BUG: Fix Timestamp('now') and Timestamp.now unit inconsistency

Author: yuanx749

doc/source/whatsnew/v2.1.4.rst

@@ -22,6 +22,7 @@ Fixed regressions
 Bug fixes
 ~~~~~~~~~
 - Bug in :class:`Series` constructor raising DeprecationWarning when ``index`` is a list of :class:`Series` (:issue:`55228`)
+- Bug in :class:`Timestamp` construction with ``ts_input=now`` or ``ts_input=today`` giving a different unit from :meth:`Timestamp.now` or :meth:`Timestamp.today` (:issue:`55879`)
 - Bug in :meth:`Index.__getitem__` returning wrong result for Arrow dtypes and negative stepsize (:issue:`55832`)
 - Fixed bug in :func:`to_numeric` converting to extension dtype for ``string[pyarrow_numpy]`` dtype (:issue:`56179`)
 - Fixed bug in :meth:`DataFrame.__setitem__` casting :class:`Index` with object-dtype to PyArrow backed strings when ``infer_string`` option is set (:issue:`55638`)

pandas/_libs/tslibs/conversion.pyx

@@ -646,7 +646,7 @@ cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz,
         reso = get_supported_reso(out_bestunit)
         return convert_datetime_to_tsobject(dt, tz, nanos=nanos, reso=reso)
 
-    return convert_datetime_to_tsobject(dt, tz)
+    return convert_datetime_to_tsobject(dt, tz, nanos=0, reso=NPY_FR_us)
 
 
 cdef check_overflows(_TSObject obj, NPY_DATETIMEUNIT reso=NPY_FR_ns):

pandas/tests/scalar/timestamp/test_constructors.py

@@ -464,6 +464,20 @@ def test_constructor_str_infer_reso(self):
         ts = Timestamp("2020-01-01 00+00:00")
         assert ts.unit == "s"
 
+    def test_now_today_unit(self):
+        # GH#55879
+        ts_now_from_method = Timestamp.now()
+        ts_now_from_string = Timestamp("now")
+        ts_today_from_method = Timestamp.today()
+        ts_today_from_string = Timestamp("today")
+        assert (
+            ts_now_from_method.unit
+            == ts_now_from_string.unit
+            == ts_today_from_method.unit
+            == ts_today_from_string.unit
+            == "us"
+        )
+
 
 class TestTimestampConstructors:
     def test_weekday_but_no_day_raises(self):

pandas/tests/tools/test_to_datetime.py

@@ -1040,7 +1040,7 @@ def test_to_datetime_now(self):
         # See GH#18666
         with tm.set_timezone("US/Eastern"):
             # GH#18705
-            now = Timestamp("now")
+            now = Timestamp("now").as_unit("ns")
             pdnow = to_datetime("now")
             pdnow2 = to_datetime(["now"])[0]
 
@@ -1066,7 +1066,7 @@ def test_to_datetime_today(self, tz):
             pdtoday = to_datetime("today")
             pdtoday2 = to_datetime(["today"])[0]
 
-            tstoday = Timestamp("today")
+            tstoday = Timestamp("today").as_unit("ns")
             tstoday2 = Timestamp.today().as_unit("ns")
 
             # These should all be equal with infinite perf; this gives