A web application written in C++ before there was the STL or many libraries written for C++... in 2007 or so. It was probably 200k LOC or so of CGI application code, libraries to render PDFs, manage a filesystem of XML files, an XML parser... none of it documented or tested.
The developer who was leaving the company dropped a copy of Michael Feathers' Working Effectively with Legacy Code. There was a small amount of Python to wrap the API to the C++ code using Boost against which a small suite of unit and functional tests were being developed. I learned a lot on that project.
I never fully understood how the C++ code all worked but having that API interface in Python helped to grok parts of it (and eventually replace it with a few hundred lines of Python code at a time).
The developer who was leaving the company dropped a copy of Michael Feathers' Working Effectively with Legacy Code. There was a small amount of Python to wrap the API to the C++ code using Boost against which a small suite of unit and functional tests were being developed. I learned a lot on that project.
I never fully understood how the C++ code all worked but having that API interface in Python helped to grok parts of it (and eventually replace it with a few hundred lines of Python code at a time).