objc.error: NSInternalInconsistencyException - readFromData:ofType:error: is a subclass responsibility but has not been overridden

Question

I am using PyObj-C and am making some methods in a python file to read and write files using NSDocument, which uses the abstract NSFileCoordinater class. Accessing files this way instead of just using python's open let's these classes handle things for me such as preventing files from being edited from more than one program at a time or giving enough time for read/write operations to finish before it could get deadlocked.

These features are very important, and the app I ma building I want to be up to standard as much as I can here.

I have this code that instantiates a NSDocument object that contains the content of whatever file path you put into it, as a function:

@classmethod
def write(cls, file: str):
    path = NSURL.fileURLWithPath_(file)
    ext = file.split('.')[-1]
    doc = NSDocument.alloc().initWithContentsOfURL_ofType_error_(path, ext, None)

When I call this function with a valid file path I get this error:

  File "/Users/user123/PycharmProjects/shoutout/src/sutils/cfiles.py", line 27, in write
    doc = NSDocument.alloc().initWithContentsOfURL_ofType_error_(path, ext, None)
objc.error: NSInternalInconsistencyException - readFromData:ofType:error: is a subclass responsibility but has not been overridden.

I have tried to find forums both objective-c, swift, or pyobj-c based as it were asking any keywords such as objective-c is a subclass responsibility but has not been overridden on google, and checked stackoverflow, and github for existing posts on this error but I could find none.

As I understand it Objective-C being polymorphic, has my method initWithContentsOfURL:ofType:error: call readFromData:ofType:error, among other ones at the same time. I don't understand exactly however what it means when it's saying that "is a subclass responsibility but has not been overridden." I am not sure also about what it means to override a class or a one being a responsibility so that doesn't help on my part.

A NSInternalInconsistencyException means a "when an internal assertion fails and implies an unexpected condition within the called code." Not sure what a internal "assertion" is either or what this could mean.

Any idea of what I could do to fix this?

Answer 1

NSDocument is an abstract class that requires you to subclass and implement a number of methods to make it usable. This is document in Apple's documentation for the class.

the older Document-Baed App Programming Guide for Mac gives more information on this.