Add a convenient and correct way to compare floats.

testing/python/approx.py

@@ -0,0 +1,27 @@
+import pytest
+import doctest
+
+class MyDocTestRunner(doctest.DocTestRunner):
+
+    def __init__(self):
+        doctest.DocTestRunner.__init__(self)
+
+    def report_failure(self, out, test, example, got):
+        raise AssertionError("'{}' evaluates to '{}', not '{}'".format(
+            example.source.strip(), got.strip(), example.want.strip()))
+
+
+class TestApprox:
+
+    def test_approx(self):
+        parser = doctest.DocTestParser()
+        test = parser.get_doctest(
+                pytest.approx.__doc__,
+                {'approx': pytest.approx},
+                pytest.approx.__name__,
+                None, None,
+        )
+        runner = MyDocTestRunner()
+        runner.run(test)
+
+