correct cpdef to_offset, add tests for warning

pandas/_libs/tslibs/offsets.pyx

@@ -4641,15 +4641,6 @@ cpdef to_offset(freq, bint is_period=False):
                 if not stride:
                     stride = 1
 
-                if prefix in c_DEPR_ABBREVS:
-                    warnings.warn(
-                        f"\'{prefix}\' is deprecated and will be removed in a "
-                        f"future version. Please use \'{c_DEPR_ABBREVS.get(prefix)}\' "
-                        f"instead of \'{prefix}\'.",
-                        FutureWarning,
-                        stacklevel=find_stack_level(),
-                    )
-                    prefix = c_DEPR_ABBREVS[prefix]
                 if prefix in {"D", "H", "min", "s", "ms", "us", "ns"}:
                     # For these prefixes, we have something like "3H" or
                     #  "2.5T", so we can construct a Timedelta with the
@@ -4662,8 +4653,19 @@ cpdef to_offset(freq, bint is_period=False):
                         #  into the offset
                         offset *= stride_sign
                 else:
+                    if prefix in c_DEPR_ABBREVS:
+                        warnings.warn(
+                            f"\'{prefix}\' is deprecated and will be removed "
+                            f"in a future version. Please use "
+                            f"\'{c_DEPR_ABBREVS.get(prefix)}\' "
+                            f"instead of \'{prefix}\'.",
+                            FutureWarning,
+                            stacklevel=find_stack_level(),
+                        )
+                        prefix = c_DEPR_ABBREVS[prefix]
+
                     stride = int(stride)
-                    offset = _get_offset(name)
+                    offset = _get_offset(prefix)
                     offset = offset * int(np.fabs(stride) * stride_sign)
 
                 if delta is None:

pandas/tests/indexes/datetimes/test_date_range.py

@@ -838,14 +838,15 @@ def test_freq_dateoffset_with_relateivedelta_nanos(self):
     @pytest.mark.parametrize(
         "freq,freq_depr",
         [
+            ("Y", "A"),
             ("min", "T"),
             ("s", "S"),
             ("ms", "L"),
             ("us", "U"),
             ("ns", "N"),
         ],
     )
-    def test_frequencies_T_S_L_U_N_deprecated(self, freq, freq_depr):
+    def test_frequencies_a_t_s_l_u_n_deprecated(self, freq, freq_depr):
         # GH#52536
         msg = f"'{freq_depr}' is deprecated and will be removed in a future version."
 

pandas/tests/indexes/period/test_period.py

@@ -284,6 +284,15 @@ def test_period_index_frequency_ME_error_message(self):
         with pytest.raises(ValueError, match=msg):
             PeriodIndex(["2020-01-01", "2020-01-02"], freq="2ME")
 
+    def test_a_deprecated_from_time_series(self):
+        # GH#52536
+        msg = "'A' is deprecated and will be removed in a future version."
+
+        with tm.assert_produces_warning(FutureWarning, match=msg):
+            index = period_range(freq="A", start="1/1/2001", end="12/1/2009")
+        series = Series(1, index=index)
+        assert isinstance(series, Series)
+
 
 def test_maybe_convert_timedelta():
     pi = PeriodIndex(["2000", "2001"], freq="D")

pandas/tests/tseries/frequencies/test_freq_code.py

@@ -99,9 +99,9 @@ def test_compatibility(freqstr, expected):
     assert ts_np + do == np.datetime64(expected)
 
 
-@pytest.mark.parametrize("freq", ["T", "S", "L", "N", "U"])
-def test_units_t_l_deprecated_from__attrname_to_abbrevs(freq):
-    # GH 52536
+@pytest.mark.parametrize("freq", ["A", "T", "S", "L", "U", "N"])
+def test_units_a_t_s_l_u_n_deprecated_from_attrname_to_abbrevs(freq):
+    # GH#52536
     msg = f"'{freq}' is deprecated and will be removed in a future version."
 
     with tm.assert_produces_warning(FutureWarning, match=msg):