On 03/08/2023 11.38, Jon Ribbens via Python-list wrote:
On 2023-08-02, dn <pythonl...@danceswithmice.info> wrote:
Can you please explain why a multi-part second-argument must be a tuple
and not any other form of collection-type?
The following comment may hold a clue:
if (PyTuple_Check(cls)) {
/* Not a general sequence -- that opens up the road to
recursion and stack overflow. */
https://github.com/python/cpython/blob/main/Objects/abstract.c#L2684
Plus an almost total lack of demand for change I should think.
Thanks for the reference!
Am not proposing a change (have learned 'the rule' and accepted
life-as-it-is), but was curious about the restriction: why not any
(reasonable sequence)?
Such pondering continues in my subversive mind, given such thoughts as:
- "we're all adults here"
(eg frequent justification when arguing about Python not having "private
attributes" per-se).
So, if we introduce a complexity, on our own heads be it!
(hardly likely given that all we are likely to attempt is the
application of a simple and short 'list' of [un]acceptable types).
NB understood that the quoted-code is applied in many and other 'scalar
or tuple' situations.
- Python happily enables recursion, which "opens up the road to
recursion and stack overflow.".
So, why install a 'nanny' to save us from ourselves here?
Again: seems on-the-surface that such 'lists' (at the code-line level)
will be mono-dimensional 99.9% of the time.
NB having said that, the underlying mechanism *is* multi-dimensional:
"direct, indirect, or virtual) subclass thereof"
(https://docs.python.org/3/library/functions.html#isinstance)
Further: the 'rules' say: "classinfo is a tuple of type objects (or
recursively, other such tuples)". Thus, can write:
>>> target_object = ...
>>> inner_tuple = float, complex
>>> inner_tuple
(<class 'float'>, <class 'complex'>)
>>> isinstance( target_object, ( str, inner_tuple, int, ), )
False
I can't say I've ever used such construction in-the-wild - either
defining and then using an "inner_tuple", or even nesting. YMMV!
Any thoughts?
--
Regards,
=dn
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