diff --git a/source/python.js b/source/python.js
index b1e9d65df7..e2e4f4958a 100644
--- a/source/python.js
+++ b/source/python.js
@@ -756,7 +756,24 @@ python.Execution = class {
                     if (decorator_list) {
                         decorator_list = Array.from(decorator_list);
                     }
-                    const bases = this._tokenizer.peek().type === '(' ? this._arguments() : [];
+                    const bases = [];
+                    if (this._tokenizer.eat('(')) {
+                        while (!this._tokenizer.eat(')')) {
+                            if (this._tokenizer.eat('\n')) {
+                                continue;
+                            }
+                            const expression = this._expression(-1, [], false);
+                            if (expression === null) {
+                                throw new python.Error(`Expected expression ${this._tokenizer.location()}`);
+                            }
+                            bases.push(expression);
+                            if (!this._tokenizer.eat(',')) {
+                                this._tokenizer.eat('\n');
+                                this._tokenizer.expect(')');
+                                break;
+                            }
+                        }
+                    }
                     this._tokenizer.expect(':');
                     const body = this._suite();
                     const node = new ast.ClassDef(name.id, bases, null, body, decorator_list, null);
@@ -776,7 +793,7 @@ python.Execution = class {
                         decorator_list = Array.from(decorator_list);
                     }
                     this._tokenizer.expect('(');
-                    const args = this._parameters(')');
+                    const args = this._arguments(')');
                     let returns = null;
                     if (this._tokenizer.eat('->')) {
                         returns = this._type();
@@ -1153,7 +1170,7 @@ python.Execution = class {
                     }
                     position = this._eat('id', 'lambda');
                     if (position) {
-                        const args = this._parameters(':');
+                        const args = this._arguments(':');
                         const body = this._expression(-1, terminal, false);
                         const node = new ast.Lambda(args, body);
                         this._mark(node, position);
@@ -1195,24 +1212,37 @@ python.Execution = class {
                         continue;
                     }
                     if (this._tokenizer.peek().type === '(') {
+                        const position = this._position();
+                        const args = [];
+                        this._tokenizer.expect('(');
+                        while (!this._tokenizer.eat(')')) {
+                            if (this._tokenizer.eat('\n')) {
+                                continue;
+                            }
+                            const expression = this._expression(-1, [], false);
+                            if (expression === null) {
+                                throw new python.Error(`Expected expression ${this._tokenizer.location()}`);
+                            }
+                            args.push(expression);
+                            if (!this._tokenizer.eat(',')) {
+                                this._tokenizer.eat('\n');
+                                this._tokenizer.expect(')');
+                                break;
+                            }
+                        }
                         if (stack.length === 0) {
-                            const position = this._position();
-                            const args = this._arguments();
                             if (args.length === 1) {
-                                stack.push(args[0]);
+                                [node] = args;
                             } else {
                                 node = new ast.Tuple(args);
-                                node = this._mark(node, position);
-                                stack.push(node);
+                                this._mark(node, position);
                             }
                         } else {
-                            const location = this._tokenizer.location();
                             const func = stack.pop();
-                            const args = this._arguments();
                             node = new ast.Call(func, args);
-                            node.location = location;
-                            stack.push(node);
+                            this._mark(node, position);
                         }
+                        stack.push(node);
                         continue;
                     }
                     if (this._tokenizer.peek().type === '[') {
@@ -1483,36 +1513,85 @@ python.Execution = class {
                 }
                 return null;
             }
-            _parameters(terminal) {
-                const list = [];
+            _arguments(terminal) {
+                let posonlyargs = [];
+                let args = [];
+                let vararg = null;
+                const kwonlyargs = [];
+                const kw_defaults = [];
+                let kwarg = null;
+                const defaults = [];
+                let is_slash = false;
+                let is_vararg = false; // '*'
+                let is_kwarg = false; // '**'
                 while (!this._tokenizer.eat(terminal)) {
                     this._tokenizer.eat('\n');
-                    if (this._tokenizer.eat('(')) {
-                        list.push(this._parameters(')'));
-                    } else {
-                        const node = this._node('parameter');
-                        if (this._tokenizer.eat('/')) {
-                            node.name = '/';
-                        } else {
-                            if (this._tokenizer.eat('**')) {
-                                node.annotation = '**';
-                            }
-                            if (this._tokenizer.eat('*')) {
-                                node.annotation = '*';
-                            }
-                            const name = this._name();
-                            if (name === null) {
-                                throw new python.Error(`Expected parameter ${this._tokenizer.location()}`);
-                            }
-                            node.name = name.id;
-                            if (terminal !== ':' && this._tokenizer.eat(':')) {
-                                node.annotation = this._type();
+                    if (this._tokenizer.eat('/')) {
+                        if (is_slash || is_vararg || is_kwarg) {
+                            throw new python.Error(`Invalid '/' in arguments ${this._tokenizer.location()}`);
+                        }
+                        is_slash = true;
+                        if (!this._tokenizer.eat(',')) {
+                            this._tokenizer.expect(terminal);
+                            break;
+                        }
+                        continue;
+                    }
+                    if (this._tokenizer.eat('**')) {
+                        if (is_kwarg) {
+                            throw new python.Error(`Multiple '**' arguments ${this._tokenizer.location()}`);
+                        }
+                        is_kwarg = true;
+                        const name = this._name(true);
+                        const annotation = terminal !== ':' && this._tokenizer.eat(':') ? this._type() : null;
+                        kwarg = new ast.arg(name.id, annotation, null);
+                        if (!this._tokenizer.eat(',')) {
+                            this._tokenizer.expect(terminal);
+                            break;
+                        }
+                        continue;
+                    }
+                    if (this._tokenizer.eat('*')) {
+                        if (is_vararg) {
+                            throw new python.Error(`Multiple '*' arguments ${this._tokenizer.location()}`);
+                        }
+                        is_vararg = true;
+                        const name = this._name(false);
+                        if (name) {
+                            const annotation = terminal !== ':' && this._tokenizer.eat(':') ? this._type() : null;
+                            vararg = new ast.arg(name.id, annotation, null);
+                        }
+                        if (!this._tokenizer.eat(',')) {
+                            this._tokenizer.expect(terminal);
+                            break;
+                        }
+                        continue;
+                    }
+                    const name = this._name();
+                    if (!name) {
+                        this._tokenizer.expect(terminal);
+                        break;
+                    }
+                    const annotation = terminal !== ':' && this._tokenizer.eat(':') ? this._type() : null;
+                    const arg = new ast.arg(name.id, annotation, null);
+                    const default_value = this._tokenizer.eat('=') ? this._expression() : null;
+                    if (!is_vararg && !is_kwarg) {
+                        if (is_slash) {
+                            args.push(arg);
+                            if (default_value !== null) {
+                                defaults.push(default_value);
                             }
-                            if (this._tokenizer.eat('=')) {
-                                node.initializer = this._expression();
+                        } else {
+                            posonlyargs.push(arg);
+                            if (default_value !== null) {
+                                defaults.push(default_value);
                             }
                         }
-                        list.push(node);
+                    } else if (is_vararg && !is_kwarg) {
+                        kwonlyargs.push(arg);
+                        kw_defaults.push(default_value);
+                    } else {
+                        throw new python.Error(`Argument after '**' parameter ${this._tokenizer.location()}`);
                     }
                     this._tokenizer.eat('\n');
                     if (!this._tokenizer.eat(',')) {
@@ -1520,27 +1599,11 @@ python.Execution = class {
                         break;
                     }
                 }
-                return list;
-            }
-            _arguments() {
-                const list = [];
-                this._tokenizer.expect('(');
-                while (!this._tokenizer.eat(')')) {
-                    if (this._tokenizer.eat('\n')) {
-                        continue;
-                    }
-                    const expression = this._expression(-1, [], false);
-                    if (expression === null) {
-                        throw new python.Error(`Expected expression ${this._tokenizer.location()}`);
-                    }
-                    list.push(expression);
-                    if (!this._tokenizer.eat(',')) {
-                        this._tokenizer.eat('\n');
-                        this._tokenizer.expect(')');
-                        break;
-                    }
+                if (!is_slash) {
+                    args = posonlyargs.concat(args);
+                    posonlyargs = [];
                 }
-                return list;
+                return new ast.arguments(posonlyargs, args, vararg, kwonlyargs, kw_defaults, kwarg, defaults);
             }
             _node(type) {
                 const node = {};
@@ -8009,7 +8072,7 @@ python.Execution = class {
             }
             parseArgsFromDecl(decl, skip_self) {
                 const retval = [];
-                const params = skip_self ? decl.args.slice(1) : decl.args.slice();
+                const params = skip_self ? decl.args.args.slice(1) : decl.args.args.slice();
                 for (const decl_arg of params) {
                     const N = null;
                     const default_value = null;
@@ -9453,9 +9516,9 @@ python.Execution = class {
                         args.push(this.data); // self
                     }
                     if (this.data.forward.__code__ && this.data.forward.__code__.args) {
-                        for (const arg of this.data.forward.__code__.args) {
-                            if (execution.traceAttr || arg.name !== 'self') {
-                                const value = execution.graph.addInput(arg.name);
+                        for (const arg of this.data.forward.__code__.args.args) {
+                            if (execution.traceAttr || arg.arg !== 'self') {
+                                const value = execution.graph.addInput(arg.arg);
                                 value.setType(execution.type(arg.annotation));
                                 if (isTensor(value)) {
                                     value.__variable__ = arg.name;
@@ -11932,12 +11995,19 @@ python.Execution = class {
     apply(method, args, context) {
         const locals = Array.prototype.slice.call(args);
         context = new python.Execution.Context(context.globals, {});
-        for (const argument of method.args) {
-            let value = locals.shift();
-            if (value === undefined && argument.initializer) {
-                value = this.expression(argument.initializer, context);
+        args = method.args.posonlyargs.concat(method.args.args);
+        const default_pos = args.length - method.args.defaults.length;
+        for (let i = 0; i < method.args.args.length; i++) {
+            const arg = method.args.args[i];
+            let value = null;
+            if (locals.length > 0) {
+                value = locals.shift();
+            } else if (i >= default_pos) {
+                value = this.expression(method.args.defaults[i - default_pos], context);
+            } else {
+                throw new python.Error('Missing required positional argument.');
             }
-            context.set(argument.name, value);
+            context.set(arg.arg, value);
         }
         return this.block(method.body, context);
     }