[Python-ideas] Default arguments in Python - the return - running out of ideas but...

[spir]

Le Wed, 13 May 2009 20:10:42 -0400,
George Sakkis <george.sakkis at gmail.com> s'exprima ainsi:

> Subproposal (2): If subproposal (1) is accepted, we could get for free
> (in terms of syntax at least) dynamic args depending on previous ones.
> That is,
> 
>     def myfunc(a, b, *m=(a+b)/2):
> 
> would mean
> 
>     def myfunc(a, b, *m = lambda a,b: (a+b)/2):
> 
> with the lambda being passed the values of a and b at runtime.

While I understand the intent, this seems complicated to me. I find clearer to express m in the func body; using a sentinel if m is a real default arg (meaning it could possibly be passed by the user).

UNDEF = object()
def myfunc(a, b, m=UNDEF):
    if m is UNDEF:
        m = (a+b)/2)

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>

Denis
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