assertions.py

class Assertions(object):                     
    """Custom class to deal with assertions. it provides some hight level          
    methods that you can call to get valid or invalid assertions.                  
                                                                                   
    If many assertions are contained, then it will loop on them.                   
    """                                                                            
    def __init__(self, assertions):                                                
        self._assertions = assertions                                              
        
                                                                                   
    def get_assertion(self, key="valid"):                                          
        """return an assertion"""                                                  
        return random.choice(self._assertions[key])
                                                                                   
    valid = functools.partial(get_assertion, key="valid")                          
    invalid_cert = functools.partial(get_assertion, key="invalid_cert")            
    invalid_issuer = functools.partial(get_assertion, key="invalid_issuer")  

usage.py

assertions = Assertions(dict_of_assertions)
assertions.valid() # should return the same thing than assertions.get_assertion(key="valid")

So, this will not work as-is. I need to put the extra definitions of valid and invalid_* in the init method.