blog.py

from peewee import CharField, TextField, ForeignKeyField
from peewee import Model, SqliteDatabase

import post    # !!!! imports post which imports blog

class Blog(Model):
    title = CharFiled()
    body = TextField()
  
    def import_posts(self, backup):
        for p in backup:
            Post.from_backup(self, backup)   # !!! requires the post class
            

post.py

from peewee import CharField, TextField, ForeignKeyField
from peewee import Model, SqliteDatabase

import blog  # !!!! imports blog which imports post

class Post(Model):
    blog = ForeignKeyField(Blog, related_name='posts')  # !!!! requires the blog class
    
    title = CharFiled()
    body = TextField()
    
    @classmethod
    def from_backup(blog, backup):
        p = Post(backup)
        p.blog = blog
        p.save()