SEMANTICS for 'variables' produced by a decorator
There's been a discussion in this list on extending Python to provide SYNTAX such as @decorator name = EXPRESSION and also suitable semantics. (Here 'name' is an identifier, in the discussion called a 'variable'.)
This post is about providing SEMANTICS for such decorator syntax. We can do this within the existing Python syntax for decorators.
Recall that @decorator def fn(....): # body is usually equivalent to def fn(....): # body fn = decorator(fn) and that decorator is just a callable that returns something. It need not return another function. It could return a 'variable', such as the result of calling fn.
Here's a proof of concept. Consider the following BEGIN $ cat work.py from collections import namedtuple Locn = namedtuple('Locn', 'doc module name')
def locn_from_fn(fn): name = fn.__name__ module = fn.__module__ doc = fn.__doc__ return Locn(name=name, module=module, doc=doc)
def decovar(fn): locn = locn_from_fn(fn) return fn(locn)
@decovar def variable(locn): return ('value', locn)
def deconamedtuple(fn): locn = locn_from_fn(fn) nt = namedtuple(locn.name, fn())
nt.__doc__ = locn.doc nt.__module__ = locn.module nt.__name__ = locn.name
@deconamedtuple def Point(): '''Return a point (x, y) in the plane.''' return 'x y'
print(variable) print(Point) print(Point.__doc__) END
Here's what we get when we run the script. BEGIN $ python3 work.py ('value', Locn(doc=None, module='__main__', name='variable')) <class '__main__.Point'> Return a point (x, y) in the plane. END
It should now be clear that this approach allows us to pass much useful information to the decorator.
I claim that this approach gives most or all of the semantic benefits of 'decorators on variables', and within the current Python syntax. If so, and the semantic benefits are strong, then here's part of a good case for extending the syntax in a future version of Python.