What extras names are treated as equal and why?

[uranusjr (Tzu-ping Chung)]

Sorry for the late reply (I was actually reminded by an entirely unrelated thread elsewhere that wants to add a uncommon extra name…)

@hroncok

I need to know what conversions (should) happen in installers when they resolve dependencies. Does pip follow the same logic?

Yes, pip normalises all extra names when they come in with the same safe_extra() function. So

1. Only ASCII alphanumerics, -, _, and . are allowed in an extra name.
2. - and . are normalised to _ for comparison. So foo[my-bar], foo[my_bar], and foo[my.bar] are equivalent.
3. Case is folded.

@hroncok

If we are not afraid of changes, we could define it to what PEP 503 does for names:

def normalize(extra_name):
    return re.sub(r"[-_.]+", "-", extra_name).lower()

I think this would actually not cause any backwards compatibility issues for pip. Can’t say for other installers, but I don’t really care (they should’ve asked for a clarification like this thread before doing something different).

@hroncok

Should I PEP this? The PEP would define the rules of “valid” extra names (defined by current packaging parser) and their normalized form (one of the above).

Does this count as a “small specification change” described in PyPA Governance - Specification Updates? I’d say let’s first send a clarification PR to PyPA specifications first, and write a PEP only if someone thinks it is required.