Mapping class methods of python class

Question

I created a message class, which stores an arbitrary message like so:

class MessageBase:
    """Wrapper class for messages from different resources"""
    
    def __init__(
        self,     
        subject,
        body,
        sender,
        recipient,
        received,
        to
    ):

        self.subject = subject
        self.body = body
        self.sender = sender
        self.recipient = recipient
        self.received = received
        self.to = to

Messages can come from different sources, and I want my class to be flexible with these resources. I want to create such a syntax:

message1 = Message.from_source(source='outlook', raw_outlook_message)
message2 = Message.from_source(source='sendinblue_email', raw_sendinblue_message)
message1.perform_action()
message2.perform_action()

od way to code a class, or whether this goes against some practices.

To achieve this, I created several class methods, and want to map them as such:

        self.sources = {
            'outlook': self.from_outlook,
            'sendinblue_email': self.from_sendinblue_email,
        }

    @classmethod
    def from_source(cls, source_name, *args):
        return cls.sources[source_name.lower()](*args)

    @classmethod
    def from_outlook(cls, raw_message: dict):
        return cls(<parse_outlook_message>)

    @classmethod
    def from_sendinblue_email(cls, raw_message: dict):
        return cls(<parse_outlook_message>)

I have one issue with this setup and one concern. The issue is that I cannot access the sources dictionary of self.sources, and I do not know how to achieve this functionality. The concern is whether this is a go

Sounds like you want a class variable, not an instance variable. But if the sources are going to be entirely distinct anyway, why not just have different classmethods for them to begin with? What's the benefit to having the from_source composite in the first place? — Silvio Mayolo
– Silvio Mayolo, Commented Mar 17, 2023 at 16:34
@SilvioMayolo Thanks, Class variable is indeed what I needed. The idea is that I can specify the resources that I want to collect in a separate Resource class, and I have to map the right classmethod to the specified resource. Perhaps it is better to place the mapping and/or classmethods elsewhere, but I want to try different things. — Jeroen Vermunt
– Jeroen Vermunt, Commented Mar 18, 2023 at 11:40

chepner · Accepted Answer · 2023-03-17 16:48:43Z

You want a class attribute that maps strings to class methods.

Since the objects are instances of classmethod, you can't call them directly in from_source; you'll have to extract the underlying function and call it with an explicit class argument.

class MessageBase:
    @classmethod
    def from_outlook(cls, ...):
        ...

    @classmethod
    def from_sendinblue_email(cls, ...):
        ...

    # When this dict is defined, the above are still just names
    # in the current namespace, not class attributes.
    sources = {
        'outlook': from_outlook,
        'sendinblue_email': from_sendinblue_email,
    }

    @classmethod
    def from_source(cls, source, ...):
        f = cls.sources[source.lower()].__func__
        return f(cls, ...)

An alternative is to add the name of the class method to the mapping, and letting from_source perform attribute look up on the class.

class MessageBase:
    @classmethod
    def from_outlook(cls, ...):
        ...

    @classmethod
    def from_sendinblue_email(cls, ...):
        ...

    # When this dict is defined, the above are still just names
    # in the current namespace, not class attributes.
    sources = {
        'outlook': 'from_outlook',
        'sendinblue_email': 'from_sendinblue_email'
    }

    @classmethod
    def from_source(cls, source, ...):
        method_name = cls.sources[source.lower()].__func__
        return getattr(cls, method_name)(...)

HansQ · Accepted Answer · 2023-03-17 16:52:47Z

1

Why not just define another class method

>>> class MessageBase:    
...     @classmethod
...     def sources(cls):
...         return { 'sendinblue_email': cls.from_sendinblue_email } 
...     @classmethod      
...     def from_sendinblue_email(cls, raw_message: dict):
...         return cls()                                             
...

answered Mar 17, 2023 at 16:52

HansQ

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1 Comment

Jeroen Vermunt Over a year ago

Thanks, this works. I marked the other answer as the solution because the resulting syntax is a little bit better.

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