On 3/25/2011 4:49 AM, Andrea Crotti wrote:
Terry Reedy<tjre...@udel.edu> writes:
def fib_iter(n, _cache = [1,1]):
k = len(_cache)
if n>= k:
for i in range(k, n+1):
_cache.append(_cache[i-2] + _cache[i-1])
return _cache[n]
I just realized that the signature really ought to be
def fib_iter(n, *, _cache = [1,1]):
so that the _cache parameter is keyword only (relatively new feature),
so that it cannot be overwritten by accident but only by intentional
resetting of what by its name is obviously intended to be a private, do
not touch value. This pretty much eliminates one objection people have
had to this sort of thing.
Actually, it should be
def fib_iter(n, *, _cache = [0,1]):
to match the standard definition that fib(0)=0, etc.
https://secure.wikimedia.org/wikipedia/en/wiki/Fibonacci_number
I just happened to copy the your initialization.
People who omit 0 from the series also usually leave fib(0) undefined.
If it had a more "expected" behaviour of instantiating a new cache
dictionary every time this trick would not be possible.
Next time someone claims that mutable defaults are not useful, I will
try to remember to show this example again.
--
Terry Jan Reedy
--
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