default argument in method

[Steven D'Aprano]

On Wed, 15 Dec 2010 21:10:05 +0100, Hans-Peter Jansen wrote:

> Since this is a major pitfall, it might be worth mentioning, that
> mutable default arguments are generally a bad idea, as the default
> arguments are evaluated just once, hence e.g. using an empty list might
> contain the items, that were appended in earlier calls of this method..

It's only a pitfall for users who expect that default arguments are re-
created every time you call the function; it's only a bad idea for code 
which relies on the default arguments being re-created each time.

If you hold misunderstandings about the behaviour of a language, you'll 
have trouble understanding what code does. Default arguments are no 
different from any other feature.



> Code, that _relies_ on such behavior should be yanked instantaneous and
> the producer of such code should be punished with coding APL¹ on a
> dubeolsik hangul keyboard² for a year at least..

Python code that relies on default arguments to *not* be re-created on 
each function call is no worse than (say) Ruby code that relies on 
default arguments *to* be re-created each time.

I don't mean to be elitist (ah, who am I fooling, of course I do), but 
when coders of the skill and experience of the Effbot and Guido use 
mutable defaults, who are you to say they shouldn't?

http://effbot.org/zone/default-values.htm
http://www.python.org/doc/essays/graphs/



-- 
Steven