bpo-33826: add __filename__ to Python-defined classes for introspection

[t-vi]

This adds an attribute __filename__ to classes. It enables introspection in IPython/Jupyter using inspect.getsource().

https://bugs.python.org/issue33826

[gvanrossum]

I'm sorry, I don't read consensus in the ticket that this is the solution. You may have to make a quick detour through python-dev or Discourse (discuss.python.org) to see what other core devs think.

[t-vi]

I'll do that next year. Thanks!

[t-vi]

So a year passed, I sent the following to python-dev, but I'm not sure whether it is filtered (it doesn't show up in the archives).

I would love to hear your opinion on the following aspect of inspect that I believe might be worth improving:

Consider the following program saved in a file (say hello.py):

import inspect

def hello():
    print("Hello World")

print(inspect.getsource(hello))

class Hello:
    def __init__(self):
        print("Hello World")

print(inspect.getsource(Hello))

Running hello.py will, unsurprisingly, print the source of hello and Hello.

Now, some of us use an Jupyter (with the capabilities provided by IPython) notebooks, which are a great tool and awesome match with Python. These notebooks can be large and complex enough to want to use introspection on methods defined in itself (also, I'm prototyping things I might want to use as a library in Notebooks a lot, and I think I'm not alone).

IPython enhances the interactive console to enable introspection (by providing "files" for the cells).
As a result, the following will work as expected:

def hello():
    print("Hello World")
print(inspect.getsource(hello))

However, it does not work for classes:

class Hello:
    def __init__(self):
        print("Hello World")
print(inspect.getsource(Hello))

will run into an error in a Jupyter notebook, more precisely

TypeError: <class '__main__.Hello'> is a built-in class

The reason why the latter does not work is because inspect cannot find a source file.

The technical background is that for a function hello, inspect.getfile finds the file through hello.__code__.co_filename which IPython can arrange for, while for the class Hello, it tries Hello.__module__, which is __main__ and then would see if sys.modules[Hello.__module__] has a __file__ attribute, which it does not (and which could not be disambiguated into cell-level).

I once made this PR in github and sent a bug and patch earlier but got, let's say, reactions that were not entirely encouraging. I still think that it is a useful feature and I don't think that there are readily available solutions and after another year has passed, I humbly submit this for your considerations.

[gvanrossum]

Oh, your message showed up. It's here: https://mail.python.org/archives/list/python-dev@python.org/message/AC5B5VORS6FF6KJ3EG5DIV3ZLAQB6DIY/