From 9698da8a50e81e2dcf6f0bc097a1839ca7e0b17a Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Mon, 21 Jun 2021 08:10:13 -0700
Subject: [PATCH] REF: implement PeriodDtypeBase.resolution

---
 pandas/_libs/tslibs/dtypes.pyx | 13 +++++++++++++
 pandas/core/indexes/period.py  | 12 ++----------
 2 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/pandas/_libs/tslibs/dtypes.pyx b/pandas/_libs/tslibs/dtypes.pyx
index 415bdf74db80a..f79ffd2d425c4 100644
--- a/pandas/_libs/tslibs/dtypes.pyx
+++ b/pandas/_libs/tslibs/dtypes.pyx
@@ -27,6 +27,11 @@ cdef class PeriodDtypeBase:
         # See also: libperiod.get_freq_group
         return (self._dtype_code // 1000) * 1000
 
+    @property
+    def resolution(self) -> "Resolution":
+        fgc = self.freq_group_code
+        return Resolution.from_freq_group(FreqGroup(fgc))
+
     @property
     def date_offset(self):
         """
@@ -259,6 +264,14 @@ class Resolution(Enum):
 
         return cls.from_attrname(attr_name)
 
+    @classmethod
+    def from_freq_group(cls, freq_group: FreqGroup) -> "Resolution":
+        abbrev = _reverse_period_code_map[freq_group.value].split("-")[0]
+        if abbrev == "B":
+            return cls.RESO_DAY
+        attrname = _abbrev_to_attrnames[abbrev]
+        return cls.from_attrname(attrname)
+
 
 cdef dict _reso_str_map = {
     Resolution.RESO_NS.value: "nanosecond",
diff --git a/pandas/core/indexes/period.py b/pandas/core/indexes/period.py
index 731958e363b40..91488b41af6a1 100644
--- a/pandas/core/indexes/period.py
+++ b/pandas/core/indexes/period.py
@@ -424,17 +424,9 @@ def get_loc(self, key, method=None, tolerance=None):
                 raise KeyError(f"Cannot interpret '{key}' as period") from err
 
             reso = Resolution.from_attrname(reso_str)
-            grp = reso.freq_group.value
-            freqn = self.dtype.freq_group_code
 
-            # _get_string_slice will handle cases where grp < freqn
-            assert grp >= freqn
-
-            # BusinessDay is a bit strange. It has a *lower* code, but we never parse
-            # a string as "BusinessDay" resolution, just Day.
-            if grp == freqn or (
-                reso == Resolution.RESO_DAY and self.dtype.freq.name == "B"
-            ):
+            if reso == self.dtype.resolution:
+                # the reso < self.dtype.resolution case goes through _get_string_slice
                 key = Period(asdt, freq=self.freq)
                 loc = self.get_loc(key, method=method, tolerance=tolerance)
                 return loc