[Python-Dev] Re: PEP 616 -- String methods to remove prefixes and suffixes

2020年3月21日 12:35:08 -0700 

On 21 Mar 2020, at 19:09, Steven D'Aprano wrote:

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
And it is different for string subclasses, because the method always returns an instance of the baseclass: 
class str2(str):

... pass
...

isinstance(str2('a b c').strip(), str2)

False

isinstance(str2('a b c').strip(), str2)

False
Servus,
 Walter
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