Generator objects are defined using `PyObject_HEAD` rather than `PyObject_VAR_HEAD`

[jbower-fb]

Bug report

Bug description:

#29891 converted the generator objects to be variable in size so they could incorporate interpreter frames. However, the object layout is still defined using PyObject_HEAD. Technically I think this is incorrect. In practice probably nobody notices because the generator type is not a base class and there are other ways to work out the variable sized part where needed, so ob_size is never read after it is written as part of make_gen().

Long story short, I want to fix this because I would like to use ob_size to find the size of generator objects as part of work on Meta's JIT.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-126596: Use PyObject_VAR_HEAD for generator objects #126597

[jbower-fb]

cc @markshannon who authored #29891.

[markshannon]

The PyVarObject docs state that "This is only used for objects that have some notion of length".
Code objects have no notion of length.

What advantage would there be to changing to PyVarObject?
It wouldn't provide any new capabilities, but it would use an additional 8 bytes per generator.

[jbower-fb]

Primarily it would allow me to stuff JIT-related data at the end of the generator object and not have to a bunch of work each time to calculate where that starts. I do appreciate that may not be so as useful to anyone else though so I'll see if I can find another way to mitigate the cost.

I am still left wondering though, if these objects don't have a notion of length then why don't we set tp_itemsize to 0 in the generator types?

cc @DinoV @markshannon

[markshannon]

why don't we set tp_itemsize to 0 in the generator types?

Because tp_itemsize is to tell the allocation function(s) how big the object is. Everything has a size, not everything has a length.