config: new field 'holidays_info' to show generated holidays in config-form

config.py

@@ -9,6 +9,9 @@ from dateutil.easter import (
     easter, EASTER_JULIAN, EASTER_ORTHODOX, EASTER_WESTERN)
 from trytond.pool import PoolMeta, Pool
 from trytond.model import fields
+from trytond.transaction import Transaction
+from trytond.report import Report
+
 
 holidays = fields.Char(
     string='Holidays', help='Semicolon separate list of dates: ' +
@@ -21,6 +24,32 @@ class Configuration(metaclass=PoolMeta):
     __name__ = 'cashbook.configuration'
 
     holidays = fields.MultiValue(holidays)
+    holidays_info = fields.Function(fields.Char(
+        string='Holidays', readonly=True,
+        help='Holidays in the current year.'), 'on_change_with_holidays_info')
+
+    @fields.depends('holidays')
+    def on_change_with_holidays_info(self, name=None):
+        """ get string of generated holidays for years in
+        context-value 'holiday_years' or current year
+
+        Args:
+            name (str, optional): field. Defaults to None.
+
+        Returns:
+            str: formatted holidays in language of user
+        """
+        pool = Pool()
+        Config = pool.get('cashbook.configuration')
+        context = Transaction().context
+
+        years = context.get('holiday_years', [])
+        cfg1 = Config.get_singleton()
+        if cfg1:
+            dates = cfg1.holiday_dates(years)
+            dates.sort()
+            return '|'.join([
+                Report.format_date(x) for x in dates])
 
     def holiday_dates(self, years=[]):
         """ get list of dates for list of years