short path evaluation, why is f() called here: dict(a=1).get('a', f())

Tim Chase python.list at tim.thechases.com
Mon Jan 14 23:07:12 CET 2008


> But how can Python determine when you want the result to be *the 
> callable* and when you want it to be *the result of calling the 
> callable*? 
> 
> Functions and other callables are first-class objects, and it is quite 
> reasonable to have something like this:
> 
> map = {'a': Aclass, 'b': Bclass, 'c': Cclass}
> class_ = map.get(astring, default=Zclass)
> 
> The result I want is the class, not the result of calling the class 
> (which would be an instance). If I wanted the other semantics, I'd be 
> using defaultdict instead.

For an example of the defaultdict usage without calling it the 
first time:

   from collections import defaultdict
   def f():
     print "Doing some expensive calculation"
     return 42

   d = defaultdict(f)
   d['hello'] = 3.14159
   print 'Hello:', d['hello']
   print 'World:', d['world']
   print 'World (again):', d['world']

This results in the expensive calculation only being executed 
once and having the result stored in the defaultdict.  This is a 
good thing.

If you're doing as Steven suggests, you can pass and store 
function objects or class objects with the same ease:

> map = {'a': Aclass, 'b': Bclass, 'c': Cclass}
> class_ = map.get(astring, default=Zclass)

Other than tromping on the "map" built-in, one can then 
instantiate the given class with

   my_instance = map.get(astring, default=Zclass)(params)

Perfect for the factory pattern if you groove on that sort of thing.

-tkc






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