Issue #23185: add math.inf and math.nan constants.

Doc/library/math.rst

@@ -383,6 +383,22 @@ Constants
    The mathematical constant e = 2.718281..., to available precision.
 
 
+.. data:: inf
+
+   A floating-point positive infinity.  (For negative infinity, use
+   ``-math.inf``.)  Equivalent to the output of ``float('inf')``.
+
+   .. versionadded:: 3.5
+
+
+.. data:: nan
+
+   A floating-point "not a number" (NaN) value.  Equivalent to the output of
+   ``float('nan')``.
+
+   .. versionadded:: 3.5
+
+
 .. impl-detail::
 
    The :mod:`math` module consists mostly of thin wrappers around the platform C

Doc/whatsnew/3.5.rst

@@ -243,6 +243,12 @@ re
 * Now unmatched groups are replaced with empty strings in :func:`re.sub`
   and :func:`re.subn`.  (Contributed by Serhiy Storchaka in :issue:`1519638`.)
 
+math
+----
+
+* :data:`math.inf` and :data:`math.nan` constants added.  (Contributed by Mark
+  Dickinson in :issue:`23185`.)
+
 shutil
 ------