DEPR: to_datetime('now') match Timestamp('now') (#49346)

* DEPR: to_datetime('now') match Timestamp('now')

* perf

* try again

Author: jbrockmendel

doc/source/whatsnew/v2.0.0.rst

@@ -253,6 +253,7 @@ Removal of prior version deprecations/changes
 - Removed the ``display.column_space`` option in favor of ``df.to_string(col_space=...)`` (:issue:`47280`)
 - Removed the deprecated method ``mad`` from pandas classes (:issue:`11787`)
 - Removed the deprecated method ``tshift`` from pandas classes (:issue:`11631`)
+- Changed the behavior of :func:`to_datetime` with argument "now" with ``utc=False`` to match ``Timestamp("now")`` (:issue:`18705`)
 - Changed behavior of :class:`DataFrame` constructor given floating-point ``data`` and an integer ``dtype``, when the data cannot be cast losslessly, the floating point dtype is retained, matching :class:`Series` behavior (:issue:`41170`)
 - Changed behavior of :class:`DataFrame` constructor when passed a ``dtype`` (other than int) that the data cannot be cast to; it now raises instead of silently ignoring the dtype (:issue:`41733`)
 - Changed the behavior of :class:`Series` constructor, it will no longer infer a datetime64 or timedelta64 dtype from string entries (:issue:`41731`)

pandas/_libs/tslib.pyx

@@ -1,5 +1,3 @@
-import warnings
-
 cimport cython
 from cpython.datetime cimport (
     PyDate_Check,
@@ -9,8 +7,6 @@ from cpython.datetime cimport (
     tzinfo,
 )
 
-from pandas.util._exceptions import find_stack_level
-
 # import datetime C API
 import_datetime()
 
@@ -855,17 +851,12 @@ cdef inline bint _parse_today_now(str val, int64_t* iresult, bint utc):
     # We delay this check for as long as possible
     # because it catches relatively rare cases
     if val == "now":
-        iresult[0] = Timestamp.utcnow().value
-        if not utc:
+        if utc:
+            iresult[0] = Timestamp.utcnow().value
+        else:
             # GH#18705 make sure to_datetime("now") matches Timestamp("now")
-            warnings.warn(
-                "The parsing of 'now' in pd.to_datetime without `utc=True` is "
-                "deprecated. In a future version, this will match Timestamp('now') "
-                "and Timestamp.now()",
-                FutureWarning,
-                stacklevel=find_stack_level(),
-            )
-
+            # Note using Timestamp.now() is faster than Timestamp("now")
+            iresult[0] = Timestamp.now().value
         return True
     elif val == "today":
         iresult[0] = Timestamp.today().value

pandas/tests/tools/test_to_datetime.py

@@ -626,20 +626,15 @@ def test_to_datetime_unparsable_ignore(self):
     def test_to_datetime_now(self):
         # See GH#18666
         with tm.set_timezone("US/Eastern"):
-            msg = "The parsing of 'now' in pd.to_datetime"
-            with tm.assert_produces_warning(
-                FutureWarning, match=msg, check_stacklevel=False
-            ):
-                # checking stacklevel is tricky because we go through cython code
-                # GH#18705
-                npnow = np.datetime64("now").astype("datetime64[ns]")
-                pdnow = to_datetime("now")
-                pdnow2 = to_datetime(["now"])[0]
+            # GH#18705
+            now = Timestamp("now")
+            pdnow = to_datetime("now")
+            pdnow2 = to_datetime(["now"])[0]
 
             # These should all be equal with infinite perf; this gives
             # a generous margin of 10 seconds
-            assert abs(pdnow.value - npnow.astype(np.int64)) < 1e10
-            assert abs(pdnow2.value - npnow.astype(np.int64)) < 1e10
+            assert abs(pdnow.value - now.value) < 1e10
+            assert abs(pdnow2.value - now.value) < 1e10
 
             assert pdnow.tzinfo is None
             assert pdnow2.tzinfo is None
@@ -673,12 +668,7 @@ def test_to_datetime_today(self, tz):
 
     @pytest.mark.parametrize("arg", ["now", "today"])
     def test_to_datetime_today_now_unicode_bytes(self, arg):
-        warn = FutureWarning if arg == "now" else None
-        msg = "The parsing of 'now' in pd.to_datetime"
-        with tm.assert_produces_warning(warn, match=msg, check_stacklevel=False):
-            # checking stacklevel is tricky because we go through cython code
-            # GH#18705
-            to_datetime([arg])
+        to_datetime([arg])
 
     @pytest.mark.parametrize(
         "dt", [np.datetime64("2000-01-01"), np.datetime64("2000-01-02")]