REF: de-duplicate code in libparsing/libperiod (#38915)

Author: jbrockmendel

pandas/_libs/tslibs/parsing.pxd

@@ -1,2 +1,3 @@
 
 cpdef str get_rule_month(str source)
+cpdef quarter_to_myear(int year, int quarter, str freq)

pandas/_libs/tslibs/parsing.pyx

@@ -378,7 +378,7 @@ cpdef bint _does_string_look_like_datetime(str py_string):
 
 
 cdef inline object _parse_dateabbr_string(object date_string, datetime default,
-                                          object freq):
+                                          str freq=None):
     cdef:
         object ret
         # year initialized to prevent compiler warnings
@@ -438,21 +438,13 @@ cdef inline object _parse_dateabbr_string(object date_string, datetime default,
                                      f'quarter must be '
                                      f'between 1 and 4: {date_string}')
 
-            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):
-                    raise DateParseError(f'Unable to retrieve month '
-                                         f'information from given '
-                                         f'freq: {freq}')
-
-                month = (mnum + (quarter - 1) * 3) % 12 + 1
-                if month > mnum:
-                    year -= 1
-            else:
-                month = (quarter - 1) * 3 + 1
+            try:
+                # GH#1228
+                year, month = quarter_to_myear(year, quarter, freq)
+            except KeyError:
+                raise DateParseError("Unable to retrieve month "
+                                     "information from given "
+                                     f"freq: {freq}")
 
             ret = default.replace(year=year, month=month)
             return ret, 'quarter'
@@ -482,6 +474,41 @@ cdef inline object _parse_dateabbr_string(object date_string, datetime default,
     raise ValueError(f'Unable to parse {date_string}')
 
 
+cpdef quarter_to_myear(int year, int quarter, str freq):
+    """
+    A quarterly frequency defines a "year" which may not coincide with
+    the calendar-year.  Find the calendar-year and calendar-month associated
+    with the given year and quarter under the `freq`-derived calendar.
+
+    Parameters
+    ----------
+    year : int
+    quarter : int
+    freq : str or None
+
+    Returns
+    -------
+    year : int
+    month : int
+
+    See Also
+    --------
+    Period.qyear
+    """
+    if quarter <= 0 or quarter > 4:
+        raise ValueError("Quarter must be 1 <= q <= 4")
+
+    if freq is not None:
+        mnum = c_MONTH_NUMBERS[get_rule_month(freq)] + 1
+        month = (mnum + (quarter - 1) * 3) % 12 + 1
+        if month > mnum:
+            year -= 1
+    else:
+        month = (quarter - 1) * 3 + 1
+
+    return year, month
+
+
 cdef dateutil_parse(
     str timestr,
     object default,

pandas/_libs/tslibs/period.pyx

@@ -74,8 +74,7 @@ from pandas._libs.tslibs.dtypes cimport (
     PeriodDtypeBase,
     attrname_to_abbrevs,
 )
-from pandas._libs.tslibs.parsing cimport get_rule_month
-
+from pandas._libs.tslibs.parsing cimport quarter_to_myear
 from pandas._libs.tslibs.parsing import parse_time_string
 
 from pandas._libs.tslibs.nattype cimport (
@@ -2461,40 +2460,6 @@ cdef int64_t _ordinal_from_fields(int year, int month, quarter, int day,
                           minute, second, 0, 0, base)
 
 
-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
-    with the given year and quarter under the `freq`-derived calendar.
-
-    Parameters
-    ----------
-    year : int
-    quarter : int
-    freqstr : str
-        Equivalent to freq.freqstr
-
-    Returns
-    -------
-    year : int
-    month : int
-
-    See Also
-    --------
-    Period.qyear
-    """
-    if quarter <= 0 or quarter > 4:
-        raise ValueError('Quarter must be 1 <= q <= 4')
-
-    mnum = c_MONTH_NUMBERS[get_rule_month(freqstr)] + 1
-    month = (mnum + (quarter - 1) * 3) % 12 + 1
-    if month > mnum:
-        year -= 1
-
-    return year, month
-    # TODO: This whole func is really similar to parsing.pyx L434-L450
-
-
 def validate_end_alias(how):
     how_dict = {'S': 'S', 'E': 'E',
                 'START': 'S', 'FINISH': 'E',

pandas/core/arrays/period.py

@@ -12,6 +12,7 @@
     delta_to_nanoseconds,
     dt64arr_to_periodarr as c_dt64arr_to_periodarr,
     iNaT,
+    parsing,
     period as libperiod,
     to_offset,
 )
@@ -1074,7 +1075,7 @@ def _range_from_fields(
         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, freqstr)
+            y, m = parsing.quarter_to_myear(y, q, freqstr)
             val = libperiod.period_ordinal(y, m, 1, 1, 1, 1, 0, 0, base)
             ordinals.append(val)
     else: