> I found safe_repr() from Lib/unittest/util.py.

The functions in Lib/unittest/util.py shouldn't be used outside unittest.

> We would require a similar function, just implemented in C.
> What is a good place to define such C helpers that could be used everywhere?

You could start by adding it in the file where you need it.  If it starts becoming useful elsewhere too, it can then be moved somewhere else.  I would expect such function to be private, so we are free to move it whenever we need it.