mutable default parameter problem [Prothon]

[Andrea Griffini]

On Tue, 15 Jun 2004 22:01:19 -0800, Troy Melhase <troy at gci.net> wrote:

>Seriously, you see a "wart" and a "problem".  I see a pleasant side-effect of 
>the documented semantics.  True, new folks are surprised by the behavior, but 
>once it's understood, it becomes more powerful.  
>
>How do you intend to account for code like this:
>
>def F(a, b, cache={}):
>    try:
>        return cache[(a,b)]
>    except (IndexError, ):
>        value = cache[(a,b)] = do_some_long_calc(a,b)
>        return value

I'm new to python. To my eyes this is a pretty poor attempt to
have static variables. I've implemented in the past a few
scripting languages, and it's not really difficult to
implement static variables... it's quite surprising for me
there's no such a concept in python and just that wart...
hmmm... excuse me... that bad smelling wart has to be used instead.

>Or even this:
>
>shared_cache = {}
>
>def F(a, b, cache=shared_cache):
>    ...

A global you mean ? Why not just saying "global shared_cache"
at the start of the function ?

If you need more power than a global then you probably a
(callable) class is going to serve you better than a function.

Is really a feature that if someone passes (by mistake) an extra
parameter to the function the script silently swallows it and
behaves strangely ? Or you also double-wrap the function, so
that a(x,y) calls real_a(x,y,cache=[]) ?

>Of course you can argue that this is bad style, but the counter argument is 
>just as strong:  this is quite pythonic and quite readable.

Pythonic ? In the sense that this is for example more explicit ?
Are you kidding ?

>Python is a tool, and you decrease the utility of that tool when you limit 
>it's idioms.
>
>> How much Python code would these different proposals break?
>
>A lot.  I ran this:

This doesn't surprise me. Static variables are useful when you
don't really need the power of a class instance. Too bad that
them are missing in python, and for unknown (to me) reasons
they haven't been added in the evolution of the language.

>$  find /usr/lib/python2.3/ -name "*.py" -exec grep "def.*=\[\]" {} \; | wc
>
>And see 67 instances just in the standard library.  Multiply that by a factor 
>of 1000, 10000 or more to reflect code in the field, and you might start to 
>understand the significance of changing the language definition.

That something is used is one thing. That something is elegant
is another.

Andrea