diff -r 76e9c3e876d4 Doc/library/math.rst
--- a/Doc/library/math.rst	Sun May 29 20:46:27 2011 +0200
+++ b/Doc/library/math.rst	Sun Jun 26 17:48:48 2011 +0900
@@ -33,9 +33,22 @@
 
 .. function:: copysign(x, y)
 
-   Return *x* with the sign of *y*.  On a platform that supports
-   signed zeros, ``copysign(1.0, -0.0)`` returns *-1.0*.
+   Return a float with the magnitude (absolute value) of *x* but the sign of
+   *y*.  On platforms that supports signed zeros, ``copysign(1.0, -0.0)``
+   returns *-1.0*.
 
+        in case the arguments are NaN
+
+        >>> import math
+        >>> x = float('nan')
+        >>> math.copysign(1., x)
+        1.0
+        >>> math.copysign(-1., x)
+        1.0
+        >>> math.copysign(x, -1)
+        nan
+        >>> math.copysign(x, x)
+        nan
 
 .. function:: fabs(x)
 
diff -r 76e9c3e876d4 Modules/mathmodule.c
--- a/Modules/mathmodule.c	Sun May 29 20:46:27 2011 +0200
+++ b/Modules/mathmodule.c	Sun Jun 26 17:48:48 2011 +0900
@@ -905,7 +905,9 @@
              "This is the smallest integral value >= x.");
 
 FUNC2(copysign, copysign,
-      "copysign(x, y)\n\nReturn x with the sign of y.")
+      "copysign(x, y)\n\nReturn a float with the magnitude (absolute value) "
+      "of x but the sign \nof y. On platforms that supports signed zeros, "
+      "copysign(1.0, -0.0) \nreturns -1.0.")
 FUNC1(cos, cos, 0,
       "cos(x)\n\nReturn the cosine of x (measured in radians).")
 FUNC1(cosh, cosh, 1,