O04: the order things are found in

[tamnd]

The fourth lesson of the object model part, and the one that finishes what O03 started. O03 explained which dictionary a name gets found in. This one explains the order those dictionaries get looked at, which is the only interesting question once a class has more than one base.

The answer is a list, computed once when the class is made and stored on the type. It is not a search, and that is the thing most people are surprised by.

What is in it:

• the diamond, and the fact that D().where() gives you C rather than A, straight out of the flat __mro__ tuple
• swapping the declared bases gives a different list, so the bases tuple is an input to the computation rather than a record of what you typed
• the three constraints the list satisfies, checked on classes from the notebook and from the standard library
• the C3 merge written out in twenty lines of Python, with an assertion that it reproduces __mro__ exactly for eleven classes including bool, io.StringIO, collections.OrderedDict and type
• the single base fast path, which skips the merge entirely
• the two ways a class statement fails: contradictory orders, where the error names the classes the merge got stuck between rather than the bases you wrote, and a repeated base, which is caught earlier by a different check with a different message
• super reading the MRO of the instance rather than of the class the call was written in, so the same unchanged method in Left returns Base for one instance and dispatches to Right for another
• and the two ways the list changes afterwards: assigning __bases__, which rebuilds it for the class and every subclass while existing instances pick up the new method at once, and a metaclass with its own mro(), which can hand back an order C3 would never produce

Thirteen citations against v3.15.0rc1: pmerge, tail_contains, check_duplicates, set_mro_error, mro_implementation_unlocked and its fast path, mro_invoke, mro_hierarchy_for_complete_type, type_mro_modified, _PySuper_LookupDescr and find_name_in_mro.

Six diagrams. Every cell prints the same thing on 3.14, on 3.15.0rc1 and in the Pyodide browser probe, so this lesson needed no version notes beyond the standard banner.

Two glossary terms: MRO and C3 linearization. GLOSSARY.md is now 115.

Part of #23.