[Python-ideas] Make len() usable on a generator

[Cathal Garvey]

Well, he said for _containers_, for which we assume a length attribute
has been tallied all along; calling len() on a container should just
access this tally, making it minimally intensive.

For iteration-enabled classes, you can implement a __len__() magic
method which returns whatever you like, so if you can make predictions
about the length of an iterable class, even if the values are generated
on the fly, then you can implement this to make it return the value
without much computation.

Some builtin generators appear to do just this:

In [10]: %timeit  len(range(100))
1000000 loops, best of 3: 1.27 µs per loop

In [11]: %timeit  len(range(1000))
1000000 loops, best of 3: 1.51 µs per loop

In [12]: %timeit  len(range(100000))
1000000 loops, best of 3: 1.59 µs per loop

In [13]: %timeit  len(range(10000000))
1000000 loops, best of 3: 1.58 µs per loop

On 03/10/14 16:36, Mark Young wrote:
> Why do you think len is an inherently O(1) operation? Sure, on array-based
> things it is, but for an arbitrary collection, the only logical assertion
> is that it's O(n). (For some containers, like arrays, it might be O(1) as
> well, but you can't assume that in general)​
> 
> 
> 
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