REF: move get_rule_month to libparsing to simplify dep structure

pandas/_libs/tslibs/frequencies.pxd

@@ -1,7 +1,5 @@
 cdef dict attrname_to_abbrevs
 
-cpdef str get_rule_month(object source, str default=*)
-
 cpdef get_freq_code(freqstr)
 cpdef int get_to_timestamp_base(int base)
 cpdef str get_freq_str(base, mult=*)

pandas/_libs/tslibs/frequencies.pyx

@@ -7,6 +7,7 @@ from pandas._libs.tslibs.util cimport is_integer_object
 
 from pandas._libs.tslibs.ccalendar cimport c_MONTH_NUMBERS
 from pandas._libs.tslibs.offsets cimport is_offset_object
+from pandas._libs.tslibs.parsing cimport get_rule_month
 
 # ----------------------------------------------------------------------
 # Constants
@@ -490,35 +491,3 @@ cdef bint _is_monthly(str rule):
 cdef bint _is_weekly(str rule):
     rule = rule.upper()
     return rule == 'W' or rule.startswith('W-')
-
-
-# ----------------------------------------------------------------------
-
-cpdef str get_rule_month(object source, str default="DEC"):
-    """
-    Return starting month of given freq, default is December.
-
-    Parameters
-    ----------
-    source : object
-    default : str, default "DEC"
-
-    Returns
-    -------
-    rule_month: str
-
-    Examples
-    --------
-    >>> get_rule_month('D')
-    'DEC'
-
-    >>> get_rule_month('A-JAN')
-    'JAN'
-    """
-    if hasattr(source, 'freqstr'):
-        source = source.freqstr
-    source = source.upper()
-    if '-' not in source:
-        return default
-    else:
-        return source.split('-')[1]

pandas/_libs/tslibs/parsing.pxd

@@ -0,0 +1,2 @@
+
+cpdef str get_rule_month(object source, str default=*)

pandas/_libs/tslibs/parsing.pyx

@@ -41,7 +41,6 @@ from pandas._libs.tslibs.util cimport (
     is_array,
     get_c_string_buf_and_size,
 )
-from pandas._libs.tslibs.frequencies cimport get_rule_month
 from pandas._libs.tslibs.offsets cimport is_offset_object
 
 cdef extern from "../src/headers/portable.h":
@@ -1019,3 +1018,34 @@ def concat_date_cols(tuple date_cols, bint keep_trivial_numbers=True):
             result_view[row_idx] = " ".join(list_to_join)
 
     return result
+
+
+# TODO: `default` never used?
+cpdef str get_rule_month(object source, str default="DEC"):
+    """
+    Return starting month of given freq, default is December.
+
+    Parameters
+    ----------
+    source : object
+    default : str, default "DEC"
+
+    Returns
+    -------
+    rule_month: str
+
+    Examples
+    --------
+    >>> get_rule_month('D')
+    'DEC'
+
+    >>> get_rule_month('A-JAN')
+    'JAN'
+    """
+    if is_offset_object(source):
+        source = source.freqstr
+    source = source.upper()
+    if "-" not in source:
+        return default
+    else:
+        return source.split("-")[1]

pandas/_libs/tslibs/period.pyx

@@ -54,9 +54,9 @@ from pandas._libs.tslibs.frequencies cimport (
     attrname_to_abbrevs,
     get_freq_code,
     get_freq_str,
-    get_rule_month,
     get_to_timestamp_base,
 )
+from pandas._libs.tslibs.parsing cimport get_rule_month
 from pandas._libs.tslibs.parsing import parse_time_string
 from pandas._libs.tslibs.nattype cimport (
     _nat_scalar_rules,
@@ -2506,6 +2506,7 @@ def quarter_to_myear(year: int, quarter: int, freq):
         year -= 1
 
     return year, month
+    # TODO: This whole func is really similar to parsing.pyx L434-L450
 
 
 def validate_end_alias(how):

pandas/tests/tslibs/test_libfrequencies.py

@@ -3,10 +3,10 @@
 from pandas._libs.tslibs.frequencies import (
     INVALID_FREQ_ERR_MSG,
     _period_str_to_code,
-    get_rule_month,
     is_subperiod,
     is_superperiod,
 )
+from pandas._libs.tslibs.parsing import get_rule_month
 
 from pandas.tseries import offsets