I'm curious, which public codebases would you consider to be examples of well written Python?
Also, one thing that made a negative impression on me is https://github.com/antirez/redis/blob/unstable/src/adlist.c:
/* Free the whole list.
* This function can't fail. */
void listRelease(list *list)
unsigned long len;
listNode *current, *next;
current = list->head;
Others mentioned kernel, GNOME. Redis is nice because it is fairly self contained. It has a good set of networking code, some command parsing, storage. Kind of a happy medium.
Anyway, I didn't look in depth at understanding the semantics of every module.
Here are some more things (besides NULL check I can see that are worrisome in that particular piece of code):
* can len be out of sync with actual list length
* can list->free be defined (non null) but value wasn't allocated with an malloc perhaps
* what if an element is inserted twice in list, will list->free be called on already free-ed area
* on 64 bit machine unsigned long will be 64 bit and on 32 bit it will be 32 bit, is that problem?
Which ones do you see?
Now those are issues I see by looking at the module in isolation. But it is not quite an isolated. It is part of a large module. Sometimes there are invariants that are enforced at the input (at the system boundary) and so it is not necessary to always keep checking for NULL or validating inputs in every single internal function.
That is one good lesson I learned from Erlang. Check your inputs at the boundary then code for the "happy" path and let error result in quick and early failure. Maybe prefer a quick segfault rather than a dangling pointer or wondering later how exactly to handle NULL pointer if it is not really expected to be NULL.