Static Variables in Python C API

Question

How would one expose "static" variables like this

class MyClass:
    X = 1
    Y = 2

via the C API? The only variable on the PyTypeObject that looks like it would work is tp_members, but I see no flag in the PyMemberDef to indicate that the member should be per-class, not per-instance.

For a bit more clarification, since it may change the answer, I'm trying to expose a C enum to Python such that the enumeration

enum MyFlags {
    Alpha = 0,
    Beta = 1
};

Can be accessed in Python as:

module.MyFlags.Alpha
module.MyFlags.Beta

I only found this SO question after I had figured out the answer myself. Prior to that I searched for things like "cpython class constants". I wonder if we could add more words to this question that could help other people find this. — user1411349
– user1411349, Commented Dec 1, 2021 at 5:57

user2357112 · Accepted Answer · 2020-07-19 21:40:38Z

12

Just put them in the type's tp_dict e.g. with PyDict_SetItemString.

edited Jul 19, 2020 at 21:40

user2357112

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answered Mar 4, 2010 at 0:19

Alex Martelli

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4 Comments

Toji Over a year ago

Excellent! That's what I was looking for. (You'll excuse me if I take a moment to test it out before marking this as answered.)

Toji Over a year ago

Works as advertised. :) Thanks again.

user2357112 Over a year ago

Since the time this answer was originally posted, the following warning has been added to the docs: "Warning: It is not safe to use PyDict_SetItem() on or otherwise modify tp_dict with the dictionary C-API."

user2357112 Over a year ago

I think that warning is a bit broader than it should be - it should be okay to set entries in tp_dict as long as the names don't have any corresponding slots (so no setting __add__ or __new__ or stuff like that) and you call PyType_Modified on the type afterward.

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