[Breaking/Fix] Skip isotopes when iterating through `core.Element`

[DanielYang59]

Summary

• Skip isotopes when iterating through core.Element, to fix #4178, rationale:
  • I might expect looping through Element to behave similarly to looping through the periodic table, i.e. isotopes should not sneak in here, do you like this idea @esoteric-ephemera?
• Implementation based on https://docs.python.org/3/library/enum.html:

  EnumType is responsible for setting the correct repr(), str(), format(), and reduce() methods on the final enum, as well as creating the enum members, properly handling duplicates, providing iteration over the enum class, etc.

• Add property Element.named_isotopes for all named_isotopes so far.
• Clean up core.periodic_table docstring (remove non-existent PeriodicTable class).

This would be a breaking change (also might be called a fix), so comments are hugely appreciated.

---

from pymatgen.core import Element

for idx, ele in enumerate(Element):
    print(idx, ele)

Before

0 H
1 H  # Deuterium
2 H  # Tritium
3 He
4 Li
5 Be
6 B
7 C
......

Now (nothing special, just skip isotopes of hydrogen)

0 H
1 He
2 Li
3 Be
4 B
5 C
......

[shyuep]

I agree isotopes shouldn't appear as part of default behavior.

[esoteric-ephemera]

That was an oversight on my part, thanks for the fix! Also agree that enumerating over elements shouldn't list isotopes. Maybe adding a method to Element to show people which isotopes are available would be useful tho?

[DanielYang59]

That was an oversight on my part, thanks for the fix!

No problem at all, cannot blame anyone, it's the test that is missing, together we improve test coverage :)

Maybe adding a method to Element to show people which isotopes are available would be useful tho?

Fair point, is current implementation looking good to you @esoteric-ephemera (I don't have much experience overwriting an Enum so not sure if there's any pitfall like builtin dict/list/..., do review with a pinch of salt)?

Element.named_isotopes # ---> (Element.D, Element.T)

[DanielYang59]

@janosh I believe you're very experienced with Enum (while I'm not), perhaps you could help me double-check the implementation?