bpo-38693: Use f-strings instead of str.format() within importlib by gpshead · Pull Request #17058 · python/cpython

This is a small performance improvement, especially for one or two hot
places such as _handle_fromlist() that are called a lot and the
.format() method was being used just to join two strings with a dot.

Otherwise it is merely a readability improvement.

This could be backported to 3.8 as it changes no functionality.

I kept `_ERR_MSG` as an attribute in `importlib._bootstrap` and `imp`
as I wasn't sure if there were other things in the world that might
refer to those.  They're private and could go away in 3.9 but should
not within 3.8 just in case.

…thon#17058)

This is a small performance improvement, especially for one or two hot
places such as _handle_fromlist() that are called a lot and the
.format() method was being used just to join two strings with a dot.

Otherwise it is merely a readability improvement.

We keep `_ERR_MSG` and `_ERR_MSG_PREFIX` as those may be used elsewhere for canonical looking error messages.