diff --git a/vm/src/stdlib/warnings.rs b/vm/src/stdlib/warnings.rs
index cb97ead48..da3c0339c 100644
--- a/vm/src/stdlib/warnings.rs
+++ b/vm/src/stdlib/warnings.rs
@@ -1,6 +1,7 @@
 use crate::function::OptionalArg;
 use crate::obj::objstr::PyStringRef;
-use crate::pyobject::PyObjectRef;
+use crate::obj::objtype::{self, PyClassRef};
+use crate::pyobject::{PyObjectRef, PyResult, TypeProtocol};
 use crate::vm::VirtualMachine;
 
 #[derive(FromArgs)]
@@ -8,23 +9,27 @@ struct WarnArgs {
     #[pyarg(positional_only, optional = false)]
     message: PyStringRef,
     #[pyarg(positional_or_keyword, optional = true)]
-    category: OptionalArg<PyObjectRef>,
+    category: OptionalArg<PyClassRef>,
     #[pyarg(positional_or_keyword, optional = true)]
     stacklevel: OptionalArg<u32>,
 }
 
-fn warnings_warn(args: WarnArgs, _vm: &VirtualMachine) {
+fn warnings_warn(args: WarnArgs, vm: &VirtualMachine) -> PyResult<()> {
     // TODO: Implement correctly
-    let level = match args.stacklevel {
-        OptionalArg::Present(l) => l,
-        OptionalArg::Missing => 1,
+    let level = args.stacklevel.unwrap_or(1);
+    let category = if let OptionalArg::Present(category) = args.category {
+        if !objtype::issubclass(&category, &vm.ctx.exceptions.warning) {
+            return Err(vm.new_type_error(format!(
+                "category must be a Warning subclass, not '{}'",
+                category.class().name
+            )));
+        }
+        category
+    } else {
+        vm.ctx.exceptions.user_warning.clone()
     };
-    eprintln!(
-        "Warning: {} , category: {:?}, level: {}",
-        args.message.as_str(),
-        args.category,
-        level
-    )
+    eprintln!("level:{}: {}: {}", level, category.name, args.message);
+    Ok(())
 }
 
 pub fn make_module(vm: &VirtualMachine) -> PyObjectRef {