From 06277b7459ce2b140f6260b933936046318653ae Mon Sep 17 00:00:00 2001
From: MarcoGorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Fri, 12 Jan 2024 19:14:20 +0000
Subject: [PATCH 1/3] dont raise if users use (undocumented) "m" for "month
 end", warn to use "ME" instead

---
 pandas/_libs/tslibs/offsets.pyx                   | 6 +++---
 pandas/tests/indexes/datetimes/test_date_range.py | 9 +++++++++
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/pandas/_libs/tslibs/offsets.pyx b/pandas/_libs/tslibs/offsets.pyx
index 84544322b57a8..446088821b10d 100644
--- a/pandas/_libs/tslibs/offsets.pyx
+++ b/pandas/_libs/tslibs/offsets.pyx
@@ -4860,15 +4860,15 @@ 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):
-                if is_period is False and name in c_OFFSET_DEPR_FREQSTR:
+                if is_period is False and name.upper() in c_OFFSET_DEPR_FREQSTR:
                     warnings.warn(
                         f"\'{name}\' is deprecated and will be removed "
                         f"in a future version, please use "
-                        f"\'{c_OFFSET_DEPR_FREQSTR.get(name)}\' instead.",
+                        f"\'{c_OFFSET_DEPR_FREQSTR.get(name.upper())}\' instead.",
                         FutureWarning,
                         stacklevel=find_stack_level(),
                     )
-                    name = c_OFFSET_DEPR_FREQSTR[name]
+                    name = c_OFFSET_DEPR_FREQSTR[name.upper()]
                 if is_period is True and name in c_REVERSE_OFFSET_DEPR_FREQSTR:
                     if name.startswith("Y"):
                         raise ValueError(
diff --git a/pandas/tests/indexes/datetimes/test_date_range.py b/pandas/tests/indexes/datetimes/test_date_range.py
index ec158f7b194a0..69f2ba8a4deab 100644
--- a/pandas/tests/indexes/datetimes/test_date_range.py
+++ b/pandas/tests/indexes/datetimes/test_date_range.py
@@ -818,6 +818,15 @@ def test_frequencies_A_deprecated_Y_renamed(self, freq, freq_depr):
             result = date_range("1/1/2000", periods=2, freq=freq_depr)
         tm.assert_index_equal(result, expected)
 
+    def test_to_offset_with_lowercase_deprecated_freq(self) -> None:
+        # https://github.com/pandas-dev/pandas/issues/56847
+        msg = "'m' is deprecated and will be removed in a future version, please use "
+        "'ME' instead."
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            result = date_range("2010-01-01", periods=2, freq="m")
+        expected = DatetimeIndex(["2010-01-31", "2010-02-28"], freq="ME")
+        tm.assert_index_equal(result, expected)
+
     def test_date_range_bday(self):
         sdate = datetime(1999, 12, 25)
         idx = date_range(start=sdate, freq="1B", periods=20)

From 77a3bbbf65b91964bb0f59ffdf2e3377c2da7e20 Mon Sep 17 00:00:00 2001
From: MarcoGorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Fri, 12 Jan 2024 19:41:08 +0000
Subject: [PATCH 2/3] fixup test

---
 pandas/tests/tslibs/test_to_offset.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pandas/tests/tslibs/test_to_offset.py b/pandas/tests/tslibs/test_to_offset.py
index 204775347e47a..c5a2f08933392 100644
--- a/pandas/tests/tslibs/test_to_offset.py
+++ b/pandas/tests/tslibs/test_to_offset.py
@@ -44,6 +44,7 @@ def test_to_offset_negative(freqstr, expected):
     assert result.n == expected
 
 
+@pytest.mark.filterwarnings("ignore:.*'m' is deprecated.*:FutureWarning")
 @pytest.mark.parametrize(
     "freqstr",
     [

From 42c3bb6875bb519e75b9c20d0506ae948d426ace Mon Sep 17 00:00:00 2001
From: MarcoGorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Fri, 12 Jan 2024 20:12:08 +0000
Subject: [PATCH 3/3] lint

---
 pandas/tests/indexes/datetimes/test_date_range.py | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/pandas/tests/indexes/datetimes/test_date_range.py b/pandas/tests/indexes/datetimes/test_date_range.py
index 69f2ba8a4deab..e26f35f4e8258 100644
--- a/pandas/tests/indexes/datetimes/test_date_range.py
+++ b/pandas/tests/indexes/datetimes/test_date_range.py
@@ -820,8 +820,10 @@ def test_frequencies_A_deprecated_Y_renamed(self, freq, freq_depr):
 
     def test_to_offset_with_lowercase_deprecated_freq(self) -> None:
         # https://github.com/pandas-dev/pandas/issues/56847
-        msg = "'m' is deprecated and will be removed in a future version, please use "
-        "'ME' instead."
+        msg = (
+            "'m' is deprecated and will be removed in a future version, please use "
+            "'ME' instead."
+        )
         with tm.assert_produces_warning(FutureWarning, match=msg):
             result = date_range("2010-01-01", periods=2, freq="m")
         expected = DatetimeIndex(["2010-01-31", "2010-02-28"], freq="ME")