Add section about floating points to Python

rules/0270-Python.md

@@ -13,4 +13,38 @@ If you have some ideas for this section, head over to [GitHub][GH-python-issue].
 [python-pep20]: https://www.python.org/dev/peps/pep-0020/ "The Zen of Python"
 [GH-python-issue]: https://github.com/bkaestner/codewars-rules/issues/7
 
+### Floating point tests
+
+As written above, one shouldn't use `assert_equals` on floating point
+values, since the user might use another order of addition, and adding
+floating point numbers isn't associative:
+
+``` python
+print ("Is + associative in Python on double values?")
+print (1 + (1e-12 + 3e-12) == (1 + 1e-12) + 3e-12)
+
+print ("    :(    ")
+```
+
+Therefore, you should plan a little threshold. I usually use `1e-12`,
+which is *good enough*, or `1e-6` if the kata is about approximative
+results. Just change the value below to whatever you feel is best.
+Just keep in mind that any value lower than `1e-15` is bogus.
+
+``` python
+def assertFuzzyEquals(actual, expected, msg=""):
+    import math
+
+    if expected == 0:
+        inrange = math.fabs(actual) <= 1e-12
+    else:
+        inrange = math.fabs((actual - expected) / expected) <= 1e-12
+
+    if msg == "":
+        msg = "Expected value near {:.12f}, but got {:.12f}"
+        msg = msg.format(expected, actual)
+
+    return Test.expect(inrange, msg)
+```
+
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