Parameterized Functions without Classes

[Christian Tismer]

Hi Pythonistas,

just as a small comment on the side-effects of the
rather new concept of local functions with access to
their scope:

This concept can be used to avoid having extra
classes just for keeping some read-only state.

Since this feature is not so obvious in the first
place, I thought to share the obervation.

In the ancient times, I saw myself writing classes
to parameterize functions, like this:

class Converter:
     def __init__(self, scale):
         self.scale = scale
     def __call__(self, arg):
         return self.scale * arg

inch_to_cm = Converter(2.54)
cm_to_inch = Converter(1 / 2.54)
...

 >>> inch_to_cm(20)
50.799999999999997
 >>> cm_to_inch(20)
7.8740157480314954
 >>>

This can be easily done without an extra class, just by
local variables which access the outer scope:

def _converter(scale):
     def convert(arg):
         return scale * arg
     return convert

inch_to_cm = _converter(2.54)
cm_to_inch = _converter(1 / 2.54)

 >>> inch_to_cm(20)
50.799999999999997
 >>> cm_to_inch(20)
7.8740157480314954
 >>>

This trick (and I don't consider it a trick) works for
all cases, where you don't need write access to an instance
variable, but read access, only.
It is a very clean way to parameterize functions, and it
is very effective since there is no attribute lookup
necessary.

cheers - chris

-- 
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