Issue #19543: Implementation of isclose as per PEP 485

For details, see:
PEP 0485 -- A Function for testing approximate equality

Functions added: math.isclose() and cmath.isclose().

Original code by Chris Barker. Patch by Tal Einat.

3 files changed, 85 insertions, 0 deletions

diff --git a/Doc/library/cmath.rst b/Doc/library/cmath.rst
index a981d94..ab619a0 100644
--- a/Doc/library/cmath.rst
+++ b/Doc/library/cmath.rst
@@ -207,6 +207,38 @@ Classification functions
    and ``False`` otherwise.
 
 
+.. function:: isclose(a, b, *, rel_tol=1e-09, abs_tol=0.0)
+
+   Return ``True`` if the values *a* and *b* are close to each other and
+   ``False`` otherwise.
+
+   Whether or not two values are considered close is determined according to
+   given absolute and relative tolerances.
+
+   *rel_tol* is the relative tolerance -- it is the maximum allowed difference
+   between *a* and *b*, relative to the larger absolute value of *a* or *b*.
+   For example, to set a tolerance of 5%, pass ``rel_tol=0.05``.  The default
+   tolerance is ``1e-09``, which assures that the two values are the same
+   within about 9 decimal digits.  *rel_tol* must be greater than zero.
+
+   *abs_tol* is the minimum absolute tolerance -- useful for comparisons near
+   zero. *abs_tol* must be at least zero.
+
+   If no errors occur, the result will be:
+   ``abs(a-b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)``.
+
+   The IEEE 754 special values of ``NaN``, ``inf``, and ``-inf`` will be
+   handled according to IEEE rules.  Specifically, ``NaN`` is not considered
+   close to any other value, including ``NaN``.  ``inf`` and ``-inf`` are only
+   considered close to themselves.
+
+   .. versionadded:: 3.5
+
+   .. seealso::
+
+      :pep:`485` -- A function for testing approximate equality
+
+
 Constants
 ---------
 
diff --git a/Doc/library/math.rst b/Doc/library/math.rst
index a88d1ac..244663e 100644
--- a/Doc/library/math.rst
+++ b/Doc/library/math.rst
@@ -110,6 +110,38 @@ Number-theoretic and representation functions
    .. versionadded:: 3.5
 
 
+.. function:: isclose(a, b, *, rel_tol=1e-09, abs_tol=0.0)
+
+   Return ``True`` if the values *a* and *b* are close to each other and
+   ``False`` otherwise.
+
+   Whether or not two values are considered close is determined according to
+   given absolute and relative tolerances.
+
+   *rel_tol* is the relative tolerance -- it is the maximum allowed difference
+   between *a* and *b*, relative to the larger absolute value of *a* or *b*.
+   For example, to set a tolerance of 5%, pass ``rel_tol=0.05``.  The default
+   tolerance is ``1e-09``, which assures that the two values are the same
+   within about 9 decimal digits.  *rel_tol* must be greater than zero.
+
+   *abs_tol* is the minimum absolute tolerance -- useful for comparisons near
+   zero. *abs_tol* must be at least zero.
+
+   If no errors occur, the result will be:
+   ``abs(a-b) <= max(rel_tol * max(abs(a), abs(b)), abs_tol)``.
+
+   The IEEE 754 special values of ``NaN``, ``inf``, and ``-inf`` will be
+   handled according to IEEE rules.  Specifically, ``NaN`` is not considered
+   close to any other value, including ``NaN``.  ``inf`` and ``-inf`` are only
+   considered close to themselves.
+
+   .. versionadded:: 3.5
+
+   .. seealso::
+
+      :pep:`485` -- A function for testing approximate equality
+
+
 .. function:: isfinite(x)
 
    Return ``True`` if *x* is neither an infinity nor a NaN, and
diff --git a/Doc/whatsnew/3.5.rst b/Doc/whatsnew/3.5.rst
index ee0e5d1..085ade7 100644
--- a/Doc/whatsnew/3.5.rst
+++ b/Doc/whatsnew/3.5.rst
@@ -285,6 +285,18 @@ rather than being restricted to ASCII.
 
    :pep:`488` -- Multi-phase extension module initialization
 
+PEP 485: A function for testing approximate equality
+----------------------------------------------------
+
+:pep:`485` adds the :func:`math.isclose` and :func:`cmath.isclose`
+functions which tell whether two values are approximately equal or
+"close" to each other.  Whether or not two values are considered
+close is determined according to given absolute and relative tolerances.
+
+.. seealso::
+
+   :pep:`485` -- A function for testing approximate equality
+
 Other Language Changes
 ======================
 
@@ -346,6 +358,13 @@ cgi
 * :class:`~cgi.FieldStorage` now supports the context management protocol.
   (Contributed by Berker Peksag in :issue:`20289`.)
 
+cmath
+-----
+
+* :func:`cmath.isclose` function added.
+  (Contributed by Chris Barker and Tal Einat in :issue:`24270`.)
+
+
 code
 ----
 
@@ -578,6 +597,8 @@ math
 
 * :data:`math.inf` and :data:`math.nan` constants added.  (Contributed by Mark
   Dickinson in :issue:`23185`.)
+* :func:`math.isclose` function added.
+  (Contributed by Chris Barker and Tal Einat in :issue:`24270`.)
 
 shutil
 ------