Re: Functional/Best?

[Terry Reedy]

Tim Cook wrote:

I guess I can classify my application(s) as more procedural than
anything else. But I have a question about the best way to handle
something in Python.
>
When given a mapping of keywords, I want to call a function based on a
certain keyword found when parsing a text file. The mapping looks like
this:
>
definClassMap={ 'SECTION':'bldS ection','COMPOS ITION':'bldComp osition','OBSER VATION':'bldObs ervation','ITEM _TREE':'bldItem Tree'}
>
So if the text file contains 'ITEM_TREE' I want to call bldItemTree
which creates an instance of the class ItemTree.
>
I currently use an if ..., elif ... construct.
Is there a better, more efficient, more Pythonic way of doing this?

Yes. Create a mapping of keywords to function objects rather than to
function names!

def bldSection(): <whatever>
....
def bldItemTree(): <whatever else>

class_map={
'SECTION':bldSe ction,
'COMPOSITION':b ldComposition,
'OBSERVATION':b ldObservation,
'ITEM_TREE':bld ItemTree, # trailing comma allows easy additions
}

for word in parselist:
try;
class_map[word]()
except KeyError:
<whatever for non-keys>

tjr

[Paul Hankin]

Re: Functional/Best?

On Jul 13, 7:00 pm, Terry Reedy <tjre...@udel.e duwrote:

Tim Cook wrote:

I guess I can classify my application(s) as more procedural than
anything else.  But I have a question about the best way to handle
something in Python.

>

When given a mapping of keywords, I want to call a function based on a
certain keyword found when parsing a text file.  The mapping looks like
this:

>

definClassMap={ 'SECTION':'bldS ection','COMPOS ITION':'bldComp osition','OBSER VATION':'bldObs ervation','ITEM _TREE':'bldItem Tree'}

>

So if the text file contains 'ITEM_TREE'  I want to call bldItemTree
which creates an instance of the class ItemTree.  

>

I currently use an if ..., elif ... construct.
Is there a better, more efficient, more Pythonic way of doing this?

>
Yes. Create a mapping of keywords to function objects rather than to
function names!
>
def bldSection(): <whatever>
...
def bldItemTree(): <whatever else>
>
class_map={
   'SECTION':bldSe ction,
   'COMPOSITION':b ldComposition,
   'OBSERVATION':b ldObservation,
   'ITEM_TREE':bld ItemTree, # trailing comma allows easy additions
>
}
>
for word in parselist:
   try;
     class_map[word]()
   except KeyError:
     <whatever for non-keys>

A nice variant of this, which minimizes repetition, is to group all
the factory methods together into a class, naming them the same as the
keyword...

class Builder(object) :
def SECTION(self):
...
def COMPOSITION(sel f):
...
def OBSERVATION(sel f):
...
def ITEM_TREE(self) :
...

builder = Builder()
for word in parse_list:
item = getattr(builder , word)()
...

--
Paul Hankin