CLN: tighten types to get_rule_month (#35205)

Author: jbrockmendel

pandas/_libs/tslibs/parsing.pxd

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

pandas/_libs/tslibs/parsing.pyx

@@ -284,7 +284,7 @@ def parse_time_string(arg: str, freq=None, dayfirst=None, yearfirst=None):
 
 
 cdef parse_datetime_string_with_reso(
-    str date_string, object freq=None, bint dayfirst=False, bint yearfirst=False,
+    str date_string, str freq=None, bint dayfirst=False, bint yearfirst=False,
 ):
     """
     Parse datetime string and try to identify its resolution.
@@ -438,6 +438,7 @@ cdef inline object _parse_dateabbr_string(object date_string, datetime default,
 
             if freq is not None:
                 # TODO: hack attack, #1228
+                freq = getattr(freq, "freqstr", freq)
                 try:
                     mnum = c_MONTH_NUMBERS[get_rule_month(freq)] + 1
                 except (KeyError, ValueError):
@@ -1020,15 +1021,14 @@ def concat_date_cols(tuple date_cols, bint keep_trivial_numbers=True):
     return result
 
 
-# TODO: `default` never used?
-cpdef str get_rule_month(object source, str default="DEC"):
+cpdef str get_rule_month(str source):
     """
     Return starting month of given freq, default is December.
 
     Parameters
     ----------
-    source : object
-    default : str, default "DEC"
+    source : str
+        Derived from `freq.rule_code` or `freq.freqstr`.
 
     Returns
     -------
@@ -1042,10 +1042,8 @@ cpdef str get_rule_month(object source, str default="DEC"):
     >>> get_rule_month('A-JAN')
     'JAN'
     """
-    if is_offset_object(source):
-        source = source.freqstr
     source = source.upper()
     if "-" not in source:
-        return default
+        return "DEC"
     else:
         return source.split("-")[1]

pandas/_libs/tslibs/period.pyx

@@ -2440,13 +2440,13 @@ cdef int64_t _ordinal_from_fields(int year, int month, quarter, int day,
                                   BaseOffset freq):
     base = freq_to_dtype_code(freq)
     if quarter is not None:
-        year, month = quarter_to_myear(year, quarter, freq)
+        year, month = quarter_to_myear(year, quarter, freq.freqstr)
 
     return period_ordinal(year, month, day, hour,
                           minute, second, 0, 0, base)
 
 
-def quarter_to_myear(year: int, quarter: int, freq):
+def quarter_to_myear(year: int, quarter: int, freqstr: str):
     """
     A quarterly frequency defines a "year" which may not coincide with
     the calendar-year.  Find the calendar-year and calendar-month associated
@@ -2456,7 +2456,8 @@ def quarter_to_myear(year: int, quarter: int, freq):
     ----------
     year : int
     quarter : int
-    freq : DateOffset
+    freqstr : str
+        Equivalent to freq.freqstr
 
     Returns
     -------
@@ -2470,7 +2471,7 @@ def quarter_to_myear(year: int, quarter: int, freq):
     if quarter <= 0 or quarter > 4:
         raise ValueError('Quarter must be 1 <= q <= 4')
 
-    mnum = c_MONTH_NUMBERS[get_rule_month(freq)] + 1
+    mnum = c_MONTH_NUMBERS[get_rule_month(freqstr)] + 1
     month = (mnum + (quarter - 1) * 3) % 12 + 1
     if month > mnum:
         year -= 1

pandas/core/arrays/period.py

@@ -1034,9 +1034,10 @@ def _range_from_fields(
             if base != FreqGroup.FR_QTR:
                 raise AssertionError("base must equal FR_QTR")
 
+        freqstr = freq.freqstr
         year, quarter = _make_field_arrays(year, quarter)
         for y, q in zip(year, quarter):
-            y, m = libperiod.quarter_to_myear(y, q, freq)
+            y, m = libperiod.quarter_to_myear(y, q, freqstr)
             val = libperiod.period_ordinal(y, m, 1, 1, 1, 1, 0, 0, base)
             ordinals.append(val)
     else:

pandas/tests/tslibs/test_libfrequencies.py

@@ -9,19 +9,19 @@
     "obj,expected",
     [
         ("W", "DEC"),
-        (offsets.Week(), "DEC"),
+        (offsets.Week().freqstr, "DEC"),
         ("D", "DEC"),
-        (offsets.Day(), "DEC"),
+        (offsets.Day().freqstr, "DEC"),
         ("Q", "DEC"),
-        (offsets.QuarterEnd(startingMonth=12), "DEC"),
+        (offsets.QuarterEnd(startingMonth=12).freqstr, "DEC"),
         ("Q-JAN", "JAN"),
-        (offsets.QuarterEnd(startingMonth=1), "JAN"),
+        (offsets.QuarterEnd(startingMonth=1).freqstr, "JAN"),
         ("A-DEC", "DEC"),
         ("Y-DEC", "DEC"),
-        (offsets.YearEnd(), "DEC"),
+        (offsets.YearEnd().freqstr, "DEC"),
         ("A-MAY", "MAY"),
         ("Y-MAY", "MAY"),
-        (offsets.YearEnd(month=5), "MAY"),
+        (offsets.YearEnd(month=5).freqstr, "MAY"),
     ],
 )
 def test_get_rule_month(obj, expected):