diff -r 586195685aaf Lib/doctest.py
--- a/Lib/doctest.py	Tue Aug 18 14:30:15 2015 -0400
+++ b/Lib/doctest.py	Mon Aug 24 12:15:06 2015 -0400
@@ -1099,6 +1099,25 @@
                 if pat.match(source_lines[lineno]):
                     return lineno
 
+        if isinstance(obj, str) and source_lines is not None:
+            # This will find __test__ string doctests if and only if they are
+            # formatted as triple quoted strings in the source.
+            lines = obj.splitlines(keepends=True)
+            index = 1  # skip first line in case it directly follows the """
+            while index < len(lines) and not lines[index].strip():
+                index += 1
+            if index < len(lines):
+                lineno = 0
+                while lineno < len(source_lines):
+                    if lines[index] == source_lines[lineno]:
+                        print('match')
+                        for i in range(index+1, len(lines)-1):
+                            if lines[i] != source_lines[lineno+i-index]:
+                                break
+                        else:
+                            return lineno-index
+                    lineno += 1
+
         # We couldn't find the line number.
         return None