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Bools and explicitness [was Re: PyWart: The problem with "print"]

rusi rustompmody at gmail.com
Thu Jun 6 23:24:41 EDT 2013 
On Jun 7, 8:14 am, Mark Janssen <dreamingforw... at gmail.com> wrote:
> >> I am aware of what it means, but Python doesn't really have it (although
> >> it may evolve to it with annotations).
>
> > No polymorphism huh?
>
> > py> len([1, 2, 3])  # len works on lists
> > 3
> > py> len((1, 2))  # and on tuples
> > 2
> > py> len({})  # and on dicts
> > 0
> > py> len('I pity the fool')  # and on strings
> > 15
> > py> len(b'\x23')  # and on bytes
> > 1
> > py> len(set(range(2)))  # and on sets
> > 2
> > py> len(frozenset(range(4)))  # and on frozensets
> > 4
> > py> len(range(1000))  # and on range objects
> > 1000
>
> Okay, wow, it looks like we need to define some new computer science
> terms here.

Fairly definitive terms have existed since 1985:
http://lucacardelli.name/Papers/OnUnderstanding.A4.pdf

>
> You are making an "outside view of a function" (until a better term is
> found).  So that give you one possible view of polymorphism.  However,
> *within* a class that I would write, you would not see polymorphism
> like you have in C++,  where it is within the *function closure*
> itself.   Instead you would see many if/then combinations to define
> the behavior given several input types.  I would call this simulated
> polymorphism.

Cardelli and Wegner cited above call this ad-hoc polymorphism.
What you are calling polymorphism, they call universal polymorphism.

See sect 1.3 for a summary diagram
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