bad recursion, still works

[mdsherry at gmail.com]

On Jul 15, 4:12 pm, iu2 <isra... at elbit.co.il> wrote:
> On Jul 15, 9:30 pm, mdshe... at gmail.com wrote:
>
>
>
> > On Jul 15, 2:59 pm, iu2 <isra... at elbit.co.il> wrote:
>
> > > Hi,
>
> > > I wrote this wrong recursive function that flattens a list:
>
> > > def flatten(lst, acc=[]):
> > >     #print 'acc =', acc, 'lst =', lst
> > >     if type(lst) != list:
> > >         acc.append(lst)
> > >     else:
> > >         for item in lst:
> > >             flatten(item)
> > >     return acc
>
> > > a = [1, 2, [3, 4, 5], [6, [7, 8, [9, 10], 11], 12], 13, 14]
> > > b = flatten(a)
> > > print b
>
> > > I was amazed to realize that it flattens the list alright. Why? 'acc'
> > > should be an empty list on each invocation of flatten, but is seems to
> > > accumulate anyway...
>
> > When you say acc=[] in the function declaration, it binds acc to a
> > particular list object, rather than to a concept of an empty list.
> > Thus, all operations being performed on acc are being performed on the
> > same list. If, after the sample code you provided, were to call
>
> > c = flatten([15,16,17,[18,19]])
> > print c
>
> > you would get back the list [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
> > 13, 14, 15, 16, 17, 18, 19].
>
> > Mark Sherry
>
> I still don't understand: In each recursive call to flatten, acc
> should be bound to a new [], shouldn't it? Why does the binding happen
> only on the first call to flatten?

Default values are bound when the function is defined, not when it's
called. For example,

>>> import random
>>> def foo(bar = random.random()):
...     print bar
...
>>> foo()
0.632312549821312
>>> foo()
0.632312549821312

If you view [...] just as shorthand for list(...), it might make a bit
more sense. For immutable values, one can't change the value bound to
the name, only what the name is bound to, so this behaviour is less
obvious. But still there.

As to why default values are evaluated at define time vs. call time,
I'd argue reasons of scope and speed - if the default value is a
computed constant, it makes little sense to recompute it every time
the function is called.

Mark Sherry