BUG: fix to_json on period

Author: xiaohuanlin

pandas/_libs/tslibs/offsets.pyx

@@ -2079,6 +2079,22 @@ def e(self):
         series = Series([_TestObject(a=1, b=2, _c=3, d=4)])
         assert json.loads(series.to_json()) == {"0": {"a": 1, "b": 2, "d": 4}}
 
+    def test_to_json_with_period(self):
+        # GH55490
+        ser = Series(pd.period_range(start=2021, freq="Y", periods=1))
+        result = ser.to_json()
+        expected = (
+            '{"0":{"day":31,"day_of_week":4,"day_of_year":365, '
+            '"dayofweek":4,"dayofyear":365,"days_in_month":31, '
+            '"daysinmonth":31,"end_time":1640995199999, '
+            '"freq":{"n":1,"normalize":false,"month":12}, '
+            '"freqstr":"Y-DEC","hour":0,"is_leap_year":false, '
+            '"minute":0,"month":12,"ordinal":51,"quarter":4, '
+            '"qyear":2021,"second":0,"start_time":1609459200000, '
+            '"week":52,"weekday":4,"weekofyear":52,"year":2021}}'
+        )
+        assert result == expected
+
     @pytest.mark.parametrize(
         "data,expected",
         [

pandas/tests/io/json/test_pandas.py

@@ -1227,3 +1227,12 @@ def test_is_yqm_start_end():
 def test_multiply_dateoffset_typeerror(left, right):
     with pytest.raises(TypeError, match="Cannot multiply"):
         left * right
+
+
+def test_toDict(offset_types):
+    offset = offset_types(n=2)
+    d = offset.toDict()
+
+    for attr in offset._attributes:
+        if hasattr(offset, attr):
+            assert d[attr] == getattr(offset, attr)