clean: simplify lite-rule-alias and dont-uppercase

pandas/_libs/tslibs/dtypes.pyx

@@ -359,6 +359,8 @@ cdef dict c_PERIOD_AND_OFFSET_DEPR_FREQSTR = {
     "b": "B",
     "c": "C",
     "MIN": "min",
+    "US": "us",
+    "NS": "ns",
 }
 
 cdef str INVALID_FREQ_ERR_MSG = "Invalid frequency: {0}"

pandas/_libs/tslibs/offsets.pyx

@@ -4697,13 +4697,9 @@ _lite_rule_alias = {
     "BYS": "BYS-JAN",  # BYearBegin(month=1),
 
     "Min": "min",
-    "min": "min",
-    "ms": "ms",
-    "us": "us",
-    "ns": "ns",
 }
 
-_dont_uppercase = {"h", "bh", "cbh", "MS", "ms", "s"}
+_dont_uppercase = {"min", "h", "bh", "cbh", "s", "ms", "us", "ns"}
 
 
 INVALID_FREQ_ERR_MSG = "Invalid frequency: {0}"
@@ -4713,6 +4709,37 @@ INVALID_FREQ_ERR_MSG = "Invalid frequency: {0}"
 _offset_map = {}
 
 
+def _warn_about_deprecated_aliases(name: str, is_period: bool) -> str:
+    if name in _lite_rule_alias:
+        return name
+    if name in c_PERIOD_AND_OFFSET_DEPR_FREQSTR:
+        warnings.warn(
+            f"\'{name}\' is deprecated and will be removed "
+            f"in a future version, please use "
+            f"\'{c_PERIOD_AND_OFFSET_DEPR_FREQSTR.get(name)}\' "
+            f" instead.",
+            FutureWarning,
+            stacklevel=find_stack_level(),
+            )
+        return c_PERIOD_AND_OFFSET_DEPR_FREQSTR[name]
+
+    for _name in (name.lower(), name.upper()):
+        if name == _name:
+            continue
+        if _name in c_PERIOD_AND_OFFSET_DEPR_FREQSTR.values():
+            warnings.warn(
+                f"\'{name}\' is deprecated and will be removed "
+                f"in a future version, please use "
+                f"\'{_name}\' "
+                f" instead.",
+                FutureWarning,
+                stacklevel=find_stack_level(),
+                )
+            return _name
+
+    return name
+
+
 def _validate_to_offset_alias(alias: str, is_period: bool) -> None:
     if not is_period:
         if alias.upper() in c_OFFSET_RENAMED_FREQSTR:
@@ -4750,35 +4777,6 @@ def _get_offset(name: str) -> BaseOffset:
     --------
     _get_offset('EOM') --> BMonthEnd(1)
     """
-    if (
-        name not in _lite_rule_alias
-        and (name.upper() in _lite_rule_alias)
-        and name != "ms"
-    ):
-        warnings.warn(
-            f"\'{name}\' is deprecated and will be removed "
-            f"in a future version, please use \'{name.upper()}\' instead.",
-            FutureWarning,
-            stacklevel=find_stack_level(),
-        )
-    elif (
-        name not in _lite_rule_alias
-        and (name.lower() in _lite_rule_alias)
-        and name != "MS"
-    ):
-        warnings.warn(
-            f"\'{name}\' is deprecated and will be removed "
-            f"in a future version, please use \'{name.lower()}\' instead.",
-            FutureWarning,
-            stacklevel=find_stack_level(),
-        )
-    if name not in _dont_uppercase:
-        name = name.upper()
-        name = _lite_rule_alias.get(name, name)
-        name = _lite_rule_alias.get(name.lower(), name)
-    else:
-        name = _lite_rule_alias.get(name, name)
-
     if name not in _offset_map:
         try:
             split = name.split("-")
@@ -4880,39 +4878,31 @@ cpdef to_offset(freq, bint is_period=False):
 
             tups = zip(split[0::4], split[1::4], split[2::4])
             for n, (sep, stride, name) in enumerate(tups):
+                name = _warn_about_deprecated_aliases(name, is_period)
                 _validate_to_offset_alias(name, is_period)
+                name = _lite_rule_alias.get(name, name)
                 if is_period:
                     if name.upper() in c_PERIOD_TO_OFFSET_FREQSTR:
                         if name.upper() != name:
                             raise ValueError(
                                 f"\'{name}\' is no longer supported, "
                                 f"please use \'{name.upper()}\' instead.",
                             )
-                        name = c_PERIOD_TO_OFFSET_FREQSTR.get(name.upper())
-
-                if name in c_PERIOD_AND_OFFSET_DEPR_FREQSTR:
-                    warnings.warn(
-                        f"\'{name}\' is deprecated and will be removed "
-                        f"in a future version, please use "
-                        f"\'{c_PERIOD_AND_OFFSET_DEPR_FREQSTR.get(name)}\' "
-                        f"instead.",
-                        FutureWarning,
-                        stacklevel=find_stack_level(),
-                        )
-                    name = c_PERIOD_AND_OFFSET_DEPR_FREQSTR.get(name)
+                        name = c_PERIOD_TO_OFFSET_FREQSTR[name.upper()]
+                name = _lite_rule_alias.get(name, name)
+
                 if sep != "" and not sep.isspace():
                     raise ValueError("separator must be spaces")
-                prefix = _lite_rule_alias.get(name) or name
                 if stride_sign is None:
                     stride_sign = -1 if stride.startswith("-") else 1
                 if not stride:
                     stride = 1
 
-                if prefix in {"D", "h", "min", "s", "ms", "us", "ns"}:
+                if name in {"D", "h", "min", "s", "ms", "us", "ns"}:
                     # For these prefixes, we have something like "3h" or
                     #  "2.5min", so we can construct a Timedelta with the
                     #  matching unit and get our offset from delta_to_tick
-                    td = Timedelta(1, unit=prefix)
+                    td = Timedelta(1, unit=name)
                     off = delta_to_tick(td)
                     offset = off * float(stride)
                     if n != 0:
@@ -4921,7 +4911,7 @@ cpdef to_offset(freq, bint is_period=False):
                         offset *= stride_sign
                 else:
                     stride = int(stride)
-                    offset = _get_offset(prefix)
+                    offset = _get_offset(name)
                     offset = offset * int(np.fabs(stride) * stride_sign)
 
                 if result is None:
@@ -4931,7 +4921,7 @@ cpdef to_offset(freq, bint is_period=False):
         except (ValueError, TypeError) as err:
             raise ValueError(INVALID_FREQ_ERR_MSG.format(
                 f"{freq}, failed to parse with error message: {repr(err)}")
-            )
+            ) from err
     else:
         result = None
 

pandas/tests/arrays/test_datetimes.py

@@ -760,7 +760,7 @@ def test_date_range_frequency_M_Q_Y_raises(self, freq):
         with pytest.raises(ValueError, match=msg):
             pd.date_range("1/1/2000", periods=4, freq=freq)
 
-    @pytest.mark.parametrize("freq_depr", ["2MIN", "2mS", "2Us"])
+    @pytest.mark.parametrize("freq_depr", ["2MIN", "2nS", "2Us"])
     def test_date_range_uppercase_frequency_deprecated(self, freq_depr):
         # GH#9586, GH#54939
         depr_msg = f"'{freq_depr[1:]}' is deprecated and will be removed in a "

pandas/tests/tseries/offsets/test_offsets.py

@@ -788,9 +788,7 @@ def test_get_offset():
 
     pairs = [
         ("B", BDay()),
-        ("b", BDay()),
-        ("bme", BMonthEnd()),
-        ("Bme", BMonthEnd()),
+        ("BME", BMonthEnd()),
         ("W-MON", Week(weekday=0)),
         ("W-TUE", Week(weekday=1)),
         ("W-WED", Week(weekday=2)),