Initial commit of Python queries and QL libraries.

python/ql/src/external/DuplicateFunction.qhelp

@@ -0,0 +1,43 @@
+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+
+<overview>
+<p>A function should never be duplicated verbatim in several places in the code. Of course
+the severity of this anti-pattern is higher for longer functions than for extremely short
+functions of one or two statements, but there are usually better ways of achieving the same
+effect.</p>
+
+</overview>
+<recommendation>
+<p>Code duplication in general is highly undesirable for a range of reasons: The artificially
+inflated amount of code hinders comprehension, and ranges of similar but subtly different lines
+can mask the real purpose or intention behind a function. There's also an omnipresent risk of
+update anomalies, where only one of several copies of the code is updated to address a defect or
+add a feature.</p>
+
+<p>In the case of function duplication, how to address the issue depends on the functions themselves
+and on the precise classes or modules in which the duplication occurs. At its simplest, the duplication can
+be addressed by simply removing all but one of the duplicate function definitions and making
+callers of the removed functions refer to the (now canonical) single remaining definition
+instead.</p>
+
+<p>This may not be possible for reasons of accessibility. A common example might
+be where two classes implement the same functionality but neither is a subtype of the other,
+so it is not possible to inherit a single method definition. In such cases, introducing a
+common superclass to share the duplicated code is a viable option. Alternatively, if the methods
+don't need access to private object state, they can be moved to a module-level function.</p>
+
+
+</recommendation>
+<references>
+
+<li>Elmar Juergens, Florian Deissenboeck, Benjamin Hummel, and Stefan Wagner. 2009.
+Do code clones matter?  In <em>Proceedings of the 31st International Conference on
+Software Engineering</em> (ICSE '09). IEEE Computer Society, Washington, DC, USA,
+485-495.</li>
+
+</references>
+</qhelp>