# Interop with PyVar

In pkpy, any python object is represented by a PyVar.

# Create PyVar from C type

A set of overloaded function PyVar py_var(VM* vm, ...) were implemented to create a PyVar from a supported C type.

Assume we have a VM* vm instance. You can create a python int object from a C i64 type:

i64 i = 2;
PyVar obj = py_var(vm, i);

Each python type has a corresponding C type, for example, int in python is i64 in C. python's list corresponds to List, str corresponds to Str, etc. For strings, we have defined a set of overloaded version including const char*, std::string, std::string_view, Str, etc.

PyVar obj = py_var(vm, "abc");		// create a python str object

A more complex example is to create a python list. In the following code, we create a list equals to [0, 1, 2, 3].

List list;
for (i64 i = 0; i < 4; i++) {
    list.push_back(py_var(vm, i));

obj = py_var(vm, std::move(list));		// create a python list object

Please note that std::move is used here to avoid unnecessary copy. Most types have both a rvalue and a lvalue version of py_var function.

# Access internal C type of PyVar

A set of template function T py_cast<T>(VM* vm, PyVar obj) were implemented.

i64 i = 2;
PyVar obj = py_var(vm, i);

// cast a PyVar to C i64
i64 j = py_cast<i64>(vm, obj);

The py_cast function will check the type of obj before casting. If the type is not matched, a TypeError will be thrown.

However, this type check has a cost. If you are sure about the type of obj, you can use the underscore version _py_cast to skip the type check.

// cast a PyVar to C i64 (unsafe but faster)
i64 j = _py_cast<i64>(vm, obj);		

For complex objects like list, we can use reference cast to avoid unnecessary copy.

PyVar obj = py_var(vm, List());
// reference cast (no copy)
List& list = py_cast<List&>(vm, obj);

# Check type of PyVar

Each PyVar has a Type type field to indicate its type. Type is just an integer which is the global index in vm->_all_types.

VM class has a set of predefined Type constants for quick access. They are prefixed by tp_. For example, tp_object(object), tp_int(int), tp_str(str), tp_list(list), etc.

Types are divided into tagged type and non-tagged type.

  • small int objects are tagged.
  • Other types are non-tagged type.

To determine whether a PyVar is of a specific type, you can use the following functions:

  • bool is_type(PyVar obj, Type type)
  • bool is_int(PyVar obj)
  • bool is_float(PyVar obj)
  • bool is_tagged(PyVar obj)
PyVar obj = py_var(vm, 1);

bool ok = is_type(obj, vm->tp_int);		// true
ok = is_int(obj);						// true
ok = is_tagged(obj);					// true

ok = is_type(obj, vm->tp_float);		// false
ok = is_float(obj);						// false

Simply put, is_type is the most general function and can check any types. Other variants are designed for specific types and are faster.

You can also use check_ prefix functions assert the type of a PyVar, which will throw TypeError on failure.

  • void check_type(PyVar obj, Type type)