== Python's complicating concept of a callable

Through [[Planet Python http://planetpython.org/]] I recently wound
up reading [[this article on Python decorators
https://pythonconquerstheuniverse.wordpress.com/2012/04/29/python-decorators/]].
It has the commendable and generally achieved goal of a clear,
easily followed explanation of decorators, starting out by talking
about how functions can return other functions and then defining
decorators as:

> A ~~decorator~~ is a function (such as _foobar_ in the above example)
> that takes a function object as an argument, and returns a function
> object as a return value.

Some experienced Python people are now jumping up and and down to
say that this definition is not complete and thus not correct. To be
complete and correct, you have to change *function* to *callable*.
These people are correct, but at the same time this correctness creates
a hassle in this sort of explanation.

In Python, it's pretty easy to understand what a function is. We have
an intuitive view that pretty much matches reality; if you write '_def
foobar(...):_' (in module scope), you have a function. It's not so easy
to inventory and understand all of the things in Python that can be
callables. Can you do it? I'm not sure that I can:

* functions, including lambdas
* classes
* objects if they're instances of a class with a ((__call__)) special
  method
* [[bound methods HowFunctionsToMethods]] of an object (eg _anobj.afunc_)
* methods of classes in general, especially class methods and static
  methods

(I don't think you can make modules callable, and in any case it would
be perverse.)

Being technically correct in this sort of explanation exacts an annoying
price. Rather than say 'function', you must say 'callable' and then
probably try to give some sort of brief and useful explanation of just
what a callable is beyond a function (which should cover at least
classes and callable objects, which are the two common cases for things
like decorators). This is inevitably going to complicate your writing
and put at least a speed bump in the way for readers.

The generality of Python accepting callables for many things is
important, but it does drag a relatively complicated concept into any
number of otherwise relatively straightforward explanations of things. I
don't have any particular way to square the circle here; even web
specific hacks like writing {{AB:function:actually, any callable}} as an
<ABBR> element with its own pseudo-footnote seem kind of ugly.

(You could try to separate the idea of 'many things can be callable'
from the specifics of just what they are beyond functions. I'm not sure
that would work well, but I'm probably too close to the issue; it's
quite possible that people who don't already know about callables would
be happy with that.)