[Python-ideas] deferred default arguments

[Greg Ewing]

Steven D'Aprano wrote:

> def mean(data, missing=False): pass
> def variance(data, missing=False):  pass
> def pvariance(data, missing=False):  pass
> def stdev(data, missing=False):  pass
> def pstdev(data, missing=False):  pass
> 
> For API consistency, a change to one signature requires corresponding 
> changes to the others.

If you think you might change your mind about the default
value of "missing", you can do

default_missing = False

def mean(data, missing=default_missing): ...
def variance(data, missing=default_missing): ...

etc.

> If the caller doesn't supply a value for name, 
> and f tries to use it (except as below), then you get an unambiguous 
> UnboundLocalError.
> 
> The exception is, calling another function with it as an argument is 
> permitted.

That just leads to another version of the "None-as-sentinel"
problem, though. What happens if you don't want to *directly*
pass it to another function, but via some intermediate name
or data structure? And how do debuggers deal with it?

-- 
Greg