diff --git a/openfisca_core/periods/period_.py b/openfisca_core/periods/period_.py
index 707bc43c0e..7411d9f72c 100644
--- a/openfisca_core/periods/period_.py
+++ b/openfisca_core/periods/period_.py
@@ -121,42 +121,10 @@ class Period(tuple):
 
     """
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return '{}({})'.format(self.__class__.__name__, super(Period, self).__repr__())
 
-    def __str__(self):
-        """Transform period to a string.
-
-        Examples:
-            >>> str(Period(("year", Instant((2021, 1, 1)), 1)))
-            '2021'
-
-            >>> str(Period(("year", Instant((2021, 2, 1)), 1)))
-            'year:2021-02'
-
-            >>> str(Period(("month", Instant((2021, 2, 1)), 1)))
-            '2021-02'
-
-            >>> str(Period(("year", Instant((2021, 1, 1)), 2)))
-            'year:2021:2'
-
-            >>> str(Period(("month", Instant((2021, 1, 1)), 2)))
-            'month:2021-01:2'
-
-            >>> str(Period(("month", Instant((2021, 1, 1)), 12)))
-            '2021'
-
-            >>> str(Period(("year", Instant((2021, 3, 1)), 2)))
-            'year:2021-03:2'
-
-            >>> str(Period(("month", Instant((2021, 3, 1)), 2)))
-            'month:2021-03:2'
-
-            >>> str(Period(("month", Instant((2021, 3, 1)), 12)))
-            'year:2021-03'
-
-        """
-
+    def __str__(self) -> str:
         unit, start_instant, size = self
         if unit == config.ETERNITY:
             return 'ETERNITY'
diff --git a/openfisca_core/periods/tests/period/test_str.py b/openfisca_core/periods/tests/period/test_str.py
index 47ea3c390d..874a8405ab 100644
--- a/openfisca_core/periods/tests/period/test_str.py
+++ b/openfisca_core/periods/tests/period/test_str.py
@@ -30,4 +30,4 @@ def test_str_with_months(date_unit, instant, size, expected):
     [DateUnit.DAY, Instant((2022, 3, 1)), 3, "day:2022-03-01:3"],
     ])
 def test_str_with_days(date_unit, instant, size, expected):
-    assert str(Period((date_unit, instant, size))) == expected
\ No newline at end of file
+    assert str(Period((date_unit, instant, size))) == expected
diff --git a/tests/core/test_periods.py b/tests/core/test_periods.py
index 92c8b49022..58fffd7919 100644
--- a/tests/core/test_periods.py
+++ b/tests/core/test_periods.py
@@ -15,6 +15,7 @@
 
 # Years
 
+
 def test_parsing_year():
     assert period('2014') == Period((YEAR, first_jan, 1))