[Python-ideas] Fwd: Define a method or function attribute outside of a class with the dot operator

[Stephan Hoyer]

On Fri, Feb 10, 2017 at 9:20 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> What I would personally hope to see from the proposal is that given:
>
>     class Spam:
>         pass
>
>     def Spam.func(self):
>         return __class__
>
> the effective runtime behaviour would be semantically identical to:
>
>     class Spam:
>         def func(self):
>             return __class__
>

Yes, this is exactly what I would hope/expect to see.

One use case for this functionality is defining classes with an extensive
method-based API with a sane dependency graph. For example, consider
writing a class like numpy.ndarray
<https://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.html> or
pandas.DataFrame
<http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.html>
with dozens of methods. You could argue that using so many methods is an
anti-pattern, but nonetheless it's pretty common and hard to avoid in some
cases (e.g., for making number-like classes that support arithmetic and
comparisons).

For obvious reasons, the functionality for these classes does not all live
in a single module. But the modules that define helper functions for most
methods also depend on the base class, so many of them need to get imported
inside method definitions
<https://github.com/pandas-dev/pandas/blob/v0.19.2/pandas/core/frame.py#L1227>
to
avoid circular imports. The result is pretty ugly, and files defining the
class still get gigantic.

An important note is that ideally, we would still have way of indicating
that Spam.func should exists in on the Spam class itself, even if it
doesn't define the implementation. I suppose an abstractmethod overwritten
by the later definition might do the trick, e.g.,

class Spam(metaclass=ABCMeta):
    @abstractmethod
    def func(self):
        pass

def Spam.func(self):
    return __class__

And finally, it's quite possible that there's a clean metaclass based
solution for extending Spam in another file, I just don't know it yet.
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