This is a bit horrible, as it mixes up language implementation with your program logic.
Could you consider instead using a decorator to "register" the relevant subclasses as it defines them?
I mean something like this ...
```
MAPPING_TYPES = []

def make_mapping(mapping, fu_p): 
    for mapping_type in Mapping.MAPPING_TYPES:
        ...

def mapping_subtype(cls):
   MAPPING_TYPES.append(cls)
   return cls

@mapping_subtype
class StashCFMapping(Mapping):
    ...

@mapping_subtype
class FieldcodeCFMapping(Mapping):
    ...

@mapping_subtype
class CFFieldcodeMapping(Mapping):
    ...
```