On Sat, Mar 21, 2020 at 12:15:21PM -0400, Eric V. Smith wrote:
> On 3/21/2020 11:20 AM, Ned Batchelder wrote:
> >Why be so prescriptive? The semantics of these functions should be
> >about what the resulting string contains. Leave it to implementors to
> >decide when it is OK to return self or not.
I agree with Ned -- whether the string object is returned unchanged or a
copy is an implementation decision, not a language decision.
[Eric]
> The only reason I can think of is to enable the test above: did a
> suffix/prefix removal take place? That seems like a useful thing.
We don't make this guarantee about string identity for any other string
method, and CPython's behaviour varies from method to method:
py> s = 'a b c'
py> s is s.strip()
True
py> s is s.lower()
False
and version to version:
py> s is s.replace('a', 'a') # 2.7
False
py> s is s.replace('a', 'a') # 3.5
True
I've never seen anyone relying on this behaviour, and I don't expect
these new methods will change that. Thinking that `is` is another way of
writing `==`, yes, I see that frequently. But relying on object identity
to see whether a new string was created by a method, no.
If you want to know whether a prefix/suffix was removed, there's a more
reliable way than identity and a cheaper way than O(N) equality. Just
compare the length of the string before and after. If the lengths are
the same, nothing was removed.
--
Steven
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