Injecting code into a function

[Paddy]

Try searching for: 'python aspect-oriented' as aspect oriented
programming is about modifying existing class-methods (not exactly
functions which is what you asked for).
You might also do a search for "AOP considered harmful"
http://www.infosun.fmi.uni-passau.de/st/papers/EIWAS04/stoerzer04aop_harmful.pdf

The main point is that when you are reading the source you don't know
what the code is as it may be augmented by an "external" change.