diff --git a/kernel/register.cc b/kernel/register.cc
index 9ffb17c1a97..1853e94d56b 100644
--- a/kernel/register.cc
+++ b/kernel/register.cc
@@ -108,9 +108,8 @@ Pass::Pass(std::string name, std::string short_help) : pass_name(name), short_he
 
 void Pass::run_register()
 {
-	if (pass_register.count(pass_name))
+	if (pass_register.count(pass_name) && !replace_existing_pass())
 		log_error("Unable to register pass '%s', pass already exists!\n", pass_name.c_str());
-
 	pass_register[pass_name] = this;
 }
 
@@ -447,13 +446,12 @@ Frontend::Frontend(std::string name, std::string short_help) :
 
 void Frontend::run_register()
 {
-	if (pass_register.count(pass_name))
+	if (pass_register.count(pass_name) && !replace_existing_pass())
 		log_error("Unable to register pass '%s', pass already exists!\n", pass_name.c_str());
 	pass_register[pass_name] = this;
 
-	if (frontend_register.count(frontend_name))
+	if (frontend_register.count(frontend_name) && !replace_existing_pass())
 		log_error("Unable to register frontend '%s', frontend already exists!\n", frontend_name.c_str());
-
 	frontend_register[frontend_name] = this;
 }
 
diff --git a/kernel/register.h b/kernel/register.h
index 15750af2a40..08ce4b28787 100644
--- a/kernel/register.h
+++ b/kernel/register.h
@@ -70,6 +70,7 @@ struct Pass
 
 	virtual void on_register();
 	virtual void on_shutdown();
+	virtual bool replace_existing_pass() const { return false; }
 };
 
 struct ScriptPass : Pass
diff --git a/misc/py_wrap_generator.py b/misc/py_wrap_generator.py
index 7fe78e03a58..a65d24d9218 100644
--- a/misc/py_wrap_generator.py
+++ b/misc/py_wrap_generator.py
@@ -1257,6 +1257,7 @@ def from_string(str_def, containing_file, class_, line_number, namespace):
 		func.is_static = False
 		func.is_inline = False
 		func.is_virtual = False
+		func.is_const = False
 		func.ret_attr_type = attr_types.default
 		func.is_operator = False
 		func.member_of = None
@@ -1334,6 +1335,11 @@ def from_string(str_def, containing_file, class_, line_number, namespace):
 		found = find_closing(str_def, "(", ")")
 		if found == -1:
 			return None
+
+		post_qualifiers = str_def[found + 1:].lstrip().replace("{", " {") + " "
+		if post_qualifiers.startswith("const "):
+			func.is_const = True
+
 		str_def = str_def[0:found]
 		if func.name in blacklist_methods:
 			return None
@@ -1379,6 +1385,12 @@ def mangled_name(self):
 	def gen_alias(self):
 		self.alias = self.mangled_name
 
+	def gen_post_qualifiers(self, derived=False):
+		if self.member_of != None and self.member_of.link_type == link_types.derive and self.is_virtual and derived:
+			# we drop the qualifiers when deriving callbacks to be implemented in Python
+			return ''
+		return ' const' if self.is_const else ''
+
 	def gen_decl(self):
 		if self.duplicate:
 			return ""
@@ -1392,7 +1404,7 @@ def gen_decl(self):
 			text += ", "
 		if len(self.args) > 0:
 			text = text[:-2]
-		text += ");\n"
+		text += f"){self.gen_post_qualifiers()};\n"
 		return text
 
 	def gen_decl_virtual(self):
@@ -1411,12 +1423,18 @@ def gen_decl_virtual(self):
 		if len(self.args) > 0:
 			text = text[:-2]
 		text += ")"
-		if len(self.args) == 0:
+		if len(self.args) == 0 and self.ret_type.name == "void":
 			text += "{}"
 		else:
 			text += "\n\t\t{"
 			for arg in self.args:
 				text += "\n\t\t\t(void)" + arg.gen_varname() + ";"
+			if self.ret_type.name == "void":
+				pass
+			elif self.ret_type.name == "bool":
+				text += "\n\t\t\treturn false;"
+			else:
+				raise NotImplementedError(self.ret_type.name)
 			text += "\n\t\t}\n"
 		text += "\n\t\tvirtual "
 		if self.is_static:
@@ -1427,7 +1445,7 @@ def gen_decl_virtual(self):
 			text += ", "
 		if len(self.args) > 0:
 			text = text[:-2]
-		text += ") override;\n"
+		text += f"){self.gen_post_qualifiers()} override;\n"
 		return text
 
 	def gen_decl_hash_py(self):
@@ -1452,7 +1470,7 @@ def gen_def(self):
 			text += ", "
 		if len(self.args) > 0:
 			text = text[:-2]
-		text +=")\n\t{"
+		text += f"){self.gen_post_qualifiers()}\n\t{{"
 		for arg in self.args:
 			text += arg.gen_translation()
 		text += "\n\t\t"
@@ -1507,16 +1525,17 @@ def gen_def_virtual(self):
 			text += ", "
 		if len(self.args) > 0:
 			text = text[:-2]
-		text += ")\n\t{"
+		text += f"){self.gen_post_qualifiers()}\n\t{{"
 		for arg in self.args:
 			text += arg.gen_translation_cpp()
-		text += "\n\t\t"
+		return_stmt = "return " if self.ret_type.name != "void" else ""
+		text += f"\n\t\t{return_stmt}"
 		if self.member_of == None:
 			text += "::" + self.namespace + "::" + self.alias + "("
 		elif self.is_static:
 			text += self.member_of.namespace + "::" + self.member_of.name + "::" + self.name + "("
 		else:
-			text += "py_" + self.alias + "("
+			text += f"const_cast<{self.member_of.name}*>(this)->py_" + self.alias + "("
 		for arg in self.args:
 			text += arg.gen_call_cpp() + ", "
 		if len(self.args) > 0:
@@ -1547,11 +1566,13 @@ def gen_default_impl(self):
 			call_string = call_string[0:-2]
 		call_string += ");"
 
+		return_stmt = "return " if self.ret_type.name != "void" else ""
+
 		text += ")\n\t\t{"
-		text += "\n\t\t\tif(boost::python::override py_" + self.alias + " = this->get_override(\"py_" + self.alias + "\"))"
-		text += "\n\t\t\t\t" + call_string
+		text += "\n\t\t\tif (boost::python::override py_" + self.alias + " = this->get_override(\"py_" + self.alias + "\"))"
+		text += f"\n\t\t\t\t{return_stmt}" + call_string
 		text += "\n\t\t\telse"
-		text += "\n\t\t\t\t" + self.member_of.name + "::" + call_string
+		text += f"\n\t\t\t\t{return_stmt}" + self.member_of.name + "::" + call_string
 		text += "\n\t\t}"
 
 		text += "\n\n\t\t" + self.ret_type.gen_text() + " default_py_" + self.alias + "("
@@ -1559,8 +1580,8 @@ def gen_default_impl(self):
 			text += arg.gen_listitem() + ", "
 		if len(self.args) > 0:
 			text = text[:-2]
-		text += ")\n\t\t{"
-		text += "\n\t\t\tthis->" + self.member_of.name + "::" + call_string
+		text += f")\n\t\t{{"
+		text += f"\n\t\t\t{return_stmt}this->" + self.member_of.name + "::" + call_string
 		text += "\n\t\t}"
 		return text
 
@@ -1584,9 +1605,9 @@ def gen_boost_py(self):
 			for a in self.args:
 				text += a.gen_listitem_hash() + ", "
 			if len(self.args) > 0:
-				text = text[0:-2] + ")>"
+				text = text[0:-2] + f"){self.gen_post_qualifiers(True)}>"
 			else:
-				text += "void)>"
+				text += f"void){self.gen_post_qualifiers(True)}>"
 
 		if self.is_operator:
 			text += "(\"" + wrappable_operators[self.name.replace("operator","")] + "\""
diff --git a/passes/cmds/plugin.cc b/passes/cmds/plugin.cc
index 08b4aa8c4ee..4ad7c165b1e 100644
--- a/passes/cmds/plugin.cc
+++ b/passes/cmds/plugin.cc
@@ -103,7 +103,6 @@ void load_plugin(std::string filename, std::vector<std::string> aliases)
 
 			loaded_plugins[orig_filename] = hdl;
 			Pass::init_register();
-
 		}
 	}