prevent deletion of sales invoices if country is nepal

erpnext/hooks.py

@@ -201,7 +201,7 @@ doc_events = {
 	"Sales Invoice": {
 		'validate': 'erpnext.regional.india.utils.set_place_of_supply',
 		"on_submit": "erpnext.regional.france.utils.create_transaction_log",
-		"on_trash": "erpnext.regional.france.utils.check_deletion_permission"
+		"on_trash": ["erpnext.regional.france.utils.check_deletion_permission", "erpnext.regional.nepal.utils.check_deletion_permission"]
 	},
 	"Payment Entry": {
 		"on_submit": ["erpnext.regional.france.utils.create_transaction_log", "erpnext.accounts.doctype.payment_request.payment_request.make_status_as_paid"],

erpnext/regional/nepal/utils.py

@@ -0,0 +1,18 @@
+# Copyright (c) 2018, Frappe Technologies and contributors
+# For license information, please see license.txt
+
+import frappe
+from frappe import _
+from erpnext import get_region
+
+def check_deletion_permission(doc, method):
+	region = get_region()
+	if region not in ["Nepal"]:
+		return
+	else:
+		frappe.throw(_("Deletion is not permitted for country {0}".format(region)))
+
+# don't remove this function it is used in tests
+def test_method():
+	'''test function'''
+	return 'overridden'

erpnext/tests/test_regional.py

@@ -10,7 +10,10 @@ class TestInit(unittest.TestCase):
 		self.assertEqual(test_method(), 'overridden')
 
 		frappe.flags.country = 'Nepal'
-		self.assertEqual(test_method(), 'original')
+		self.assertEqual(test_method(), 'overridden')
 
 		frappe.flags.country = 'France'
 		self.assertEqual(test_method(), 'overridden')
+
+		frappe.flags.country = 'Maldives'
+		self.assertEqual(test_method(), 'original')