Resources for learning about Python internals and CPython source code
Here is a collection of articles, tutorials, and whatnot that help with understanding of CPython internals and source code.
Python Developer's Guide
Extending and Embedding the Python Interpreter
Python/C API Reference Manual
Eli Bendersky's Python Internals series
Yaniv Aknin's Python Innards series
Allison Kaptur's Python Internals Series
My Python Internals Series!
Python Compiler Internals, by Thomas Lee
How Fast Can We Make Interpreted Python?, by Russel Power and Alex Rubinsteyn
Python Attributes and Methods
Understanding Python by breaking it