Why am I not able to access an attribute from an inherited class?

Question

Given the following classes:

class DataAccessLayer():

    def __init__(self):
        self.engine = create_engine(CONN_STRING, pool_size=95, max_overflow=0)
        self.connection = self.engine.connect()


class DataAccessLayerParallel(DataAccessLayer):

    def __init__(self):
        session_factory = sessionmaker(bind=self.connection)

I try to create an instance of DataAccessLayerParallel as follows:

dal = DataAccessLayerParallel()

However, I get the error:

Exception has occurred: AttributeError
'DataAccessLayerNonParallel' object has no attribute 'connection'

I assume this is because the __init__ method from DataAccessLayer does not run when it is inherited? How would I ensure I can get access to the connection attribute?

In Python, the parent's constructor isn't called automatically; you have to do it yourself. — Etienne de Martel, Nov 16 '20 at 16:19

Diogo Silva · Answer 1 · 2020-11-16T16:29:27.713

-1

You need to call the parent constructor with super. Like so:

class DataAccessLayer():

    def __init__(self):
        self.engine = create_engine(CONN_STRING, pool_size=95, max_overflow=0)
        self.connection = self.engine.connect()


class DataAccessLayerParallel(DataAccessLayer):

    def __init__(self):
        super(DataAccessLayerParallel, self).__init__()
        self.session_factory = sessionmaker(bind=self.connection)

edited Nov 16 '20 at 16:29

answered Nov 16 '20 at 16:20

Diogo Silva

320
2
14

That code will still raise the same error. – ekhumoro Nov 16 '20 at 16:21
@ekhumoro Sorry. I had a small error. Please check the updated answer – Diogo Silva Nov 16 '20 at 16:23
@ekhumoro You need to call the parent constructor before using `self.connection` – Diogo Silva Nov 16 '20 at 16:23
This didn't work for me - used `DataAccessLayer.__init__(self)` instead of `super` – Jossy Nov 16 '20 at 16:25
Another typo! Sorry. I coppied your code, and didn't notice you didn't use `self` when assigning `session_factory`. Check the answer – Diogo Silva Nov 16 '20 at 16:29
You should use `super` – Diogo Silva Nov 16 '20 at 16:30
2

No need to write so much code. `super().__init__()` should be enough. – Matthias Nov 16 '20 at 16:31
@Matthias. The other part is not required. But any form of `super` should be used. – Diogo Silva Nov 16 '20 at 16:32
@DiogoSilva Calling the parent class `__init__` directly is perfectly fine when multiple inheritance isn't being used. However, `super().__init__()` is more convenient, because you don't have to repeat the class name. It won't work in Python2, though. – ekhumoro Nov 16 '20 at 16:42

Why am I not able to access an attribute from an inherited class?

1 Answers1