Database Router Abstract Base Class (ABC)

[Rick van Hattem (wolph)]

Hi guys,

It seems to me that an (abstract) base class for the database routers would be useful to inherit for people. Makes implementing a router slightly easier.

I was thinking something along these lines:

import abc

class ConnectionRouterBase(abc.ABC):

    @abstractmethod

    def db_for_read(self, model, **hints):

        ...

    @abstractmethod

    def db_for_write(self, model, **hints):

        ...

    @abstractmethod

    def allow_relation(self, obj1, obj2, **hints):

        ...

    @abstractmethod

    def allow_migrate(self, db, app_label, **hints):

        ...

Would a pull request like that be acceptable?

~rick

[Adam Johnson]

I don't think this would be that helpful, Django relies on ducktyping, and the router docs ( https://docs.djangoproject.com/en/2.2/topics/db/multi-db/ ) clearly document the signatures, and that the methods are all optional:

A router doesn’t have to provide all these methods – it may omit one or more of them. If one of the methods is omitted, Django will skip that router when performing the relevant check.

Additionally ABC's in Python don't place any restrictions on their subclasses. Even with an ABC it is possible to implement the wrong method names or signatures. For example:

In [1]: import abc

In [2]: class X(abc.ABC):
   ...:     @abc.abstractmethod
   ...:     def foo(self, number):
   ...:         pass
   ...:

In [3]: class Y(X):
   ...:     def foo(self, number, number2):
   ...:         return number * number2
   ...:

In [4]: Y().foo(2, 3)
Out[4]: 6

As the Python docs state ( https://docs.python.org/3/glossary.html ), ABC's complement, and don't replace, duck-typing:

abstract base class
Abstract base classes complement duck-typing by providing a way to define interfaces when other techniques like hasattr() would be clumsy or subtly wrong (for example with magic methods).

Thanks for the suggestion though. If you're interested in validating backends, I think a system check ( https://docs.djangoproject.com/en/2.2/ref/checks/ ) could be added to verify the provided routers have the correct signatures for the method names they implement.

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Adam

[Rick van Hattem]

On Sun, 28 Apr 2019 at 11:30, Adam Johnson <m...@adamj.eu> wrote:

I don't think this would be that helpful, Django relies on ducktyping, and the router docs ( https://docs.djangoproject.com/en/2.2/topics/db/multi-db/ ) clearly document the signatures, and that the methods are all optional:

A router doesn’t have to provide all these methods – it may omit one or more of them. If one of the methods is omitted, Django will skip that router when performing the relevant check.

So in that case not a abstract base class but just a regular base class (which is why I placed the abstract part between ()). The advantage of having a base class is also that your editor can automatically complete the signature if you're planning to implement it.

Would there be anything against it though? A non-abstract base class in that case :)

~rick

[Adam Johnson]

I'm a bit ambivalent on adding such a class. Many features even in core Python don't provide base classes and instead rely on duck-typing + documentation. The amount of time saved globally will probably be less than the time we've spent discussing it so far.

I think a more valuable piece of code would be the system check, since that would validate it's all spelled correctly, for all developers, whether or not their editor has auto-complete. It would also work going forwards if/when the signatures change.

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Adam