On Fri, 3 Sept 2021 at 17:32, Guido van Rossum <gu...@python.org> wrote: > > On Fri, Sep 3, 2021 at 4:24 AM Oscar Benjamin <oscar.j.benja...@gmail.com> > wrote: >> >> On Fri, 3 Sept 2021 at 08:10, Serhiy Storchaka <storch...@gmail.com> wrote: >> > >> > There are two different kinds of TypeError: if the end user passes an >> > instance of wrong type and if the author of the library makes an error >> > in implementation of some protocols. >> > >> > For example, len(obj) raises TypeError in two cases: if obj does not >> > have __len__ (user error) and if obj.returns non-integer (implementation >> > error). for x in obj raises TypeError if obj does not have __iter__ >> > (user error) and if iter(obj) does not have __next__ (implementation >> > error). >> > >> > User errors can be fixed on user side, implementation errors can only be >> > fixed by the author of the class. Even if the user and the author is the >> > same person, these errors point to different places of code. >> > >> > Would it be worth to add a special TypeError subclass for implementation >> > errors to distinguish them from user errors? How to name it >> > (ImplementationError, ProtocolError, etc)? >> >> I think that it would be good to make TypeError more fine-grained. >> Another example is: >> >> >>> sum(1, 2, 3) >> Traceback (most recent call last): >> File "<stdin>", line 1, in <module> >> TypeError: sum() takes at most 2 arguments (3 given) >> >> There can be reasons in library code to catch TypeError that might >> arise from bad user input but in those situations you would usually >> not want to catch this TypeError. The error from calling a function >> with the wrong number of arguments would usually mean a bug in the >> library code which should not be caught. Conversely if the user input >> is a callable and you do want to catch the error resulting from it >> being called with the wrong number of arguments then catching >> TypeError is too broad again. Something like BadArgumentsError would >> be better. > > The question is, would anyone ever want to make a distinction between the two > in *real* code? I find it unlikely that someone would write > > try: > sum(x, y, z) > except TypeError: > ... > > If you bury the sum() call deep inside other code, I'd say your try/except > net is too wide.
You've misunderstood. I do *not* want to catch the exception from sum(x, y, z). Sometimes a situation arises that there is a problem involving an exception being raised that is best fixed by catching the exception. Python does not provide much of a programmatic API for inspecting exceptions beyond simply choosing which class of exceptions to catch. That makes it important to limit the class of exceptions caught as carefully as possible. If the exception that needs to be caught is literally TypeError then it is not possible to be more precise than simply "except TypeError". I have seen code that parses the error messages but that seems flakey to me. The problem with "except TypeError" is that it also catches exceptions from situations where the exception should very rarely be caught e.g. sum(1, 2, 3). If the different cases that currently result in TypeError were broken down into more fine-grained exception classes then I think that would be good. A similar thing happened for OSError - in that case it was previously possible to distinguish cases using the errno attribute but it was awkward. In the case of TypeError there is nothing akin to the errno attribute so you either catch all TypeError or none. -- Oscar _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/NOWIPXWRINTFCDZKM35PBBYPGBXSDTXI/ Code of Conduct: http://python.org/psf/codeofconduct/
