O06: what property actually is

[tamnd]

O06, the sixth lesson of the object model part. O05 said a data descriptor beats the instance dict and left it at that. This one says what a descriptor is.

The angle is that four things people learn as four separate features are one protocol, and three of them are under ten lines of C. staticmethod is six lines that hand the callable back. classmethod is eight that call the same PyMethod_New a plain function does, with the class instead of the instance. func_descr_get is seven, and it is where self comes from. property is the long one at about thirty, and most of that is error messages.

Thirteen code cells, all of them run identically on 3.14, on 3.15.0rc1 and on Pyodide 3.14.2, so the only version note in the lesson is the banner.

What it covers:

• a descriptor written by hand, so you can watch __get__, __set__ and __set_name__ get called
• PyDescr_IsData, the one line that asks whether tp_descr_set is non null, and the whole precedence rule falling out of it
• functions as non data descriptors, raw.__get__(g, Greeter) doing by hand what the dot does, and a fresh bound method on every read
• classmethod binding to the class the lookup started from, which is why Sub.cm() gets Sub
• a property with a value sitting in the instance dict being ignored, and the two error messages that name which of the three functions is missing
• the five descriptor types C code builds, which two of them are data descriptors, and the type check descr_setcheck does before writing at a fixed offset
• a descriptor in an instance dict doing nothing, and the branch in _PyObject_GenericGetAttrWithDict that explains why
• type_new_set_names walking a copy of the class dict, and the descriptor added after the class statement that never learns its name

Sixteen citations, all resolving against the pin. Six diagrams, all under 950 px. Two new glossary terms, descriptor and bound method, which brings GLOSSARY.md to 118.

Checked with the full just check and with just versions.