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I played around with a few things and this works but was wondering if
there was a better way to do this.
My first thought was list comprehension but could not get a figure out
the syntax.

tag23gr is a list of lists each with two items.
g23tag is an empty dictionary when I run the for loop below.
When is is complete each key is a graphic name who's values are a list
of tags.

for item in tag23gr:
.... value, key = tuple(item)
.... if(g23tag.get(key)):
.... g23tag[key].append(value)
.... else:
.... g23tag[key] = [value]

python <> writes:

> tag23gr is a list of lists each with two items.
> g23tag is an empty dictionary when I run the for loop below.
> When is is complete each key is a graphic name who's values are a list
> of tags.
>
> for item in tag23gr:
> ... value, key = tuple(item)
> ... if(g23tag.get(key)):
> ... g23tag[key].append(value)
> ... else:
> ... g23tag[key] = [value]

for item in tag23gr:
g23tag.setdefault(item[0],[]).append(item[1])

-- Alain.

On Wed, Apr 4, 2012 at 12:36 AM, python <> wrote:
> for item in tag23gr:
> ... * * value, key = tuple(item)
> ... * * if(g23tag.get(key)):
> ... * * * * * * g23tag[key].append(value)
> ... * * else:
> ... * * * * * * g23tag[key] = [value]

Simple enhancement: Use setdefault. Instead of the if, just use:

g23tag.setdefault(key,[]).append(value)

That'll cover both cases in one.

You can leave off the explicit tuple construction; if item is a
two-element list, you can unpack it directly. You can also embed that
straight into your for loop:

for value,key in tag23gr:

Do both and you cut your loop down to two lines. Cool!

Chris Angelico

python wrote:

> I played around with a few things and this works but was wondering if
> there was a better way to do this.
> My first thought was list comprehension but could not get a figure out
> the syntax.
>
> tag23gr is a list of lists each with two items.
> g23tag is an empty dictionary when I run the for loop below.
> When is is complete each key is a graphic name who's values are a list
> of tags.
>
> for item in tag23gr:
> ... value, key = tuple(item)
> ... if(g23tag.get(key)):

That should be

if key in g23tag:

Your version means trouble for keys that evaluate to False in a boolean
context, e. g. 0, False, None, "", (),...

> ... g23tag[key].append(value)
> ... else:
> ... g23tag[key] = [value]

from collections import defaultdict
g23tag = defaultdict(list)

for value, key in tag23gr:
g23tag[key].append(value)

On Apr 3, 11:02*am, Alain Ketterlin <al...@dpt-info.u-strasbg.fr>
wrote:
> python <w.g.sned...@gmail.com> writes:
> > tag23gr is a list of lists each with two items.
> > g23tag is an empty dictionary when I run the for loop below.
> > When is is complete each key is a graphic name who's values are a list
> > of tags.
>
> > for item in tag23gr:
> > ... * * * *value, key = tuple(item)
> > ... * * * *if(g23tag.get(key)):
> > ... * * * * * * * *g23tag[key].append(value)
> > ... * * * *else:
> > ... * * * * * * * *g23tag[key] = [value]
>
> for item in tag23gr:
> * * g23tag.setdefault(item[0],[]).append(item[1])
>
> -- Alain.

Or alternatively:

from collections import defaultdict
g23tag = defaultdict(list)
for item in tag23gr:
.....g23tag[item[0]].append(item[1])

nn <> writes:

>> > for item in tag23gr:
>> > ... Â* Â* Â* Â*value, key = tuple(item)
>> > ... Â* Â* Â* Â*if(g23tag.get(key)):
>> > ... Â* Â* Â* Â* Â* Â* Â* Â*g23tag[key].append(value)
>> > ... Â* Â* Â* Â*else:
>> > ... Â* Â* Â* Â* Â* Â* Â* Â*g23tag[key] = [value]
>>
>> for item in tag23gr:
>> Â* Â* g23tag.setdefault(item[0],[]).append(item[1])

> Or alternatively:
>
> from collections import defaultdict
> g23tag = defaultdict(list)
> for item in tag23gr:
> ....g23tag[item[0]].append(item[1])

Very handy in that case, but in general I dislike the idea of silently
inserting a default value when the access is a read, e.g., in
x=g23tag[wrung]. Explicit is better than implicit, as they say. YMMV.

-- Alain.

On Apr 3, 12:26*pm, Alain Ketterlin <al...@dpt-info.u-strasbg.fr>
wrote:
> nn <prueba...@latinmail.com> writes:
> >> > for item in tag23gr:
> >> > ... * * * *value, key = tuple(item)
> >> > ... * * * *if(g23tag.get(key)):
> >> > ... * * * * * * * *g23tag[key].append(value)
> >> > ... * * * *else:
> >> > ... * * * * * * * *g23tag[key] = [value]
>
> >> for item in tag23gr:
> >> * * g23tag.setdefault(item[0],[]).append(item[1])
> > Or alternatively:
>
> > from collections import defaultdict
> > g23tag = defaultdict(list)
> > for item in tag23gr:
> > ....g23tag[item[0]].append(item[1])
>
> Very handy in that case, but in general I dislike the idea of silently
> inserting a default value when the access is a read, e.g., in
> x=g23tag[wrung]. Explicit is better than implicit, as they say. YMMV.
>
> -- Alain.

Valid point. Preferred choice depends on the access patterns to the
dict (e.g. one write and multiple reads, multiple writes and one loop
over items, etc.)