On Thu, 17 Jan 2013 06:35:29 -0800, Mark Carter wrote:
> I thought it would be interesting to try to implement Scheme SRFI 39
> (Parameter objects) in Python.
>
> The idea is that you define a function that returns a default value. If
> you call that function with no arguments, it returns the current
> default. If you call it with an argument, it resets the default to the
> value passed in. Here's a working implementation:
[...]
> Can anyone suggest a better implementation?
I don't like the decorator version, because it requires creating a do-
nothing function that just gets thrown away. He's my version, a factory
function that takes two arguments, the default value and an optional
function name, and returns a Param function:
def param(default, name=None):
SENTINEL = object()
default = [default]
def param(arg=SENTINEL):
if arg is SENTINEL:
return default[0]
else:
default[0] = arg
return arg
if name is not None:
param.__name__ = name
return param
In Python 3, it's even nicer, although no shorter:
def param(default, name=None):
SENTINEL = object()
def param(arg=SENTINEL):
nonlocal default
if arg is SENTINEL:
return default
else:
default = arg
return arg
if name is not None:
param.__name__ = name
return param
--
Steven
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