Re: [Python-ideas] dict.fromkeys() better as dict().setkeys() ? (and other suggestions)

May 30, 2007

      Steve Howell wrote:
...
--- Ron Adam <rrr@ronadam.com> wrote:
...
However, I do think there could be very useful uses
for a standard sorting 
structure of some sort.  That's the sorting as in
mail sorters, or category 
sorters, that produce several streams of output
instead of just one.
Would that be called a de-comprehension?
Here is some category-sorting code, FWIW, where every
employee, Fred or not, gets a 50% raise, and employees
are partitioned according to their Fredness.
It doesn't use a general iterator, so maybe I'm
missing your point.
Since you aren't really creating two lists, the problem below doesn't 
really fit this particular solution.  But maybe we can make it work from a 
different point of view.

emps = {
     'Fred Smith': 50.0,
     'Fred Jones': 40.0,
     'Joe Blow': 30,
     }

def pay_increase(salary): return salary * 0.5

def is_fred(emp):
     return 'Fred' in emp[0]

# give all Freds a raise
freds, notfreds = decomp(emps.keys(), is_fred):
for name in freds:
     emp[name] = pay_increas(emp[name])

#
# Then we can use the freds list again to generate a report.
#

Of course in this case the following would work just as well...

freds = []
for name in emps:
     if is_fred(name):
         emp[name] = pay_increas(emp[name])
         freds.append(name)

One reason to generating more than one list is if each list is going to be 
handled as batches, or in different ways, or at different times than you 
otherwise would by just iterating it.

Cheers,
    Ron
...
def partitions(lst):
    dct = {}
    for k, value in lst:
        dct.setdefault(k, []).append(value)
    return dct.items()
def is_fred(emp):
    return 'Fred' in emp[0]
emps = [
    ('Fred Smith', 50),
    ('Fred Jones', 40),
    ('Joe Blow', 30),
    ]
def pay_increase(salary): return salary * 0.5
emp_groups = partitions([(is_fred(emp),
        (emp[0], pay_increase(emp[1]))) for
        emp in emps])
for fredness, emps in emp_groups:
    print
    print 'is Fred?', fredness
    for name, pay_increase in emps:
        print name, pay_increase
----
is Fred? False
Joe Blow 15.0
is Fred? True
Fred Smith 25.0
Fred Jones 20.0
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