# Access Attributes

# Direct access

Some python objects have an instance dict, a.k.a, __dict__ in cpython. You can use obj->attr() to manipulate the instance dict of an object.

VM* vm = new VM();

// get the `builtin` module
PyObject* builtins = vm->builtins;
// get `dict` type
PyObject* dict = builtins->attr("dict");
// set `pi = 3.14`
builtins->attr().set("pi", py_var(vm, 3.14));

However, you cannot call attr on an object which does not have an instance dict. For example, the int object.

// create a `int` object
PyObject* obj = py_var(vm, 1);
PyObject* add = obj->attr("__add__");

To determine whether an object has instance dict or not, you can use this snippet.

// 1. call `is_tagged` to check the object supports `->` operator
// 2. call `is_attr_valid` to check the existence of instance dict
PyObject* obj = py_var(vm, 1);
bool ok = !is_tagged(obj) && obj->is_attr_valid();  // false

# General access

As you can see, direct access does not take care of derived attributes or methods. In most cases, what you need is getattr and setattr. These two methods handle all possible cases.

# PyObject* getattr(PyObject* obj, StrName name, bool throw_err=true)

This method is equivalent to getattr in python. If the attribute is not found, it will return nullptr or throw an AttributeError depending on the value of throw_err.

// create a `int` object
PyObject* obj = py_var(vm, 1);

// get its `__add__` method, which is a `bound_method` object
PyObject* add = vm->getattr(obj, "__add__");

// call it (equivalent to `1 + 2`)
PyObject* ret = vm->call(add, py_var(vm, 2););

// get the result
int result = py_cast<int>(vm, ret);
std::cout << result << std::endl; // 3

# void setattr(PyObject*, StrName, PyObject*)

This method is equivalent to setattr in python. It raises TypeError if the object does not support attribute assignment.