fix: Consistent return types in cast_fieldtype

Note: BREAKING CHANGE

frappe/database/database.py

@@ -557,10 +557,7 @@ class Database(object):
 		if not df:
 			frappe.throw(_('Invalid field name: {0}').format(frappe.bold(fieldname)), self.InvalidColumnName)
 
-		# cast only if value is "set" or is truthy?
-		# cast_fieldtype returns currnt TS value for Datetime, Date fields
-		if val:
-			val = cast_fieldtype(df.fieldtype, val)
+		val = cast_fieldtype(df.fieldtype, val)
 
 		self.value_cache[doctype][fieldname] = val
 

frappe/utils/data.py

@@ -505,7 +505,17 @@ def has_common(l1, l2):
 	"""Returns truthy value if there are common elements in lists l1 and l2"""
 	return set(l1) & set(l2)
 
-def cast_fieldtype(fieldtype, value):
+def cast_fieldtype(fieldtype, value=None):
+	"""Cast the value to the Python native object of the Frappe fieldtype provided.
+	If value is None, the first/lowest value of the `fieldtype` will be returned.
+
+	Mapping of Python types => Frappe types:
+		* float => ("Currency", "Float", "Percent")
+		* int => ("Int", "Check")
+		* datetime.datetime => ("Datetime",)
+		* datetime.date => ("Date",)
+		* datetime.time => ("Time",)
+	"""
 	if fieldtype in ("Currency", "Float", "Percent"):
 		value = flt(value)
 
@@ -517,12 +527,18 @@ def cast_fieldtype(fieldtype, value):
 		value = cstr(value)
 
 	elif fieldtype == "Date":
+		if value is None:
+			value = datetime.datetime(1, 1, 1).date()
 		value = getdate(value)
 
 	elif fieldtype == "Datetime":
+		if value is None:
+			value = datetime.datetime(1, 1, 1)
 		value = get_datetime(value)
 
 	elif fieldtype == "Time":
+		if value is None:
+			value = "0:0:0"
 		value = to_timedelta(value)
 
 	return value