slices to immutable strings by reference and not copy

Question

If you use string.split() on a Python string, it returns a list of strings. These substrings that have been split-out are copies of their part of the parent string.

Is it possible to instead get some cheaper slice object that holds only a reference, offset and length to the bits split out?

And is it possible to have some 'string view' to extract and treat these sub-strings as if they are strings yet without making a copy of their bytes?

(I ask as I have very large strings I want to slice and am running out of memory occasionally; removing the copies would be a cheap profile-guided win.)

Answer 1

buffer will give you a read-only view on a string.

>>> s = 'abcdefghijklmnopqrstuvwxyz'
>>> b = buffer(s, 2, 10)
>>> b
<read-only buffer for 0x7f935ee75d70, size 10, offset 2 at 0x7f935ee5a8f0>
>>> b[:]
'cdefghijkl'

Answer 2

String objects always point to a NUL-terminated buffer in Python, so substrings must be copied. As Ignacio pointed out, you can use buffer() to get a read-only view on the string memory. The buffer() built-in function has been superseded by the more versatile memoryview objects, though, which are available in Python 2.7 and 3.x (buffer() is gone in Python 3.x).

s = "abcd" * 50
view = memoryview(s)
subview = view[10:20]
print subview.tobytes()

This code prints

cdabcdabcd

As soon as you call tobytes(), a copy of the string will be created, but the same happens when slicing the old buffer objects as in Ignacio's answer.