INT: take DateOffset out of the inheritance tree for BaseOffset subclasses

pandas/_libs/tslibs/offsets.pyx

@@ -733,6 +733,11 @@ class _BaseOffset:
         )
         return self.is_anchored()
 
+    def is_anchored(self) -> bool:
+        # TODO: Does this make sense for the general case?  It would help
+        # if there were a canonical docstring for what is_anchored means.
+        return self.n == 1
+
 
 class BaseOffset(_BaseOffset):
     # Here we add __rfoo__ methods that don't play well with cdef classes

pandas/tseries/frequencies.py

@@ -219,7 +219,7 @@ def _get_offset(name: str) -> DateOffset:
             klass = prefix_mapping[split[0]]
             # handles case where there's no suffix (and will TypeError if too
             # many '-')
-            offset = klass._from_name(*split[1:])
+            offset = klass._from_name(*split[1:])  # type: ignore
         except (ValueError, TypeError, KeyError) as err:
             # bad prefix or suffix
             raise ValueError(libfreqs.INVALID_FREQ_ERR_MSG.format(name)) from err

pandas/tseries/offsets.py

@@ -76,7 +76,22 @@
 # DateOffset
 
 
-class DateOffset(BaseOffset):
+class OffsetMeta(type):
+    """
+    Metaclass that allows us to pretend that all BaseOffset subclasses
+    inherit from DateOffset (which is needed for backward-compatibility).
+    """
+
+    @classmethod
+    def __instancecheck__(cls, obj) -> bool:
+        return isinstance(obj, BaseOffset)
+
+    @classmethod
+    def __subclasscheck__(cls, obj) -> bool:
+        return issubclass(obj, BaseOffset)
+
+
+class DateOffset(BaseOffset, metaclass=OffsetMeta):
     """
     Standard kind of date increment used for a date range.
 
@@ -275,25 +290,16 @@ def apply_index(self, i):
                 "applied vectorized"
             )
 
-    def is_anchored(self) -> bool:
-        # TODO: Does this make sense for the general case?  It would help
-        # if there were a canonical docstring for what is_anchored means.
-        return self.n == 1
-
     def is_on_offset(self, dt):
         if self.normalize and not is_normalized(dt):
             return False
         # TODO, see #1395
         return True
 
 
-class SingleConstructorOffset(DateOffset):
-    # All DateOffset subclasses (other than Tick) subclass SingleConstructorOffset
-    __init__ = BaseOffset.__init__
-    _attributes = BaseOffset._attributes
-    apply_index = BaseOffset.apply_index
-    is_on_offset = BaseOffset.is_on_offset
-    _adjust_dst = True
+class SingleConstructorOffset(BaseOffset):
+    _params = cache_readonly(BaseOffset._params.fget)
+    freqstr = cache_readonly(BaseOffset.freqstr.fget)
 
     @classmethod
     def _from_name(cls, suffix=None):
@@ -2389,7 +2395,7 @@ def generate_range(start=None, end=None, periods=None, offset=BDay()):
 
 
 prefix_mapping = {
-    offset._prefix: offset
+    offset._prefix: offset  # type: ignore
     for offset in [
         YearBegin,  # 'AS'
         YearEnd,  # 'A'