Fixes RustPython#328, add object.__format__

Author: skinnyBat

tests/snippets/builtin_format.py

@@ -6,3 +6,12 @@
     pass
 else:
     assert False, "TypeError not raised when format is called with a number"
+
+assert format({}) == "{}"
+
+try:
+    format({}, 'b')
+except TypeError:
+    pass
+else:
+    assert False, "TypeError not raised when format_spec not empty for dict"

vm/src/obj/objobject.rs

@@ -84,6 +84,15 @@ fn object_repr(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     Ok(vm.new_str(format!("<{} object at 0x{:x}>", type_name, address)))
 }
 
+fn object_format(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(vm, args, required = [(obj, Some(vm.ctx.object())), (format_spec, Some(vm.ctx.str_type()))]);
+    if objstr::get_value(format_spec).is_empty() {
+        vm.to_str(obj)
+    } else {
+        Err(vm.new_type_error("unsupported format string passed to object.__format__".to_string()))
+    }
+}
+
 pub fn init(context: &PyContext) {
     let object = &context.object;
     let object_doc = "The most base type";
@@ -101,6 +110,7 @@ pub fn init(context: &PyContext) {
     context.set_attr(&object, "__hash__", context.new_rustfunc(object_hash));
     context.set_attr(&object, "__str__", context.new_rustfunc(object_str));
     context.set_attr(&object, "__repr__", context.new_rustfunc(object_repr));
+    context.set_attr(&object, "__format__", context.new_rustfunc(object_format));
     context.set_attr(
         &object,
         "__getattribute__",