How to update an instance variable when another variable is changed?

Question

I have the following class and I want the instance variable api_id_bytes to update.

class ExampleClass:
    def __init__(self):
        self.api_key = ""
        self.api_id = ""
        self.api_id_bytes = self.api_key.encode('utf-8')

I'd like to be able to have this outcome:

>>>conn = ExampleClass()
>>>conn.api_key = "123"
>>>conn.api_id = "abc"

>>>print(conn.api_id_bytes)
b'123'
>>>

I basically need the self.api_key.encode('utf-8') to run when an api_id is entered but it doesn't, it only does through the initial conn = ExampleClass().

I'm not sure what this is called so searching didn't find an answer.

A 'setter' could be used. Or, expose a method def use_api_auth(self, id, key), which would be my preference here as it also accounts for both id and key at the same time.. — user2864740
– user2864740, Commented Jun 15, 2020 at 18:02
For a 'setter' (and also 'getter'), python-reference.readthedocs.io/en/latest/docs/property/… , docs.python.org/3/howto/descriptor.html, docs.python.org/3/library/functions.html — user2864740
– user2864740, Commented Jun 15, 2020 at 18:04
Or you could make api_id_bytes a property that calculates the correct value on demand. — khelwood
– khelwood, Commented Jun 15, 2020 at 18:05

khelwood · Accepted Answer · 2020-06-15 19:28:34Z

4

Here's how you could do it by making api_id_bytes a property.

class ExampleClass:
    def __init__(self):
        self.api_key = ""
        self.api_id = ""
    @property
    def api_id_bytes(self):
        return self.api_key.encode('utf-8')

Now conn.api_id_bytes will always be correct for the current value of conn.api_key.

answered Jun 15, 2020 at 19:28

khelwood

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1 Comment

Johnny John Boy Over a year ago

Legend, thank you for taking the time to post that. I read the docs others had suggest but this worked perfectly!

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