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

Lib/test/test_math.py

@@ -2503,6 +2503,50 @@ def test_input_exceptions(self):
         self.assertRaises(TypeError, math.atan2, 1.0)
         self.assertRaises(TypeError, math.atan2, 1.0, 2.0, 3.0)
 
+    def test_exception_messages(self):
+        x = -1.1
+
+        with self.assertRaises(ValueError,
+                               msg=f"expected a nonnegative input, got {x}"):
+            math.sqrt(x)
+
+        with self.assertRaises(ValueError,
+                               msg=f"expected a positive input, got {x}"):
+            math.log(x)
+        with self.assertRaises(ValueError,
+                               msg=f"expected a positive input, got {x}"):
+            math.log(123, x)
+        with self.assertRaises(ValueError,
+                               msg=f"expected a positive input, got {x}"):
+            math.log2(x)
+        with self.assertRaises(ValueError,
+                               msg=f"expected a positive input, got {x}"):
+            math.log2(x)
+
+        x = decimal.Decimal(x)
+
+        with self.assertRaises(ValueError,
+                               msg=f"expected a positive input, got {x!r}"):
+            math.log(x)
+
+        x = fractions.Fraction(1, 10**400)
+
+        with self.assertRaises(ValueError,
+                               msg=f"expected a positive input, got {float(x)!r}"):
+            math.log(x)
+
+        x = -2**1000
+
+        with self.assertRaises(ValueError,
+                               msg=f"expected a positive input"):
+            math.log(x)
+
+        x = 1.0
+
+        with self.assertRaises(ValueError,
+                               msg=f"expected a number between -1 and 1, got {x}"):
+            math.atanh(x)
+
     # Custom assertions.
 
     def assertIsNaN(self, value):

Misc/NEWS.d/next/Library/2023-02-01-16-41-31.gh-issue-101410.Dt2aQE.rst

@@ -0,0 +1,3 @@
+Support custom messages for domain errors in the :mod:`math` module
+(:func:`math.sqrt`, :func:`math.log` and :func:`math.atanh` were modified as
+examples).  Patch by Charlie Zhao and Sergey B Kirpichev.

Modules/mathmodule.c

@@ -921,33 +921,41 @@ is_error(double x)
 */
 
 static PyObject *
-math_1(PyObject *arg, double (*func) (double), int can_overflow)
+math_1(PyObject *arg, double (*func) (double), int can_overflow,
+       const char *err_msg)
 {
+    char *buf;
     double x, r;
     x = PyFloat_AsDouble(arg);
     if (x == -1.0 && PyErr_Occurred())
         return NULL;
     errno = 0;
     r = (*func)(x);
-    if (isnan(r) && !isnan(x)) {
-        PyErr_SetString(PyExc_ValueError,
-                        "math domain error"); /* invalid arg */
-        return NULL;
-    }
+    if (isnan(r) && !isnan(x))
+        goto domain_err; /* domain error */
     if (isinf(r) && isfinite(x)) {
         if (can_overflow)
             PyErr_SetString(PyExc_OverflowError,
                             "math range error"); /* overflow */
         else
-            PyErr_SetString(PyExc_ValueError,
-                            "math domain error"); /* singularity */
+            goto domain_err; /* singularity */
         return NULL;
     }
     if (isfinite(r) && errno && is_error(r))
         /* this branch unnecessary on most platforms */
         return NULL;
 
     return PyFloat_FromDouble(r);
+
+domain_err:
+    buf = PyOS_double_to_string(x, 'r', 0, Py_DTSF_ADD_DOT_0, NULL);
+
+    if (buf) {
+        PyErr_Format(PyExc_ValueError,
+                     err_msg ? err_msg : "math domain error", buf);
+        PyMem_Free(buf);
+    }
+    return NULL;
 }
 
 /* variant of math_1, to be used when the function being wrapped is known to
@@ -1032,7 +1040,13 @@ math_2(PyObject *const *args, Py_ssize_t nargs,
 
 #define FUNC1(funcname, func, can_overflow, docstring)                  \
     static PyObject * math_##funcname(PyObject *self, PyObject *args) { \
-        return math_1(args, func, can_overflow);                            \
+        return math_1(args, func, can_overflow, NULL);                  \
+    }\
+    PyDoc_STRVAR(math_##funcname##_doc, docstring);
+
+#define FUNC1D(funcname, func, can_overflow, docstring, err_msg)        \
+    static PyObject * math_##funcname(PyObject *self, PyObject *args) { \
+        return math_1(args, func, can_overflow, err_msg);               \
     }\
     PyDoc_STRVAR(math_##funcname##_doc, docstring);
 
@@ -1070,9 +1084,10 @@ FUNC2(atan2, atan2,
       "atan2($module, y, x, /)\n--\n\n"
       "Return the arc tangent (measured in radians) of y/x.\n\n"
       "Unlike atan(y/x), the signs of both x and y are considered.")
-FUNC1(atanh, atanh, 0,
+FUNC1D(atanh, atanh, 0,
       "atanh($module, x, /)\n--\n\n"
-      "Return the inverse hyperbolic tangent of x.")
+      "Return the inverse hyperbolic tangent of x.",
+      "expected a number between -1 and 1, got %s")
 FUNC1(cbrt, cbrt, 0,
       "cbrt($module, x, /)\n--\n\n"
       "Return the cube root of x.")
@@ -1205,9 +1220,10 @@ FUNC1(sin, sin, 0,
 FUNC1(sinh, sinh, 1,
       "sinh($module, x, /)\n--\n\n"
       "Return the hyperbolic sine of x.")
-FUNC1(sqrt, sqrt, 0,
+FUNC1D(sqrt, sqrt, 0,
       "sqrt($module, x, /)\n--\n\n"
-      "Return the square root of x.")
+      "Return the square root of x.",
+      "expected a nonnegative input, got %s")
 FUNC1(tan, tan, 0,
       "tan($module, x, /)\n--\n\n"
       "Return the tangent of x (measured in radians).")
@@ -2190,7 +2206,7 @@ loghelper(PyObject* arg, double (*func)(double))
         /* Negative or zero inputs give a ValueError. */
         if (!_PyLong_IsPositive((PyLongObject *)arg)) {
             PyErr_SetString(PyExc_ValueError,
-                            "math domain error");
+                            "expected a positive input");
             return NULL;
         }
 
@@ -2214,7 +2230,7 @@ loghelper(PyObject* arg, double (*func)(double))
     }
 
     /* Else let libm handle it by itself. */
-    return math_1(arg, func, 0);
+    return math_1(arg, func, 0, "expected a positive input, got %s");
 }