CPython中_PyObject_GetAttrId()是什么？sysmodule.c代码相关疑问

Great question! Since _PyObject_GetAttrId() is an internal CPython function (not part of the public Python API), it won't show up in the official Python documentation—so it makes total sense you couldn't find docs for it. Let's break down what it does and why it's used:

Core Purpose #

It's a performance-optimized alternative to public attribute-getting functions like PyObject_GetAttrString() or Python's built-in getattr(). Its job is to retrieve an attribute from a Python object, but it uses a pre-cached identifier (instead of a raw string) to avoid redundant string processing.

For example, in the code you found:

stdin_encoding = _PyObject_GetAttrId(fin, &PyId_encoding);

&PyId_encoding refers to a pre-created Unicode object representing the string "encoding". This identifier is initialized once when CPython starts up, instead of creating a new string every time the attribute is accessed.

Why Use This Instead of Public APIs? #

• Speed: Reusing pre-cached identifiers cuts down on the overhead of creating Unicode objects and computing string hashes every time an attribute is looked up. This is especially useful in core CPython modules like sysmodule.c where performance matters.
• Consistency: It ensures that attribute names are referenced consistently across the codebase, avoiding typos that could happen with raw string literals.

Where to Find Its Implementation #

You can look at the source code in Objects/object.c (within the CPython repo) to see the full implementation. The function handles the attribute lookup logic, including checking the object's instance dict, type slots, and descriptor chain—much like the public attribute-getting functions, but with the identifier optimization.

Important Note #

Since this is an internal CPython API, it's not meant for use in third-party C extensions. It's not guaranteed to be stable across CPython versions; the function signature or behavior could change without warning. For your own extension code, stick to public APIs like PyObject_GetAttrString() or PyObject_GetAttr().

内容的提问来源于stack exchange，提问作者taro