BUG: GH29461 strftime() with nanoseconds for Timestamp

pandas/_libs/tslibs/timestamps.pyx

@@ -7,6 +7,7 @@ construction requirements, we need to do object instantiation in python
 shadows the python class, where we do any heavy lifting.
 """
 import warnings
+import time as _time
 
 import numpy as np
 cimport numpy as cnp
@@ -1469,6 +1470,30 @@ default 'raise'
             np.array([self.value], dtype='i8'), tz=self.tz)[0]
         return Timestamp(normalized_value).tz_localize(self.tz)
 
+    def strftime(self, format: str) -> str:
+        """
+        Override datetime.strftime() method so we can display
+        nanosecond precision.
+        """
+        freplace = None  # the string to use for %f
+
+        newformat = []
+        i, n = 0, len(format)
+        while i < n:
+            ch = format[i]
+            if ch == 'f':
+                # remove accompanying %
+                newformat.pop()
+                # and put fractional seconds in its place
+                newformat.append(f"{self.microsecond * 1000 + self.nanosecond}")
+            else:
+                newformat.append(ch)
+
+            i += 1
+
+        newformat = "".join(newformat)
+        return _time.strftime(newformat, self.timetuple())
+
 
 # Add the min and max fields at the class level
 cdef int64_t _NS_UPPER_BOUND = np.iinfo(np.int64).max

pandas/tests/scalar/timestamp/test_timestamp.py

@@ -15,7 +15,7 @@
 from pandas.compat.numpy import np_datetime64_compat
 import pandas.util._test_decorators as td
 
-from pandas import NaT, Timedelta, Timestamp
+from pandas import NaT, Timedelta, Timestamp, to_datetime
 import pandas._testing as tm
 
 from pandas.tseries import offsets
@@ -442,6 +442,18 @@ def test_nanosecond_timestamp(self):
         assert t.value == expected
         assert t.nanosecond == 10
 
+    @pytest.mark.parametrize("date", [
+        "2020-05-22 08:53:19.123456789",
+        "2020-05-22 08:53:19.123456",
+        "2020-05-22 08:53:19"
+    ])
+    @pytest.mark.parametrize("fmt", [
+        "%m/%d/%Y %H:%M:%S.%f", "%m%d%Y%H%M%S%f"
+    ])
+    def test_nanosecond_roundtrip(self, date, fmt):
+        ts = Timestamp(date)
+        string = ts.strftime(fmt)
+        assert ts == to_datetime(string, format=fmt)
 
 class TestTimestampToJulianDate:
     def test_compare_1700(self):