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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Pass return value on ValueError exceptions in the cmath/math modules","articleBody":"# Feature or enhancement\n\n### Proposal:\n\nCurrently the error handling happens this way for the cmath module (taken from module comments):\n\n\u003e Each of the c_* functions computes and returns the C99 Annex G recommended result and also sets errno as follows: errno = 0 if no floating-point exception is associated with the result; errno = EDOM if C99 Annex G recommends raising divide-by-zero or invalid for this result; and errno = ERANGE where the overflow floating-point signal should be raised.\n\nThe ValueError raised for EDOM and the OverflowError - for ERANGE, but the Annex G result is hidden from the pure-Python world.  Though, it might be helpful for applications.  E.g. `clog(-0+0i)` returns `-∞+πi` and  `clog(+0+0i)` returns `-∞+0i` - correct one-sided limits in the pole of log().  (BTW, something like PoleError could be better here, but that's another story.)\n\nThe mpmath and the gmpy2 (per default, if `trap_divzero` and/or `trap_invalid` context options aren't enabled) rather return special values per the C standard, not raise exceptions.  And the mpmath also uses builtin float's and math/cmath functions for the fp context.  Thus, to override current behavior of the stdlib - we need to catch ValueError from the called function and then process function arguments to return special values, i.e. essentially re-implement handling of special values.  But they already are computed in cmath/math functions, so why not return this information with an exception?  An example:\n```pycon\n\u003e\u003e\u003e import cmath\n\u003e\u003e\u003e try:\n...     cmath.atanh(1)\n... except ValueError as ex:\n...     print(ex.value)\n...     \n(inf+0j)\n```\n\n\u003cdetails\u003e\n\n\u003csummary\u003eInitial patch, working for most functions, not using math_error().\u003c/summary\u003e\n\n```diff\ndiff --git a/Modules/cmathmodule.c b/Modules/cmathmodule.c\nindex 81cbf0d554..d388241f94 100644\n--- a/Modules/cmathmodule.c\n+++ b/Modules/cmathmodule.c\n@@ -36,6 +36,15 @@ class Py_complex_protected_return_converter(CReturnConverter):\n         data.return_conversion.append(\"\"\"\n if (errno == EDOM) {\n     PyErr_SetString(PyExc_ValueError, \"math domain error\");\n+\n+    PyObject *exc = PyErr_GetRaisedException();\n+    PyObject *value = PyComplex_FromCComplex(_return_value);\n+\n+    if (value) {\n+        PyObject_SetAttrString(exc, \"value\", value);\n+    }\n+    Py_DECREF(value);\n+    PyErr_SetRaisedException(exc);\n     goto exit;\n }\n else if (errno == ERANGE) {\n```\n\n\u003c/details\u003e\n\nSimilar happens in the math module and fix looks simple as well.  I didn't check all cases, but it seems that most functions in the cmath/math modules actually compute correct (per C standard and Annex G) answers for special values.\n\nAlternative approach: some global flag to turn on the gmpy2-like behavior (i.e. raise no exceptions, but return special values instead).\n\n### Has this already been discussed elsewhere?\n\nThis is a minor feature, which does not need previous discussion elsewhere\n\n### Links to previous discussion of this feature:\n\n_No response_\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-134995\n* gh-135008\n* gh-135603\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/skirpichev","@type":"Person","name":"skirpichev"},"datePublished":"2025-05-11T12:30:04.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":11},"url":"https://github.com/133895/cpython/issues/133895"}