[Python-ideas] Documenting Python warts

[Michael Foord]

On 2 January 2013 08:12, Chris Angelico <rosuav at gmail.com> wrote:

> On Wed, Jan 2, 2013 at 6:29 PM, Antoine Pitrou <solipsis at pitrou.net>
> wrote:
> > Let's call them a compromise then, but calling them a language feature
> > sounds delusional. I can't remember ever taking advantage of the fact
> > that mutable default arguments are shared accross function invocations.
>
> One common use is caching, as I mentioned earlier (with a contrived
> example). Another huge benefit is efficiency - construct a heavy
> object once and keep using it. There are others.
>
> It's a feature that can bite people, but no less a feature for that.
>


A further (and important) use case is introspection. If default values were
only added at call time (rather than definition time) then you couldn't
introspect the default value - so documentation tools (and other tools)
couldn't have access to them.

Added to which, "evaluation at call time" has its own unexpected and weird
behaviour. Consider:

x = 3
def fun(a=x):
  pass
del x

With evaluation at call time this code fails - and indeed any *re-binding*
of x in the definition scope (at any subsequent time - possibly far removed
from the function definition) affects the function.

So default values being bound at definition times have advantages for
efficiency and introspection, they have use cases for caching, and it
removes some unexpected behaviour. It's definitely a language feature.

All the best,

Michael



>
> ChrisA
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