[Python-ideas] Default arguments in Python - the return - running out of ideas but...
steve at pearwood.info
Thu May 14 23:25:28 CEST 2009
On Thu, 14 May 2009 09:44:07 pm spir wrote:
> Generally speaking, I find ok the need of sentinels for clarifying
> rare and non-obvious cases such as runtime-changing default values:
> def somefunc(arg, m=UNDEF):
> if m is UNDEF:
> m = runtimeDefaultVal()
> While I do not find ok the need of a sentinel to avoid the common
> gotcha of a default value beeing "back-updated" when the
> corresponding local var is changed in the func body:
> def otherfunc(arg, l=UNDEF):
> if l is UNDEF:
> l = 
> <possibly update l>
But those two idioms are the same thing!
In the first case, if m is not provided by the caller, your function has
to produce a fresh object at runtime. It does this by calling
runtimeDefaultVal() which returns some unspecified object.
In the second case, if l is not provided by the caller, your function
has to produce a fresh object at runtime. It does this by calling .
This is merely a special case of the first case, where
runtimeDefaultVal() simply returns  every time.
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