BUG: fix to_json on period

doc/source/whatsnew/v3.0.0.rst

@@ -750,6 +750,7 @@ I/O
 - Bug in :meth:`read_stata` where the missing code for double was not recognised for format versions 105 and prior (:issue:`58149`)
 - Bug in :meth:`set_option` where setting the pandas option ``display.html.use_mathjax`` to ``False`` has no effect (:issue:`59884`)
 - Bug in :meth:`to_excel` where :class:`MultiIndex` columns would be merged to a single row when ``merge_cells=False`` is passed (:issue:`60274`)
+- Bug in :meth:`to_json` period dtype was not being converted to string (:issue:`55490`)
 
 Period
 ^^^^^^

pandas/_libs/tslibs/offsets.pyx

@@ -568,6 +568,19 @@ cdef class BaseOffset:
             out += ": " + ", ".join(attrs)
         return out
 
+    def toDict(self) -> dict:
+        """
+        Convert BaseOffset object to a dictionary representation
+        and used for JSON serialization.
+        """
+        d = {}
+        # Add all attributes defined in _attributes
+        for attr in self._attributes:
+            if hasattr(self, attr):
+                d[attr] = getattr(self, attr)
+
+        return d
+
     @property
     def name(self) -> str:
         """
@@ -5108,8 +5121,8 @@ def _warn_about_deprecated_aliases(name: str, is_period: bool) -> str:
         warnings.warn(
             f"\'{name}\' is deprecated and will be removed "
             f"in a future version, please use "
-            f"\'{c_PERIOD_AND_OFFSET_DEPR_FREQSTR.get(name)}\'"
-            f" instead.",
+            f"\'{c_PERIOD_AND_OFFSET_DEPR_FREQSTR.get(name)}\' "
+            f"instead.",
             FutureWarning,
             stacklevel=find_stack_level(),
             )
@@ -5122,8 +5135,8 @@ def _warn_about_deprecated_aliases(name: str, is_period: bool) -> str:
             warnings.warn(
                 f"\'{name}\' is deprecated and will be removed "
                 f"in a future version, please use "
-                f"\'{_name}\'"
-                f" instead.",
+                f"\'{_name}\' "
+                f"instead.",
                 FutureWarning,
                 stacklevel=find_stack_level(),
                 )

pandas/tests/io/json/test_pandas.py

@@ -2079,6 +2079,38 @@ 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/tseries/offsets/test_offsets.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)