I think my confusion is about just how precise this sort of "reference
implementation" should be. Should it behave with ``str`` and ``tuple``
subclasses exactly how it would when implemented? If so, I would expect the
following to work:
class S(str): __len__ = __getitem__ = __iter__ = None
class T(tuple): __len__ = __getitem__ = __iter__ = None
x = str.cutprefix("FooBar", T(("a", S("Foo"), 17)))
assert x == "Bar"
assert type(x) is str
and so I think the ``str.__getitem__(self, slice(str.__len__(prefix), None))``
monstrosity would be the most technically correct, unless I'm missing
something. But I've never seen Python code so ugly. And I suppose this is a
slippery slope -- should it also guard against people redefining ``len = lambda
x: 5`` and ``str = list`` in the global scope? Clearly not. I think then maybe
it would be preferred to use the something like the following in the PEP:
def cutprefix(self, prefix, /):
if isinstance(prefix, str):
if self.startswith(prefix):
return self[len(prefix):]
return self[:]
elif isinstance(prefix, tuple):
for option in prefix:
if self.startswith(option):
return self[len(option):]
return self[:]
else:
raise TypeError()
def cutsuffix(self, suffix):
if isinstance(suffix, str):
if self.endswith(suffix):
return self[:len(self)-len(suffix)]
return self[:]
elif isinstance(suffix, tuple):
for option in suffix:
if self.endswith(option):
return self[:len(self)-len(option)]
return self[:]
else:
raise TypeError()
The above would fail the assertions as written before, but would pass them for
subclasses ``class S(str): pass`` and ``class T(tuple): pass`` that do not
override any dunder methods. Is this an acceptable compromise if it appears
alongside a clarifying sentence like the following?
These methods should always return base ``str`` objects, even when called
on ``str`` subclasses.
I'm looking for guidance as to whether that's an appropriate level of precision
for a PEP. If so, I'll make that change.
All the best,
Dennis
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