[Python-Dev] Slice as a copy... by design?

[Christian Heimes]

Facundo Batista schrieb:
> Hi!
> 
> A thread in PyAr raised the question that, considering that strings
> are immutable, why a slice of a string is a copy and not a reference
> to a part of that string.

Because the reference approach is more complicated, harder to implement
and may lead to unexpected behavior.

For example:

>>> a = "a long string with 500,000 chars ..."
>>> b = a[0]
>>> del a

With the slice-as-copy design the string 'a' is immediately freed. The
slice-as-reference design would keep the 500kB string in memory although
you are only interested in the first character.

The buffer interface was designed for the slice-as-copy use case:

>>> a = "abcdefg"
>>> b = buffer(a, 2, 3)
>>> b
<read-only buffer for 0x839c2e0, size 3, offset 2 at 0x8391c40>
>>> str(b)
'cde'
>>> sys.getrefcount(a)
3
>>> del b
>>> sys.getrefcount(a)
2

Christian