Customize and improve error messages in the math module.

The default error messages on a several math functions are mostly opaque and and unhelpful.

>>> sqrt(-5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: math domain error

It would require some effort, but the code could be refactored to give a customized and helpful response, a least in common cases (falling back to the C error code message when the cause is not known):

ValueError:  math.sqrt() expects a non-negative input.  See cmath.sqrt() for variation that supports complex numbers
ValueError:  math.log() requires a non-negative input.

Linked PRs

I checked the error message of sqrt in Julia and here is the result:

julia> sqrt(-5)
ERROR: DomainError with -5.0:
sqrt will only return a complex result if called with a complex argument. Try sqrt(Complex(x)).

Maybe we can also consider adding error messages for input parameters, such as

>>> sqrt(-5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError:  math.sqrt() expects a non-negative input, got '-5.0'.  
See cmath.sqrt() for variation that supports complex numbers

It seems that we need to expand the macro FUNC1 and the function math_1_to_whatever to add the parameter error_msg to them. Is this a feasible solution? I want to try to work for it. :)

Merge remote-tracking branch 'upstream/main' into pythongh-101410/mat…

Merge branch 'main' into pythongh-101410/math_error_message

pythongh-101410: support custom messages for domain errors in the mat…

Should the error message contain the value? There is a difference between sqrt() taking -5 and -2.220446049250313e-16. Knowing this difference can help to find an error. Especially if the function definition range is limited from two sides.

On other side, formatting the repr takes a time, and if the exception is common (it is the result of limited precision in other computations), this can harm the average performance. And for math.log() the input can be an arbitrary large integer. Its decimal representation can be arbitrary long and take arbitrary long time for calculation.

And for math.log() the input can be an arbitrary large integer.

math.log() is a special snowflake. I think here we should keep here just a basic message like "expected a positive input" in the branch, that handle large integers.

gh-101410: support custom messages for domain errors in the math modu…

Issue implemented in the change 75f59bb.

No, that just add some infrastructure to support this with few example. Most functions aren't converted yet, though it should be mostly mechanical.

BTW, latest review change 8d28fd8 should be, probably, reverted - see discussion in this issue right above.

@serhiy-storchaka

@serhiy-storchaka:

On other side, formatting the repr takes a time, and if the exception is common (it is the result of limited precision in other computations), this can harm the average performance. And for math.log() the input can be an arbitrary large integer. Its decimal representation can be arbitrary long and take arbitrary long time for calculation.

@skirpichev:

BTW, latest review change 8d28fd8 should be, probably, reverted - see discussion in this issue right above.

Oh. str(int) can produce long output for large integers, I forgot that.

If the change is reverted, please add a comment to explain why the error message doesn't include the argument.

It reminds me a list.remove(x) change which added repr(x) to the error message: the change introduced a loop on some special objects which produced an infinite output leading to a crash (out of memory):

commit f6cdc6b4a191b75027de342aa8b5d344fb31313e
Author: Victor Stinner <vstinner@python.org>
Date:   Mon Mar 18 14:54:45 2024 +0100

    Revert "gh-96844: Improve error message of list.remove (gh-106455)" (#116956)
    
    This reverts commit 217f47d6e5e56bca78b8556e910cd00890f6f84a.

pythongh-101410: customize error messages for 1-arg math functions

pythongh-101410: revert loghelper() change in 75f59bb for integer input

gh-101410: Revert loghelper() change in 75f59bb for integer input (GH…

gh-101410: Customize error messages for 1-arg math functions (#129497)

Navigation Menu

Repository navigation

Customize and improve error messages in the math module. #101410

Linked PRs

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Customize and improve error messages in the math module. #101410

Description

Linked PRs

Activity

CharlieZhao95 commented on Feb 1, 2023

serhiy-storchaka commented on Sep 22, 2024

skirpichev commented on Sep 22, 2024

vstinner commented on Jan 23, 2025

skirpichev commented on Jan 24, 2025

vstinner commented on Jan 24, 2025

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions