Using Python "json" module to make a class serializable

Question

I am working in Python with an Email() class that I would like to extend into a SerializeEmail() class, which simply adds two further methods, .write_email() and .read_email(). I would like this sort of behaviour:

# define email
my_email = SerializeEmail()
my_email.recipients = '[email protected]'
my_email.subject = 'RE: Master sword'
my_email.body = "Master using it and you can have this."
# write email to file system for hand inspection
my_email.write_email('my_email.txt')
...
# Another script reads in email
my_verified_email = SerializeEmail()
my_verified_email.read_email('my_email.txt')
my_verified_email.send()

I have navigated the json encode/decode process, and I can successfully write my SerializeEmail() object, and read it in, however, I can't find a satisfactory way to recreate my object via a SerializeEmail.read_email() call.

class SerializeEmail(Email):

    def write_email(self,file_name):

        with open(file_name,"w") as f:
            json.dump(self,f,cls=SerializeEmailJSONEncoder,sort_keys=True,indent=4)

    def read_email(self,file_name):

        with open(file_name,"r") as f:
           json.load(f,cls=SerializeEmailJSONDecoder)

The problem here is that the json.load() call in my read_email() method returns an instance of my SerializeEmail object, but doesn't assign that object to the current instance that I'm using to call it. So right now I'd have to do something like this,

another_email = my_verified_email.read_email('my_email.txt')

when what I want is for the call to my_veridied_email.read_email() to populate the current instance of my_verified_email with the data on the file. I've tried

self = json.load(f,cls=SerializeEmailJSONDecoder)

but that doesn't work. I could just assign each individual element of my returned object to my "self" object, but that seems ad-hoc and inelegant, and I'm looking for the "right way" to do this, if it exists. Any suggestions? If you think that my whole approach is flawed and recommend a different way of accomplishing this task, please sketch it out for me.

Answer 1

While you could jump through a number of hoops to load serialized content into an existing instance, I wouldn't recommend doing so. It's an unnecessary complication which really gains you nothing; it means that the extra step of creating a dummy instance is required every time you want to load an e-mail from JSON. I'd recommend using either a factory class or a factory method which loads the e-mail from the serialized JSON and returns it as a new instance. My personal preference would be a factory method, which you'd accomplish as follows:

class SerializeEmail(Email):

    def write_email(self,file_name):

        with open(file_name,"w") as f:
            json.dump(self,f,cls=SerializeEmailJSONEncoder,sort_keys=True,indent=4)

    @staticmethod
    def read_email(file_name):

        with open(file_name,"r") as f:
           return json.load(f,cls=SerializeEmailJSONDecoder)

# You can now create a new instance by simply doing the following:
new_email = SerializeEmail.read_email('my_email.txt')

Note the @staticmethod decorator, which allows you to call the method on the class without any implicit first argument being passed in. Normally factory methods would be @classmethods, but since you're loading the object from JSON, the implicit class argument is unnecessary.

Notice how, with this modification, you don't need to instantiate a SerializeEmail object before you can load another one from JSON. You simply call the method directly on the class and get the desired behavior.