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From: Antoon Pardon <antoon.pardon@rece.vub.ac.be>
Newsgroups: comp.lang.python
Subject: Re: What is a function parameter =[] for?
Date: Tue, 24 Nov 2015 13:38:01 +0100
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Op 19-11-15 om 13:45 schreef Steven D'Aprano:
> On Thu, 19 Nov 2015 10:41 pm, BartC wrote:
> 
> 
>> I expect the version with the default argument to be
>> exactly the same as the last lot of calls, namely for:
>>
>> fn()
>> fn()
>> fn()
>>
>> to be equivalent to:
>>
>> temp=[]
>> fn(temp)
>> temp=[]
>> fn(temp)
>> temp=[]
>> fn(temp)
> 
> Why on earth would you expect that?
> 
> I don't mean that it isn't sometimes useful. Of course it is sometimes
> useful, there's no doubt about that. But why would you expect the language
> to default to the *slow*, *expensive*, *complicated* behaviour instead of
> the *fast*, *cheap*, *simple* behaviour?

I would say because it is better understandable. The way default value
behave now, is just a little less surprising as when the following segment
of code would print: [1]

a = []
a.append(1)
b = []
print b

Let us not forget that the tutorial talks about Default Argument *Values*.
And also the language reference talks about precomputed *values*. What
we really get is a precomputed object.

> You already have one way to set the argument to a fresh list every time you
> call the function: put the code you want executed inside the body of the
> function. There's no need for a *second* way to get the same result.

Which is beside the point. The point is understandability.

> But if you want the default value to be evaluated exactly once, and once
> only, there is no real alternative to early binding. You could use a global
> variable, of course, but that is no solution -- that's a problem waiting to
> happen.

No more than the situation we have now. The situation we have now is a problem
waiting to happen too. Which is confirmed again and again. You are stacking
the deck by calling a possible alternative "a problem waiting to happen" while
ignoring the problems that happen with the current situation.