[issue43475] Worst-case behaviour of hash collision with float NaN
Christoph Groth added the comment: > What concrete action would you propose that the Python core devs take at this > point? Nothing for now. I stumbled across this issue through https://gitlab.kwant-project.org/kwant/tinyarray/-/issues/20 and had the impression that the aspect that I raised (that, for example, hash values of immutable built-in objects now no longer survive pickling) was not examined in this otherwise in-depth discussion. So I added it for reference. If problems come up that are caused by this change, I would consider reverting it a possible solution. > The result of the change linked to this PR is that the hash now also reflects > that containment depends on object identity, not just object value. This is a nice way to phrase it. Thanks for the link to the entertaining talk. :-) -- ___ Python tracker <https://bugs.python.org/issue43475> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue43475] Worst-case behaviour of hash collision with float NaN
Christoph Groth added the comment: Hello. I would like to point out a possible problem that this change to CPython has introduced. This change looks innocent, but it breaks the unwritten rule that the hash value of a number (or actually any built-in immutable type!) in Python depends only on its value. Thus, before this change, it was possible to convert a tuple of floats into a numpy array and back into a tuple, and the hash values of both tuples would be equal. This is no longer the case. Or, more generally, any hashable tuple could be pickled and unpickled, without changing its hash value. I could well imagine that this breaks real code in subtle ways. Likewise, it is now no longer possible to provide a library of sequences of numbers that always hashes like built-in tuples. (As "tinyarray", of which I am the author, did.) -- nosy: +cwg ___ Python tracker <https://bugs.python.org/issue43475> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15767] add ModuleNotFoundError
Christoph Groth <christ...@grothesque.org> added the comment: Unfortunately I do not feel competent enough to submit a documentation patch because I still do not understand why ModuleNotFoundError was added. I don't want to bother you further with this. Thank you all for your prompt replies. If you agree with me that there is indeed an issue, please open it yourself. -- ___ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue15767> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15767] add ModuleNotFoundError
Christoph Groth <christ...@grothesque.org> added the comment: Thank you, Chris, for your reply. If this is indeed the main use case of ModuleNotFoundError, I respectfully suggest to document it better. The way things are now, Python users who switch to 3.6 encounter this new exception during their work with the interpreter and invariably wonder "Should I change anything in my code because of this? If not, why was it introduced?". In my opinion the current documentation does not answer these questions well. Note that this is not about some deeply buried detail. Every Python programmer is bound to encounter this. That said, I cannot imagine many cases where user code would like to fall back to html.parser only if fancy_parser is not installed but not if an older version of fancy_parser is installed (or maybe it's an entirely *different* fancy_parser?). And in the rare cases where that is desired, it was already perfectly idiomatic to do so: try: import fancy_parser except ImportError: from html.parser import HTMLParser else: from fancy_parser import NewParser as HTMLParser -- ___ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue15767> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15767] add ModuleNotFoundError
Christoph Groth <christ...@grothesque.org> added the comment:
In the above, please replace "understand the decision" by "understand the
usefulness of it".
In the above discussion, as an alternative to a new exception, it was proposed
to add an attribute to ImportError ('reason'), but then people seemed to agree
that this is quite useless outside of importlib (msg192261). But then I don't
understand why the original idea of the exception was revived.
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[issue15767] add ModuleNotFoundError
Christoph Groth <christ...@grothesque.org> added the comment: My curiosity was piqued when I saw ModuleNotFoundError, so I decided to look it up. This led me to this page and I read the complete discussion. I still did not understand the decision, so I allowed myself to ask, also because I believe that when new features are introduced it should be clear what they are good for. That's all. -- ___ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue15767> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue15767] add ModuleNotFoundError
Christoph Groth <christ...@grothesque.org> added the comment:
> Read Eric's message before mine.
Of course I read it, I wouldn't have asked otherwise. Eric mentions an older
message ("see msg182332") that *predates* your judgment that "outside importlib
there shouldn't be a need to distinguish between the cases". Otherwise Eric
says that he finds "the exception to be very useful", but frankly, I don't see
why (outside of importlib or backports of it). What changed since msg192263?
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[issue15767] add ModuleNotFoundError
Christoph Groth <christ...@grothesque.org> added the comment: I'm trying to understand why ModuleNotFoundError was added to 3.6. The "what's new" entry links to this page. Looking at the discussion, Guido said in 2013: "Right. Outside importlib there shouldn't be a need to distinguish between the cases (especially given that the exception works differently than its name suggests).". Then, three years later, he supports the inclusion of the patch, without that any new arguments had been given. Could someone explain (or link to) what happened inbetween? -- nosy: +cwg ___ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue15767> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
