re.sub appears not to check count optional argument for integerness #73091

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1. DannyYoo commented on Dec 8, 2016

   

   DannyYoomannequin

   on Dec 8, 2016

   MannequinAuthor

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   This comes from diagnosing a beginner's question on Python-tutor.

   https://mail.python.org/pipermail/tutor/2016-December/110066.html

   It appears that re.sub is not checking whether the count argument is integer or not, and silently accepts a nonsensical argument. For example:

   >>> import re
   >>> s = "AAAcBBB\nAAAdBBB"
   >>> print(re.sub(r'^AAA', "aaa", s, re.MULTILINE))
   aaacBBB
   AAAdBBB

   Of course, the user intended to pass re.MULTILINE to flags, not to count, but the fact that this isn't raising a TypeError is error-prone.
2. DannyYoo

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Dec 8, 2016
3. DannyYoo commented on Dec 8, 2016

   

   DannyYoomannequin

   on Dec 8, 2016

   MannequinAuthor

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   Ugh. I suddenly realize that this is complicated by the fact that flag values are themselves represented as integers, and Python's type system isn't rich enough to label flag values as a distinct type for the purposes.

   It may be worthwhile to add a warning in the documentation about this, as it is an easy mistake to make.
4. DannyYoo commented on Dec 8, 2016

   

   DannyYoomannequin

   on Dec 8, 2016

   MannequinAuthor

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   Alternatively, change the representation of flag values from integers to some class extension that supports the common bitwise operators.

   As a very rough sketch:

   >>> class FlagInt(int):
   ...     def __or__(self, other):
   ...         return FlagInt(int(self) | int(other))
   ... 
   >>> f1 = FlagInt(1)
   >>> f2 = FlagInt(2)
   >>> f1 | f2
   3
   >>> isinstance(3, FlagInt)
   False
   >>> isinstance(f1 | f2, FlagInt)
   True

   That way, flag arguments can be determined at runtime to have derived from the proper flag values.

   This kind of approach may have some backwards-incompatibility, unfortunately, since other folks have been hardcoding integers rather than use the flag constants. Other concerns might include serialization, in case someone tries to save a FlagInt somewhere and pull it out at some other time.
5. bitdancer commented on Dec 8, 2016

   

   bitdancer

   on Dec 8, 2016

   Member

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   See bpo-28082.
6. bitdancer

   closed this as completedon Dec 8, 2016
7. ezio-melotti

   transferred this issue fromon Apr 10, 2022