Re: [Python-ideas] my take on mutable default arguments

27 Jan 2007


      As pointed out by Calvin Spealman in an earlier email:

Calvin Spealman wrote:
...
Deep copy of the originally created
default argument can be expensive and would not work in any useful way
with non-literals as defaults, such as function calls or subscript
lookups or even simple attributes.
However, your decorator is useful in several common cases.
- Chris Rebert

tomer filiba wrote:
...
i thought this code be solved nicely with a decorator... it needs some
more work, but it would make a good cookbook recipe:
.>>> from copy import deepcopy.
.>>>
.>>> def defaults(**kwdefs):
...     def deco(func):
...         def wrapper(*args, **kwargs):
...             for k,v in kwdefs.iteritems():
...                 if k not in kwargs:
...                     kwargs[k] = deepcopy(v)
...             return func(*args, **kwargs)
...         return wrapper
...     return deco
...
.>>> @defaults(x = [])
... def foo(a, x):
...     x.append(a)
...     print x
...
.>>> foo(5)
[5]
.>>> foo(5)
[5]
.>>> foo(5)
[5]
maybe it should be done by copying func_defaults... then it could
be written as
@copydefaults
def f(a, b = 5, c = []):
    ...
-tomer
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