diff --git a/.github/helper/.flake8_strict b/.github/helper/.flake8_strict
index 4c7f5f8..c8337a9 100644
--- a/.github/helper/.flake8_strict
+++ b/.github/helper/.flake8_strict
@@ -65,6 +65,11 @@
     E713,
     E712,
 
+enable-extensions =
+    M90
+
+select =
+    M511
 
 max-line-length = 200
 exclude=.github/helper/semgrep_rules,test_*.py
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 2b3a471..e411f11 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -20,7 +20,10 @@
     rev: 3.9.2
     hooks:
       - id: flake8
-        args: ['--config', '.github/helper/.flake8_strict']
+        additional_dependencies: [
+          'flake8-mutable',
+        ]
+        args: ['--select=M511', '--config', '.github/helper/.flake8_strict']
         exclude: ".*setup.py$"
 
   - repo: https://github.com/timothycrosley/isort
diff --git a/erpnext/accounts/doctype/pos_profile/test_pos_profile.py b/erpnext/accounts/doctype/pos_profile/test_pos_profile.py
index 83ecfb4..7c53f4a 100644
--- a/erpnext/accounts/doctype/pos_profile/test_pos_profile.py
+++ b/erpnext/accounts/doctype/pos_profile/test_pos_profile.py
@@ -33,7 +33,9 @@
 
 		frappe.db.sql("delete from `tabPOS Profile`")
 
-def get_customers_list(pos_profile={}):
+def get_customers_list(pos_profile=None):
+	if pos_profile is None:
+		pos_profile = {}
 	cond = "1=1"
 	customer_groups = []
 	if pos_profile.get('customer_groups'):
diff --git a/erpnext/accounts/doctype/pricing_rule/utils.py b/erpnext/accounts/doctype/pricing_rule/utils.py
index 12b486e..0637fda 100644
--- a/erpnext/accounts/doctype/pricing_rule/utils.py
+++ b/erpnext/accounts/doctype/pricing_rule/utils.py
@@ -398,7 +398,9 @@
 				pricing_rules[0].apply_rule_on_other_items = items
 				return pricing_rules
 
-def get_qty_amount_data_for_cumulative(pr_doc, doc, items=[]):
+def get_qty_amount_data_for_cumulative(pr_doc, doc, items=None):
+	if items is None:
+		items = []
 	sum_qty, sum_amt = [0, 0]
 	doctype = doc.get('parenttype') or doc.doctype
 
diff --git a/erpnext/accounts/doctype/promotional_scheme/promotional_scheme.py b/erpnext/accounts/doctype/promotional_scheme/promotional_scheme.py
index d09f7dc..f5391ca 100644
--- a/erpnext/accounts/doctype/promotional_scheme/promotional_scheme.py
+++ b/erpnext/accounts/doctype/promotional_scheme/promotional_scheme.py
@@ -69,7 +69,9 @@
 			{'promotional_scheme': self.name}):
 			frappe.delete_doc('Pricing Rule', rule.name)
 
-def get_pricing_rules(doc, rules = {}):
+def get_pricing_rules(doc, rules=None):
+	if rules is None:
+		rules = {}
 	new_doc = []
 	for child_doc, fields in {'price_discount_slabs': price_discount_fields,
 		'product_discount_slabs': product_discount_fields}.items():
@@ -78,7 +80,9 @@
 
 	return new_doc
 
-def _get_pricing_rules(doc, child_doc, discount_fields, rules = {}):
+def _get_pricing_rules(doc, child_doc, discount_fields, rules=None):
+	if rules is None:
+		rules = {}
 	new_doc = []
 	args = get_args_for_pricing_rule(doc)
 	applicable_for = frappe.scrub(doc.get('applicable_for'))
diff --git a/erpnext/accounts/report/cash_flow/cash_flow.py b/erpnext/accounts/report/cash_flow/cash_flow.py
index d527188..bb8138b 100644
--- a/erpnext/accounts/report/cash_flow/cash_flow.py
+++ b/erpnext/accounts/report/cash_flow/cash_flow.py
@@ -139,9 +139,9 @@
 	data["total"] = total
 	return data
 
-def get_account_type_based_gl_data(company, start_date, end_date, account_type, filters={}):
+def get_account_type_based_gl_data(company, start_date, end_date, account_type, filters=None):
 	cond = ""
-	filters = frappe._dict(filters)
+	filters = frappe._dict(filters or {})
 
 	if filters.include_default_book_entries:
 		company_fb = frappe.db.get_value("Company", company, 'default_finance_book')
diff --git a/erpnext/education/doctype/student/student.py b/erpnext/education/doctype/student/student.py
index ae498ba..be4ee56 100644
--- a/erpnext/education/doctype/student/student.py
+++ b/erpnext/education/doctype/student/student.py
@@ -138,7 +138,9 @@
 			enrollment.submit()
 			return enrollment
 
-	def enroll_in_course(self, course_name, program_enrollment, enrollment_date=frappe.utils.datetime.datetime.now()):
+	def enroll_in_course(self, course_name, program_enrollment, enrollment_date=None):
+		if enrollment_date is None:
+			enrollment_date = frappe.utils.datetime.datetime.now()
 		try:
 			enrollment = frappe.get_doc({
 					"doctype": "Course Enrollment",
diff --git a/erpnext/hr/doctype/holiday_list/holiday_list.py b/erpnext/hr/doctype/holiday_list/holiday_list.py
index f46f14d..7d1b991 100644
--- a/erpnext/hr/doctype/holiday_list/holiday_list.py
+++ b/erpnext/hr/doctype/holiday_list/holiday_list.py
@@ -1,4 +1,3 @@
-
 # Copyright (c) 2015, Frappe Technologies Pvt. Ltd. and Contributors
 # License: GNU General Public License v3. See license.txt
 
@@ -94,9 +93,11 @@
 		update={"allDay": 1})
 
 
-def is_holiday(holiday_list, date=today()):
+def is_holiday(holiday_list, date=None):
 	"""Returns true if the given date is a holiday in the given holiday list
 	"""
+	if date is None:
+		date = today()
 	if holiday_list:
 		return bool(frappe.get_all('Holiday List',
 			dict(name=holiday_list, holiday_date=date)))
diff --git a/erpnext/hr/doctype/shift_assignment/shift_assignment.py b/erpnext/hr/doctype/shift_assignment/shift_assignment.py
index 69af5c5..05b74a0 100644
--- a/erpnext/hr/doctype/shift_assignment/shift_assignment.py
+++ b/erpnext/hr/doctype/shift_assignment/shift_assignment.py
@@ -139,7 +139,7 @@
 	return shift_timing_map
 
 
-def get_employee_shift(employee, for_date=nowdate(), consider_default_shift=False, next_shift_direction=None):
+def get_employee_shift(employee, for_date=None, consider_default_shift=False, next_shift_direction=None):
 	"""Returns a Shift Type for the given employee on the given date. (excluding the holidays)
 
 	:param employee: Employee for which shift is required.
@@ -147,6 +147,8 @@
 	:param consider_default_shift: If set to true, default shift is taken when no shift assignment is found.
 	:param next_shift_direction: One of: None, 'forward', 'reverse'. Direction to look for next shift if shift not found on given date.
 	"""
+	if for_date is None:
+		for_date = nowdate()
 	default_shift = frappe.db.get_value('Employee', employee, 'default_shift')
 	shift_type_name = None
 	shift_assignment_details = frappe.db.get_value('Shift Assignment', {'employee':employee, 'start_date':('<=', for_date), 'docstatus': '1', 'status': "Active"}, ['shift_type', 'end_date'])
@@ -200,9 +202,11 @@
 	return get_shift_details(shift_type_name, for_date)
 
 
-def get_employee_shift_timings(employee, for_timestamp=now_datetime(), consider_default_shift=False):
+def get_employee_shift_timings(employee, for_timestamp=None, consider_default_shift=False):
 	"""Returns previous shift, current/upcoming shift, next_shift for the given timestamp and employee
 	"""
+	if for_timestamp is None:
+		for_timestamp = now_datetime()
 	# write and verify a test case for midnight shift.
 	prev_shift = curr_shift = next_shift = None
 	curr_shift = get_employee_shift(employee, for_timestamp.date(), consider_default_shift, 'forward')
@@ -220,7 +224,7 @@
 	return prev_shift, curr_shift, next_shift
 
 
-def get_shift_details(shift_type_name, for_date=nowdate()):
+def get_shift_details(shift_type_name, for_date=None):
 	"""Returns Shift Details which contain some additional information as described below.
 	'shift_details' contains the following keys:
 		'shift_type' - Object of DocType Shift Type,
@@ -234,6 +238,8 @@
 	"""
 	if not shift_type_name:
 		return None
+	if not for_date:
+		for_date = nowdate()
 	shift_type = frappe.get_doc('Shift Type', shift_type_name)
 	start_datetime = datetime.combine(for_date, datetime.min.time()) + shift_type.start_time
 	for_date = for_date + timedelta(days=1) if shift_type.start_time > shift_type.end_time else for_date
diff --git a/erpnext/hr/doctype/staffing_plan/staffing_plan.py b/erpnext/hr/doctype/staffing_plan/staffing_plan.py
index 57a92b0..93cd4e1 100644
--- a/erpnext/hr/doctype/staffing_plan/staffing_plan.py
+++ b/erpnext/hr/doctype/staffing_plan/staffing_plan.py
@@ -155,7 +155,11 @@
 	return employee_counts
 
 @frappe.whitelist()
-def get_active_staffing_plan_details(company, designation, from_date=getdate(nowdate()), to_date=getdate(nowdate())):
+def get_active_staffing_plan_details(company, designation, from_date=None, to_date=None):
+	if from_date is None:
+		from_date = getdate(nowdate())
+	if to_date is None:
+		to_date = getdate(nowdate())
 	if not company or not designation:
 		frappe.throw(_("Please select Company and Designation"))
 
diff --git a/erpnext/stock/doctype/material_request/material_request.py b/erpnext/stock/doctype/material_request/material_request.py
index cf98b19..17df977 100644
--- a/erpnext/stock/doctype/material_request/material_request.py
+++ b/erpnext/stock/doctype/material_request/material_request.py
@@ -296,7 +296,7 @@
 
 		return d.ordered_qty < d.stock_qty and child_filter
 
-	doclist = get_mapped_doc("Material Request", source_name, 	{
+	doclist = get_mapped_doc("Material Request", source_name, {
 		"Material Request": {
 			"doctype": "Purchase Order",
 			"validation": {
@@ -323,7 +323,7 @@
 
 @frappe.whitelist()
 def make_request_for_quotation(source_name, target_doc=None):
-	doclist = get_mapped_doc("Material Request", source_name, 	{
+	doclist = get_mapped_doc("Material Request", source_name, {
 		"Material Request": {
 			"doctype": "Request for Quotation",
 			"validation": {
diff --git a/erpnext/stock/doctype/serial_no/serial_no.py b/erpnext/stock/doctype/serial_no/serial_no.py
index 82d8aae..a9254fb 100644
--- a/erpnext/stock/doctype/serial_no/serial_no.py
+++ b/erpnext/stock/doctype/serial_no/serial_no.py
@@ -611,7 +611,9 @@
 
 	return reserved_sr_nos
 
-def fetch_serial_numbers(filters, qty, do_not_include=[]):
+def fetch_serial_numbers(filters, qty, do_not_include=None):
+	if do_not_include is None:
+		do_not_include = []
 	batch_join_selection = ""
 	batch_no_condition = ""
 	batch_nos = filters.get("batch_no")
diff --git a/erpnext/stock/get_item_details.py b/erpnext/stock/get_item_details.py
index 19597c3..cbff214 100644
--- a/erpnext/stock/get_item_details.py
+++ b/erpnext/stock/get_item_details.py
@@ -382,7 +382,7 @@
 
 	return out
 
-def get_item_warehouse(item, args, overwrite_warehouse, defaults={}):
+def get_item_warehouse(item, args, overwrite_warehouse, defaults=None):
 	if not defaults:
 		defaults = frappe._dict({
 			'item_defaults' : get_item_defaults(item.name, args.company),