From 5b30943cbb02a01bae2901964ed330213dffd7b4 Mon Sep 17 00:00:00 2001
From: MarcoGorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Mon, 25 Sep 2023 11:11:24 +0100
Subject: [PATCH] dont map MS to period

---
 pandas/_libs/tslibs/dtypes.pyx               | 11 ++---------
 pandas/tests/resample/test_datetime_index.py | 13 +++++++++++++
 pandas/tseries/frequencies.py                | 16 +++++-----------
 3 files changed, 20 insertions(+), 20 deletions(-)

diff --git a/pandas/_libs/tslibs/dtypes.pyx b/pandas/_libs/tslibs/dtypes.pyx
index cca379c620aeb..48d70d8b2c0b0 100644
--- a/pandas/_libs/tslibs/dtypes.pyx
+++ b/pandas/_libs/tslibs/dtypes.pyx
@@ -189,13 +189,8 @@ OFFSET_TO_PERIOD_FREQSTR: dict = {
     "WEEKDAY": "D",
     "EOM": "M",
     "BM": "M",
-    "BQS": "Q",
-    "QS": "Q",
     "BQ": "Q",
     "BA": "A",
-    "AS": "A",
-    "BAS": "A",
-    "MS": "M",
     "D": "D",
     "B": "B",
     "min": "min",
@@ -210,18 +205,16 @@ OFFSET_TO_PERIOD_FREQSTR: dict = {
     "ME": "M",
     "Y": "A",
     "BY": "A",
-    "YS": "A",
-    "BYS": "A",
 }
 cdef dict c_OFFSET_TO_PERIOD_FREQSTR = OFFSET_TO_PERIOD_FREQSTR
 
 cpdef freq_to_period_freqstr(freq_n, freq_name):
     if freq_n == 1:
         freqstr = f"""{c_OFFSET_TO_PERIOD_FREQSTR.get(
-            freq_name, freq_name)}"""
+            freq_name, None)}"""
     else:
         freqstr = f"""{freq_n}{c_OFFSET_TO_PERIOD_FREQSTR.get(
-            freq_name, freq_name)}"""
+            freq_name, None)}"""
     return freqstr
 
 # Map deprecated resolution abbreviations to correct resolution abbreviations
diff --git a/pandas/tests/resample/test_datetime_index.py b/pandas/tests/resample/test_datetime_index.py
index d929dfa6e1e59..9be27edf50cf3 100644
--- a/pandas/tests/resample/test_datetime_index.py
+++ b/pandas/tests/resample/test_datetime_index.py
@@ -2051,3 +2051,16 @@ def test_resample_M_deprecated():
     with tm.assert_produces_warning(UserWarning, match=depr_msg):
         result = s.resample("2M").mean()
     tm.assert_series_equal(result, expected)
+
+
+def test_resample_ms_closed_right():
+    # https://github.com/pandas-dev/pandas/issues/55271
+    dti = date_range(start="2020-01-31", freq="1min", periods=6000)
+    df = DataFrame({"ts": dti}, index=dti)
+    grouped = df.resample("MS", closed="right")
+    result = grouped.last()
+    expected = DataFrame(
+        {"ts": [datetime(2020, 2, 1), datetime(2020, 2, 4, 3, 59)]},
+        index=DatetimeIndex([datetime(2020, 1, 1), datetime(2020, 2, 1)], freq="MS"),
+    )
+    tm.assert_frame_equal(result, expected)
diff --git a/pandas/tseries/frequencies.py b/pandas/tseries/frequencies.py
index 7aa245341cbdd..471904cc5b60b 100644
--- a/pandas/tseries/frequencies.py
+++ b/pandas/tseries/frequencies.py
@@ -59,13 +59,6 @@
 # ---------------------------------------------------------------------
 # Offset related functions
 
-_need_suffix = ["QS", "BQ", "BQS", "YS", "AS", "BY", "BA", "BYS", "BAS"]
-
-for _prefix in _need_suffix:
-    for _m in MONTHS:
-        key = f"{_prefix}-{_m}"
-        OFFSET_TO_PERIOD_FREQSTR[key] = OFFSET_TO_PERIOD_FREQSTR[_prefix]
-
 for _prefix in ["A", "Q"]:
     for _m in MONTHS:
         _alias = f"{_prefix}-{_m}"
@@ -502,10 +495,10 @@ def is_superperiod(source, target) -> bool:
     -------
     bool
     """
-    if target is None or source is None:
-        return False
     source = _maybe_coerce_freq(source)
     target = _maybe_coerce_freq(target)
+    if target is None or source is None:
+        return False
 
     if _is_annual(source):
         if _is_annual(target):
@@ -544,7 +537,7 @@ def is_superperiod(source, target) -> bool:
         return False
 
 
-def _maybe_coerce_freq(code) -> str:
+def _maybe_coerce_freq(code) -> str | None:
     """we might need to coerce a code to a rule_code
     and uppercase it
 
@@ -557,9 +550,10 @@ def _maybe_coerce_freq(code) -> str:
     -------
     str
     """
-    assert code is not None
     if isinstance(code, DateOffset):
         code = freq_to_period_freqstr(1, code.name)
+    if code is None:
+        return None
     if code in {"min", "s", "ms", "us", "ns"}:
         return code
     else: