From: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
Date: Sun, 31 May 2026 10:04:18 +0000 (+0200)
Subject: [3.15] Correct frexp() docs for zero and non-finite numbers (GH-149753) (GH-150652)
X-Git-Tag: v3.15.0b2~32
X-Git-Url: http://git.ipfire.org/gitweb/index.cgi?a=commitdiff_plain;h=c5655dfe51764e7e4dc2ba515acd03b8c96823a9;p=thirdparty%2FPython%2Fcpython.git

[3.15] Correct frexp() docs for zero and non-finite numbers (GH-149753) (GH-150652)

0.5 <= abs(m) < 1 is only true for finite nonzero numbers
(cherry picked from commit 5b5ffce05c211a5b0b74cd95eeb4c463614ee136)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
---

diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index 9cc8c5d68863..41a9a0ab55d8 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -255,10 +255,12 @@ Floating point manipulation functions
 
 .. function:: frexp(x)
 
-   Return the mantissa and exponent of *x* as the pair ``(m, e)``.  *m* is a float
-   and *e* is an integer such that ``x == m * 2**e`` exactly. If *x* is zero,
-   returns ``(0.0, 0)``, otherwise ``0.5 <= abs(m) < 1``.  This is used to "pick
-   apart" the internal representation of a float in a portable way.
+   Return the mantissa and exponent of *x* as the pair ``(m, e)``.
+   If *x* is a finite nonzero number, then *m* is a float with
+   ``0.5 <= abs(m) < 1.0`` and an integer *e* is such that
+   ``x == m * 2**e`` exactly.  Else, return ``(x, 0)``.
+   This is used to "pick apart" the internal representation of
+   a float in a portable way.
 
    Note that :func:`frexp` has a different call/return pattern
    than its C equivalents: it takes a single argument and return a pair of
diff --git a/Modules/clinic/mathmodule.c.h b/Modules/clinic/mathmodule.c.h
index 6a1b1a45d1d9..a5e583c180de 100644
--- a/Modules/clinic/mathmodule.c.h
+++ b/Modules/clinic/mathmodule.c.h
@@ -198,8 +198,9 @@ PyDoc_STRVAR(math_frexp__doc__,
 "\n"
 "Return the mantissa and exponent of x, as pair (m, e).\n"
 "\n"
-"m is a float and e is an int, such that x = m * 2.**e.\n"
-"If x is 0, m and e are both 0.  Else 0.5 <= abs(m) < 1.0.");
+"If x is a finite nonzero number, then m is a float with\n"
+"0.5 <= abs(m) < 1.0 and an integer e is such that\n"
+"x == m * 2**e exactly.  Else, return (x, 0).");
 
 #define MATH_FREXP_METHODDEF    \
     {"frexp", (PyCFunction)math_frexp, METH_O, math_frexp__doc__},
@@ -1163,4 +1164,4 @@ math_ulp(PyObject *module, PyObject *arg)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=80c666aef8d2df36 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=3452ce8caa2d1bd7 input=a9049054013a1b77]*/
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index a7616ad70e4a..5636a00afe10 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -1438,13 +1438,14 @@ math.frexp
 
 Return the mantissa and exponent of x, as pair (m, e).
 
-m is a float and e is an int, such that x = m * 2.**e.
-If x is 0, m and e are both 0.  Else 0.5 <= abs(m) < 1.0.
+If x is a finite nonzero number, then m is a float with
+0.5 <= abs(m) < 1.0 and an integer e is such that
+x == m * 2**e exactly.  Else, return (x, 0).
 [clinic start generated code]*/
 
 static PyObject *
 math_frexp_impl(PyObject *module, double x)
-/*[clinic end generated code: output=03e30d252a15ad4a input=96251c9e208bc6e9]*/
+/*[clinic end generated code: output=03e30d252a15ad4a input=215cf8ea28a0959b]*/
 {
     int i;
     /* deal with special cases directly, to sidestep platform