Support __len__

example/classes.zig

@@ -45,6 +45,11 @@ pub const Dog = py.subclass("Dog", &.{Animal}, struct {
         self.name.decref();
     }
 
+    pub fn __len__(self: *const Self) usize {
+        _ = self;
+        return 4;
+    }
+
     pub fn get_name(self: *const Self) !py.PyString {
         return self.name;
     }

pydust/src/builtins.zig

@@ -1,4 +1,14 @@
 const py = @import("./pydust.zig");
+const ffi = @import("./ffi.zig");
+const PyError = @import("./errors.zig").PyError;
+
+/// Get the length of the given object. Equivalent to len(obj) in Python.
+pub fn len(object: anytype) !usize {
+    const obj = try py.PyObject.from(object);
+    const length = ffi.PyObject_Length(obj.py);
+    if (length < 0) return PyError.Propagate;
+    return length;
+}
 
 /// Import a module by fully-qualified name returning a PyObject.
 pub fn import(module_name: [:0]const u8) !py.PyObject {

pydust/src/functions.zig

@@ -18,6 +18,7 @@ pub const Signature = struct {
 const reservedNames = .{
     "__new__",
     "__init__",
+    "__len__",
     "__del__",
 };
 

pydust/src/pytypes.zig

@@ -87,7 +87,14 @@ fn Slots(comptime name: [:0]const u8, comptime definition: type, comptime Instan
             if (@hasDecl(definition, "__del__")) {
                 slots_ = slots_ ++ .{ffi.PyType_Slot{
                     .slot = ffi.Py_tp_finalize,
-                    .pfunc = @ptrCast(@constCast(&finalize)),
+                    .pfunc = @ptrCast(@constCast(&tp_finalize)),
+                }};
+            }
+
+            if (@hasDecl(definition, "__len__")) {
+                slots_ = slots_ ++ .{ffi.PyType_Slot{
+                    .slot = ffi.Py_mp_length,
+                    .pfunc = @ptrCast(@constCast(&mp_length)),
                 }};
             }
 
@@ -101,11 +108,21 @@ fn Slots(comptime name: [:0]const u8, comptime definition: type, comptime Instan
             break :blk slots_;
         };
 
+        fn mp_length(pyself: *ffi.PyObject) callconv(.C) isize {
+            const lenFunc = @field(definition, "__len__");
+            const self: *const Instance = @ptrCast(pyself);
+            const result = @as(isize, @intCast(lenFunc(&self.state)));
+            if (@typeInfo(@typeInfo(@TypeOf(lenFunc)).Fn.return_type.?) == .ErrorUnion) {
+                return result catch return -1;
+            }
+            return result;
+        }
+
         /// Wrapper for the user's __del__ function.
         /// Note: tp_del is deprecated in favour of tp_finalize.
         ///
         /// See https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_finalize.
-        fn finalize(pyself: *ffi.PyObject) void {
+        fn tp_finalize(pyself: *ffi.PyObject) void {
             // The finalize slot shouldn't alter any exception that is currently set.
             // So it's recommended we save the existing one (if any) and restore it afterwards.
             // TODO(ngates): we may want to move this logic to PyErr if it happens more?

test/test_classes.py

@@ -21,3 +21,7 @@ def test_super():
     assert adopted.get_name() == "Dug"
     assert adopted.get_kind_name() == "Dog named Dug"
     assert adopted.get_kind() == 1
+
+
+def test_length():
+    assert len(classes.Dog("foo")) == 4