BUG: inconsistency between PeriodIndex.get_value vs get_loc (#31172)

doc/source/whatsnew/v1.1.0.rst

@@ -144,7 +144,7 @@ Interval
 Indexing
 ^^^^^^^^
 - Bug in slicing on a :class:`DatetimeIndex` with a partial-timestamp dropping high-resolution indices near the end of a year, quarter, or month (:issue:`31064`)
--
+- Bug in :meth:`PeriodIndex.get_loc` treating higher-resolution strings differently from :meth:`PeriodIndex.get_value` (:issue:`31172`)
 -
 
 Missing

pandas/core/indexes/period.py

@@ -486,7 +486,7 @@ def get_value(self, series, key):
             try:
                 loc = self._get_string_slice(key)
                 return series[loc]
-            except (TypeError, ValueError):
+            except (TypeError, ValueError, OverflowError):
                 pass
 
             asdt, reso = parse_time_string(key, self.freq)
@@ -567,18 +567,34 @@ def get_loc(self, key, method=None, tolerance=None):
         """
 
         if isinstance(key, str):
+
             try:
-                return self._get_string_slice(key)
-            except (TypeError, KeyError, ValueError, OverflowError):
+                loc = self._get_string_slice(key)
+                return loc
+            except (TypeError, ValueError):
                 pass
 
             try:
                 asdt, reso = parse_time_string(key, self.freq)
-                key = asdt
             except DateParseError:
                 # A string with invalid format
                 raise KeyError(f"Cannot interpret '{key}' as period")
 
+            grp = resolution.Resolution.get_freq_group(reso)
+            freqn = resolution.get_freq_group(self.freq)
+
+            # _get_string_slice will handle cases where grp < freqn
+            assert grp >= freqn
+
+            if grp == freqn:
+                key = Period(asdt, freq=self.freq)
+                loc = self.get_loc(key, method=method, tolerance=tolerance)
+                return loc
+            elif method is None:
+                raise KeyError(key)
+            else:
+                key = asdt
+
         elif is_integer(key):
             # Period constructor will cast to string, which we dont want
             raise KeyError(key)

pandas/tests/indexes/period/test_indexing.py

@@ -518,12 +518,20 @@ def test_contains(self):
 
         ps0 = [p0, p1, p2]
         idx0 = pd.PeriodIndex(ps0)
+        ser = pd.Series(range(6, 9), index=idx0)
 
         for p in ps0:
             assert p in idx0
             assert str(p) in idx0
 
-        assert "2017-09-01 00:00:01" in idx0
+        # GH#31172
+        # Higher-resolution period-like are _not_ considered as contained
+        key = "2017-09-01 00:00:01"
+        assert key not in idx0
+        with pytest.raises(KeyError, match=key):
+            idx0.get_loc(key)
+        with pytest.raises(KeyError, match=key):
+            idx0.get_value(ser, key)
 
         assert "2017-09" in idx0