Fix ns precision isoformat (#53073)

* fix #53020

* add tests #53020

Author: i1oveMyse1f

pandas/_libs/tslibs/timestamps.pyx

@@ -1026,7 +1026,7 @@ cdef class _Timestamp(ABCTimestamp):
             base1, base2 = base, ""
 
         if timespec == "nanoseconds" or (timespec == "auto" and self.nanosecond):
-            if self.microsecond:
+            if self.microsecond or timespec == "nanoseconds":
                 base1 += f"{self.nanosecond:03d}"
             else:
                 base1 += f".{self.nanosecond:09d}"

pandas/tests/tslibs/test_parse_iso8601.py

@@ -73,3 +73,47 @@ def test_parsers_iso8601_leading_space():
     date_str, expected = ("2013-1-1 5:30:00", datetime(2013, 1, 1, 5, 30))
     actual = tslib._test_parse_iso8601(" " * 200 + date_str)
     assert actual == expected
+
+
+@pytest.mark.parametrize(
+    "date_str, timespec, exp",
+    [
+        ("2023-01-01 00:00:00", "auto", "2023-01-01T00:00:00"),
+        ("2023-01-01 00:00:00", "seconds", "2023-01-01T00:00:00"),
+        ("2023-01-01 00:00:00", "milliseconds", "2023-01-01T00:00:00.000"),
+        ("2023-01-01 00:00:00", "microseconds", "2023-01-01T00:00:00.000000"),
+        ("2023-01-01 00:00:00", "nanoseconds", "2023-01-01T00:00:00.000000000"),
+        ("2023-01-01 00:00:00.001", "auto", "2023-01-01T00:00:00.001000"),
+        ("2023-01-01 00:00:00.001", "seconds", "2023-01-01T00:00:00"),
+        ("2023-01-01 00:00:00.001", "milliseconds", "2023-01-01T00:00:00.001"),
+        ("2023-01-01 00:00:00.001", "microseconds", "2023-01-01T00:00:00.001000"),
+        ("2023-01-01 00:00:00.001", "nanoseconds", "2023-01-01T00:00:00.001000000"),
+        ("2023-01-01 00:00:00.000001", "auto", "2023-01-01T00:00:00.000001"),
+        ("2023-01-01 00:00:00.000001", "seconds", "2023-01-01T00:00:00"),
+        ("2023-01-01 00:00:00.000001", "milliseconds", "2023-01-01T00:00:00.000"),
+        ("2023-01-01 00:00:00.000001", "microseconds", "2023-01-01T00:00:00.000001"),
+        ("2023-01-01 00:00:00.000001", "nanoseconds", "2023-01-01T00:00:00.000001000"),
+        ("2023-01-01 00:00:00.000000001", "auto", "2023-01-01T00:00:00.000000001"),
+        ("2023-01-01 00:00:00.000000001", "seconds", "2023-01-01T00:00:00"),
+        ("2023-01-01 00:00:00.000000001", "milliseconds", "2023-01-01T00:00:00.000"),
+        ("2023-01-01 00:00:00.000000001", "microseconds", "2023-01-01T00:00:00.000000"),
+        (
+            "2023-01-01 00:00:00.000000001",
+            "nanoseconds",
+            "2023-01-01T00:00:00.000000001",
+        ),
+        ("2023-01-01 00:00:00.000001001", "auto", "2023-01-01T00:00:00.000001001"),
+        ("2023-01-01 00:00:00.000001001", "seconds", "2023-01-01T00:00:00"),
+        ("2023-01-01 00:00:00.000001001", "milliseconds", "2023-01-01T00:00:00.000"),
+        ("2023-01-01 00:00:00.000001001", "microseconds", "2023-01-01T00:00:00.000001"),
+        (
+            "2023-01-01 00:00:00.000001001",
+            "nanoseconds",
+            "2023-01-01T00:00:00.000001001",
+        ),
+    ],
+)
+def test_iso8601_formatter(date_str: str, timespec: str, exp: str):
+    # GH#53020
+    ts = Timestamp(date_str)
+    assert ts.isoformat(timespec=timespec) == exp