BUG: Fix Timestamp('now') and Timestamp.now unit inconsistency (#56281)

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

* Move return

* Update doc/source/whatsnew/v2.1.4.rst

Co-authored-by: Matthew Roeschke <[email protected]>

* Resolve

* not use tuple in cython for perf

---------

Co-authored-by: Matthew Roeschke <[email protected]>

Author: yuanx749

doc/source/whatsnew/v2.1.4.rst

@@ -23,6 +23,7 @@ Bug fixes
 ~~~~~~~~~
 - Bug in :class:`Series` constructor raising DeprecationWarning when ``index`` is a list of :class:`Series` (:issue:`55228`)
 - Bug in :class:`Series` when trying to cast date-like string inputs to :class:`ArrowDtype` of ``pyarrow.timestamp`` (:issue:`56266`)
+- 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:`.DataFrameGroupBy.min` and :meth:`.DataFrameGroupBy.max` not preserving extension dtype for empty object (:issue:`55619`)

pandas/_libs/tslibs/conversion.pyx

@@ -599,11 +599,13 @@ cdef _TSObject convert_str_to_tsobject(str ts, tzinfo tz,
         # Issue 9000, we short-circuit rather than going
         # into np_datetime_strings which returns utc
         dt = datetime.now(tz)
+        return convert_datetime_to_tsobject(dt, tz, nanos=0, reso=NPY_FR_us)
     elif ts == "today":
         # Issue 9000, we short-circuit rather than going
         # into np_datetime_strings which returns a normalized datetime
         dt = datetime.now(tz)
         # equiv: datetime.today().replace(tzinfo=tz)
+        return convert_datetime_to_tsobject(dt, tz, nanos=0, reso=NPY_FR_us)
     else:
         string_to_dts_failed = string_to_dts(
             ts, &dts, &out_bestunit, &out_local,
@@ -647,8 +649,6 @@ 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)
-
 
 cdef check_overflows(_TSObject obj, NPY_DATETIMEUNIT reso=NPY_FR_ns):
     """

pandas/tests/scalar/timestamp/test_constructors.py

@@ -464,6 +464,13 @@ def test_constructor_str_infer_reso(self):
         ts = Timestamp("2020-01-01 00+00:00")
         assert ts.unit == "s"
 
+    @pytest.mark.parametrize("method", ["now", "today"])
+    def test_now_today_unit(self, method):
+        # GH#55879
+        ts_from_method = getattr(Timestamp, method)()
+        ts_from_string = Timestamp(method)
+        assert ts_from_method.unit == ts_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