[Python-ideas] __len__() for map()

[Greg Ewing]

Adam Johnson wrote:

> def takewhile_lessthan4(x):
>     if x < 4:
>         return x
>     raise StopIteration
> 
> tuple(map(takewhile_lessthan4, range(9)))
> # (0, 1, 2, 3)
> 
> I really don't understand why this is true, under 'normal' usage, map
> shouldn't have any reason to silently swallow a StopIteration raised
> _within_ the mapped function.

It's not -- the StopIteration isn't terminating the map,
it's terminating the iteration being performed by tuple().

It's easy to show that map() is not swallowing the
StopIteration:

 >>> m = map(takewhile_lessthan4, range(9))
 >>> next(m)
0
 >>> next(m)
1
 >>> next(m)
2
 >>> next(m)
3
 >>> next(m)
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "<stdin>", line 4, in takewhile_lessthan4
StopIteration

-- 
Greg