te__ is what I was thinking of.

thats cool :)
thx
Caleb

On Thu, 11 Nov 2004 20:30:18 +0000 (UTC), Steven Bethard
<steven.bethard @gmail.com> wrote:
[color=blue]
> Caleb Hattingh <caleb1 <at> telkomsa.net> writes:[color=green]
>>
>> For interest sake, how would such a thing look with new-style classes?
>> My
>> (likely misinformed) impression is that __getattr__ for example, doesn't
>> behave in quite the same way?[/color]
>
> Just the same[1] =)
>[color=green][color=darkred]
>>>> class AllDicts(object ):[/color][/color]
> ... def __getattr__(sel f, name):
> ... d = {}
> ... setattr(self, name, d)
> ... return d
> ... def __repr__(self):
> ... items = self.__dict__.i tems()
> ... items.sort()
> ... return '\n'.join(['%s -> %r' % item for item in items])
> ...[color=green][color=darkred]
>>>> ad = AllDicts()
>>>> ad.a[1] = 99
>>>> ad.b[2] = 42
>>>> ad.b[3] = 11
>>>> ad[/color][/color]
> a -> {1: 99}
> b -> {2: 42, 3: 11}[color=green][color=darkred]
>>>>[/color][/color]
>
> I believe that __getattr__ works just the same (but to check for
> yourself, see
> http://docs.python.org/ref/attribute-access.html). I think what you're
> thinking
> of is __getattribute_ _ which new-style classes offer *in addition* to
> __getattr__. While __getattr__ is called only if an attribute is not
> found,
> __getattribute_ _ is called unconditionally for every attribute access.
>
> Steve
>
> [1] modulo my preference for list comprehensions/generator expressions
> instead
> of map
>[/color]