builder.py

class ClassBuilder:
    def __init__(self):
        self.data = {}
        
    def load_source1(self, params_for_source1):
        self.data.update(api_call_source1(params_for_source1))
        
    def load_source1(self, params_for_source2):
        self.data.update(api_call_source1(params_for_source2))
        
    def load_from_file(self, file_name):
        self.data.update(extract_data_from_file(file_name))
       
    def create():
        return MyClass(data=data)

# Should make this immutable somehow.
class MyClass:
    data: dict

my_class.py

from dataclasses import dataclass

@dataclass
class MyClass:
    name: str
    data: str
    
    def load_from_source1(self, params_for_source1):
        self.data.update(api_call_source1(params_for_source1))
        
    def load_from_source2(self, params_for_source2):
        self.data.update(api_call_source1(params_for_source2))
        
    def load_from_file(self, file_name):
        self.data.update(extract_data_from_file(file_name))