AiiDA extras

[giovannipizzi]

Currently all extras are accessible to users. If AiiDA has to store custom data, that we don't necessarily want the user to change without noticing, we don't currently have a namespacing scheme to allow this.
Two current occurrences of this are for instance the hidden flag of a code and the hash of a Node (currently stored in extras but the user might inadvertently replace everything).

Proposed solution

• namespace AiiDA-reserved keys in extras (suggestion: prefix starts with _aiida_
• possibly leave this functionality available also to plugins, reserving _aiida_core_ to AiiDA-core, and suggesting plugins to use _aiida_entrypoint_ for plugins (e.g. _aiida_quantumespresso_).
• migrate current flags under this namespace where appropriate
• have get_extras(also_private=False) where the new also_private flag, False by default, establishes if key/value pairs starting with _aiida_ should be shown or not. (I chose also_private instead of, e.g., show_private because then the same name could be used also in the set methods, see below)
• TBD: if get_extra() should have the flag as well or not (my idea: not)
• adapt also other extra methods (e.g. set_extra(s) would complain with an exception when setting a key starting with _aiida_ unless a flag is set, reset_extras by default resets only extras without prefix, unless a flag is set)
  • also, ensure that this is the only current method to change extras, otherwise adapt also the other ones
• decide if the querybuilder should hide these results (probably not, to be documented - the other option is to specify a flag in the append of a node to a QueryBuilder object, but anyway this would apply only to projecting all extras, all other cases shouldn't have a flag I believe)
• As a bonus, in the tests for extras we wouldn't need (as we do now) to ignore all extras starting with _aiida_ (that is currently done because the hashing mechasism would add "unexpected" extras).

List of known internal extras

• Code.hidden: this is being used in import/export logic
• Node._aiida_hash
• Node._aiida_cached_from

[giovannipizzi]

Todo: discuss also what happens on export

[szoupanos]

This is not urgent but there should be a discussion on what should happen when extras are exported (and what is the behaviour of private extras).
Issue #1761