DEPR: Enforce deprecation of parsing to tzlocal (pandas-dev#58002)

Author: jbrockmendel

doc/source/whatsnew/v3.0.0.rst

@@ -202,6 +202,7 @@ Removal of prior version deprecations/changes
 - Enforced deprecation of :meth:`.DataFrameGroupBy.get_group` and :meth:`.SeriesGroupBy.get_group` allowing the ``name`` argument to be a non-tuple when grouping by a list of length 1 (:issue:`54155`)
 - Enforced deprecation of :meth:`Series.interpolate` and :meth:`DataFrame.interpolate` for object-dtype (:issue:`57820`)
 - Enforced deprecation of ``axis=None`` acting the same as ``axis=0`` in the DataFrame reductions ``sum``, ``prod``, ``std``, ``var``, and ``sem``, passing ``axis=None`` will now reduce over both axes; this is particularly the case when doing e.g. ``numpy.sum(df)`` (:issue:`21597`)
+- Enforced deprecation of parsing system timezone strings to ``tzlocal``, which depended on system timezone, pass the 'tz' keyword instead (:issue:`50791`)
 - Enforced deprecation of passing a dictionary to :meth:`SeriesGroupBy.agg` (:issue:`52268`)
 - Enforced deprecation of string ``AS`` denoting frequency in :class:`YearBegin` and strings ``AS-DEC``, ``AS-JAN``, etc. denoting annual frequencies with various fiscal year starts (:issue:`57793`)
 - Enforced deprecation of string ``A`` denoting frequency in :class:`YearEnd` and strings ``A-DEC``, ``A-JAN``, etc. denoting annual frequencies with various fiscal year ends (:issue:`57699`)

pandas/_libs/tslibs/parsing.pyx

@@ -45,7 +45,6 @@ from decimal import InvalidOperation
 
 from dateutil.parser import DEFAULTPARSER
 from dateutil.tz import (
-    tzlocal as _dateutil_tzlocal,
     tzoffset,
     tzutc as _dateutil_tzutc,
 )
@@ -703,17 +702,12 @@ cdef datetime dateutil_parse(
         if res.tzname and res.tzname in time.tzname:
             # GH#50791
             if res.tzname != "UTC":
-                # If the system is localized in UTC (as many CI runs are)
-                #  we get tzlocal, once the deprecation is enforced will get
-                #  timezone.utc, not raise.
-                warnings.warn(
+                raise ValueError(
                     f"Parsing '{res.tzname}' as tzlocal (dependent on system timezone) "
-                    "is deprecated and will raise in a future version. Pass the 'tz' "
+                    "is no longer supported. Pass the 'tz' "
                     "keyword or call tz_localize after construction instead",
-                    FutureWarning,
-                    stacklevel=find_stack_level()
                 )
-            ret = ret.replace(tzinfo=_dateutil_tzlocal())
+            ret = ret.replace(tzinfo=timezone.utc)
         elif res.tzoffset == 0:
             ret = ret.replace(tzinfo=_dateutil_tzutc())
         elif res.tzoffset:

pandas/tests/tslibs/test_parsing.py

@@ -6,7 +6,6 @@
 import re
 
 from dateutil.parser import parse as du_parse
-from dateutil.tz import tzlocal
 from hypothesis import given
 import numpy as np
 import pytest
@@ -22,6 +21,10 @@
 )
 import pandas.util._test_decorators as td
 
+# Usually we wouldn't want this import in this test file (which is targeted at
+#  tslibs.parsing), but it is convenient to test the Timestamp constructor at
+#  the same time as the other parsing functions.
+from pandas import Timestamp
 import pandas._testing as tm
 from pandas._testing._hypothesis import DATETIME_NO_TZ
 
@@ -33,20 +36,21 @@
 def test_parsing_tzlocal_deprecated():
     # GH#50791
     msg = (
-        "Parsing 'EST' as tzlocal.*"
+        r"Parsing 'EST' as tzlocal \(dependent on system timezone\) "
+        r"is no longer supported\. "
         "Pass the 'tz' keyword or call tz_localize after construction instead"
     )
     dtstr = "Jan 15 2004 03:00 EST"
 
     with tm.set_timezone("US/Eastern"):
-        with tm.assert_produces_warning(FutureWarning, match=msg):
-            res, _ = parse_datetime_string_with_reso(dtstr)
+        with pytest.raises(ValueError, match=msg):
+            parse_datetime_string_with_reso(dtstr)
 
-        assert isinstance(res.tzinfo, tzlocal)
+        with pytest.raises(ValueError, match=msg):
+            parsing.py_parse_datetime_string(dtstr)
 
-        with tm.assert_produces_warning(FutureWarning, match=msg):
-            res = parsing.py_parse_datetime_string(dtstr)
-        assert isinstance(res.tzinfo, tzlocal)
+        with pytest.raises(ValueError, match=msg):
+            Timestamp(dtstr)
 
 
 def test_parse_datetime_string_with_reso():