[Python-Dev] Py3: function signatures, type checking, and related crap

SevenInchBread adamadamadamamiadam at gmail.com
Sun Apr 15 23:49:58 CEST 2007

People seem to be pushing for a consistent method for checking the "x-ness"
of objects (that is, interfaces that the object implements).

So I present an idea for a simple and straightforward type that provides a
way to construct descriptions of object structures, and I'd like some help
expanding it into a useful extension to Python's standard set of utilities.

Its a basic constructor that produces callable interface-checking predicates
(which can be use in things such as list comprehensions, filter, if
statements, or even a new syntax for function signatures that allows for
automatic interface-checking). These predicates check that an object matches
the behavior described by the constructor . Since I can't think of a name
for this constructor, and because I've never liked the term "interface",
I'll just call it "can".

Can takes an arbitrary number of keyword arguments and produces a callable
object. The keys represent object attributes, while the values are
behavior-checking predicates like the ones produced by can. Since the can
constructor produces an object that can in turn be used in other can
constructors, using previously defined interfaces in new constructions is
fairly straight-forward

    callable =  can(__call__ = object)#Returns an object that describes
objects with a  __call__ attribute
    readable =  can(read  = callable)
#                                      ...with a callable read attribute
    writable =  can(write = callable)
#                                      ...with a callable write attribute

    #a join operator can be used to combine can objects...
    #...for now I'll just use "and" and "or" to represent them.

    isfilelike = readable and writable #returns an object that matches any
type that is described
                                       #by both readable and writable

    IOable     = readable or writable       #any type that is readable or

objects that are constructed with can, when called, return True or False
based on whether or not the passed object matches the behavior described.

    callable(hash)      #returns True - as it would in the current version
of Python.

Here's some more nifty examples:

    iterable = can(__iter__=callable) or can(next=callable)
    hashable = can(__hash__=callable)

    completefile = isfilelike and iterable and can(fileno=callable,

    def outlines(f, seq):
        """Outputs a sequence of lines to a file-like object"""
        assert isfilelike(f), "%r is not a file-like object." % f
        assert isiterable(seq), "%r must be iterable"         % seq

        #a trivial example... you'd get similar error messages from Python
builtins even without the assertions.

As it stands, I don't think that this deserves to be in Python - but I think
the basic premise could be used as a foundation for  better things.

"What's money? A man is a success if he gets up in the morning and goes to
bed at night and in between does what he wants to do." ~ Bob Dylan
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