Avoid raising OverflowError in len() when __len__() returns negative large value #74025

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1. serhiy-storchaka commented on Mar 17, 2017

   

   serhiy-storchaka

   on Mar 17, 2017

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   For now len() raises ValueError if __len__() returns small negative integer and OverflowError if __len__() returns large negative integer.

   >>> class NegativeLen:
   ...     def __len__(self):
   ...         return -10
   ... 
   >>> len(NegativeLen())
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   ValueError: __len__() should return >= 0
   >>> class HugeNegativeLen:
   ...     def __len__(self):
   ...         return -sys.maxsize-10
   ... 
   >>> len(HugeNegativeLen())
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   OverflowError: cannot fit 'int' into an index-sized integer

   Proposed patch makes it always raising ValueError.
2. serhiy-storchaka

   added

   3.7 (EOL)end of lifeend of life

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   type-featureA feature request or enhancementA feature request or enhancement

   on Mar 17, 2017
3. warsaw commented on Mar 17, 2017

   

   warsaw

   on Mar 17, 2017

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   I was going to say that this is an API change, but given that without this, folks would have to catch both exceptions and now only have to catch one of them, it isn't.
4. terryjreedy commented on Mar 24, 2017

   

   terryjreedy

   on Mar 24, 2017

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   https://docs.python.org/3/library/functions.html#len * does not specify the exception. In such cases, we occasionally change exception in x.y.0 releases without prior notice other than News and What's New. Also, I think unnecessarily exposing a compile-switch dependent internal detail, as now, is almost a bug. So +1 to applying in 3.7

   • The doc does not specify that 'length' cannot be non-negative. Perhaps it should, so no-one will think that they can hijack '__len__' to return something that is not a length, as usually understood.
5. serhiy-storchaka

   self-assigned this

   on Apr 7, 2017
6. serhiy-storchaka commented on Apr 16, 2017

   

   serhiy-storchaka

   on Apr 16, 2017

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   New changeset baf9f29 by Serhiy Storchaka in branch 'master':
   bpo-29839: Raise ValueError rather than OverflowError in len() for negative values. (#701)
   baf9f29
7. serhiy-storchaka commented on Apr 16, 2017

   

   serhiy-storchaka

   on Apr 16, 2017

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   The doc does not specify that 'length' cannot be non-negative.

   It does. https://docs.python.org/3/reference/datamodel.html#object.\_\_len__

   .. method:: object.__len__(self)

   Called to implement the built-in function :func:`len`. Should return the length
   of the object, an integer ``>=`` 0.
8. serhiy-storchaka

   closed this as completedon Apr 16, 2017
9. ezio-melotti

   transferred this issue fromon Apr 10, 2022