Extend a class without access to its init method

Question

I am trying to extend a class from a given library but I do not have access to the __init__ of the class, only to a function generating an instance. Something like

class A:
    pass


def return_an_A():
    return A()


class B(A):
    # How to instantiate B with only access to return_an_A
    def extend_A():
        pass

How can I define the instanciation of class B?

Thanks for your help.

Update

As rightfully noticed, my example was poorly set up so here is, I hope, a better explanation of my real issue.

The original code I used was the following.

import gitlab


# Here gl.projects.get(project_name) is an instance of the class
# gitlab.v4.objects.Project
def project(url, private_token, project_name):
    with gitlab.Gitlab(url, private_token) as gl:
        return gl.projects.get(project_name)

# This implementation takes an instance of gitlab.v4.objects.Project
def list_files(project, commit_branch):
    current_files = []
    if not project.empty_repo:
        current_files = [
            f["path"]
            for f in project.repository_tree(
                ref=commit_branch
            )
        ]

    return current_files

I wanted to have a structure like

class MyProject:

    # Here is missing the way to instantiate like the project function
    # I don't want to pass a Project instance as a parameter to make it an
    # attribute, I would like to extend the class Project itself

    def list_files(self, commit_branch):
        current_files = []
        
        # Note here that the variables of the 
        # gitlab.v4.objects.Project are directly accessible
        if not self.empty_repo:
            current_files = [
                f["path"]
                for f in self.repository_tree(
                    ref=commit_branch
                )
            ]

        return current_files

but I can't manage to find the right way to write the __init__.

When you say "I do not have access to the `__init__`" do you mean "I don't have a reference to the class" ? — khelwood, Jul 31 '20 at 10:07
Python lacks the definition of "private/public" methods. You can always access/overwrite the `__init__` method of class `A`. However, you should be more specific on the shape of class A and what you are trying to achieve. — Riccardo Petraglia, Jul 31 '20 at 10:09
this might be helpful https://stackoverflow.com/questions/972/adding-a-method-to-an-existing-object-instance — Pratik Kumar, Jul 31 '20 at 11:43
You are totally right. My question was ill-designed. I updated the question with the real problem, hopefully making it clearer. Thanks again! — Flavien Lambert, Jul 31 '20 at 18:23

score 0 · Answer 1 · answered Aug 03 '20 at 19:33

I found a solution to my issue by using a delegation pattern.

class Project:

    def __init__(self, private_token, project_name, url):
        self.private_token = private_token
        self.project_name = project_name
        self.url = url

    # Delegate all methods and properties to the child class when those are not found
    # in the class
    def __getattr__(self, attr):
        return self._project.__getattribute__(attr)

    @property
    def _project(self):
        with gitlab.Gitlab(self.url, self.private_token) as gl:
            return gl.projects.get(self.project_name)

    def list_files(self, branch):
        current_files = []
        if not self._project.empty_repo:
            current_files = [
                f["path"]
                for f in self._project.repository_tree(
                    ref=branch, recursive=True, all=True
                )
                if f["type"] == "blob"
            ]

        return current_files

Extend a class without access to its __init__ method

Update

1 Answers1

Extend a class without access to its init method