[Python-ideas] Marvelous movable memoryview

[Matt Chaput]

By which I mean a memoryview that lets you change the start and end 
offsets of its view of the underlying object (not modifying the 
underlying object).

Let's say I want to slice a long string s into trigrams and write them 
to disk (or something).

   for i in xrange(0, len(s) - 3):
       x = s[i:i + 3]
       myfile.write(x)

At each step the slice copies the bytes of the string, even though all I 
do is write them to disk.

I could avoid copying with memoryviews...

   for i in xrange(0, len(s) - 3):
       x = memoryview(s)[i:i + 3]
       myfile.write(x)

...but this is actually much slower (3x slower in some quick tests). I'm 
guessing it's because of all the object creation (while string slicing 
probably uses fast paths).

Shouldn't I be able to do this?

   m = memoryview(s)
   for i in xrange(0, len(s) - 3):
       m.start = i
       m.end = i + 3
       myfile.write(m)


Cheers,

Matt