Python中__dir__方法的具体实现及查询方式

Awesome question! You're right that the _dir_object function just delegates to the object's __dir__ method—so let's dig into how to find the actual implementation of __dir__ for different types of objects in CPython.

How to Find __dir__ Implementations in CPython #

The implementation of __dir__ varies depending on what kind of object you're dealing with. Here's how to track it down:

1. Built-in Types (like list, dict, str)

For core Python types, their __dir__ methods are implemented in C. Follow these steps:

• Navigate to the Objects/ directory in the CPython repo—each built-in type has its own dedicated file (e.g., listobject.c for lists, dictobject.c for dicts).
• Locate the PyTypeObject struct for the target type (e.g., PyList_Type for lists). Look for the tp_dir field—this points directly to the C function that implements __dir__ for that type.
• Search for the function name linked to tp_dir to view the full implementation code.

For example:

• For list, PyList_Type.tp_dir points to list_dir. This function collects standard list methods, plus any extra attributes stored in the list instance's __dict__ (common if you've subclassed list and added custom attributes).
• If a type doesn't define a custom tp_dir, it falls back to the generic PyObject_Dir function (in object.c), which handles gathering attributes from the object's class hierarchy and its instance __dict__.

2. Custom Python Classes

• If you've explicitly defined a __dir__ method in your Python class, the implementation is right there in your code—no digging needed!
• If you haven't overridden __dir__, your class inherits it from its parent class. For most classes, this eventually traces back to the base object class's __dir__, implemented in the object_dir function in object.c. This function:
  • Collects attributes from the object's instance __dict__
  • Recursively gathers attributes from the class and all its base classes
  • Deduplicates and sorts the combined list before returning it

3. Extension Types (C-written third-party libraries)

For types from C-based third-party extensions, you'll need to inspect the library's source code. Find the PyTypeObject struct for the target type, check its tp_dir field, then look up the corresponding function implementation in the library's codebase.

Quick Deep Dive: object.__dir__ Logic #

Since most default __dir__ implementations rely on object_dir, here's a simplified breakdown of its core logic:

The object_dir function first fetches the object's class, then walks up the inheritance chain to collect all class-level attributes. It then adds any keys from the object's __dict__ (if the object supports instance dictionaries). Finally, it sorts and deduplicates the combined list to produce the final set of names you see when calling dir().

内容的提问来源于stack exchange，提问作者laike9m