Issue 28905: re.sub appears not to check count optional argument for integerness (original) (raw)

Issue28905

Created on 2016-12-08 17:02 by Danny Yoo, last changed 2022-04-11 14:58 by admin. This issue is now closed.

Messages (4)
msg282719 - (view)	Author: Danny Yoo (Danny Yoo)	Date: 2016-12-08 17:02
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.
msg282721 - (view)	Author: Danny Yoo (Danny Yoo)	Date: 2016-12-08 17:11
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.
msg282722 - (view)	Author: Danny Yoo (Danny Yoo)	Date: 2016-12-08 17:32
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.
msg282726 - (view)	Author: R. David Murray (r.david.murray) *	Date: 2016-12-08 18:37
See issue 28082.