[Tutor] Why are expressions not allowed as parameters in function definition statements?

[Steven D'Aprano]

On Sat, Jun 18, 2016 at 09:04:10PM -0700, Danny Yoo wrote:
> > You know Steve, as I was typing the beginning of a reply responding to
> > a similar question you asked earlier in your response, I suddenly
> > realized how ridiculous having a parameter of 'col/2' is!  I'll just
> > have either eat crow or attribute this to a brain fart.  You pick!
> 
> Just to play devil's advocate: it's not crazy.  Essentially, what
> you're asking for is called "pattern matching", and it  is done in a
> class of many programming languages.

Ah, I didn't think of pattern matching. Another good example is Haskell.

But still, even with pattern matching, I'm not sure that col/2 would be 
a useful pattern. That would match any number.
[...]
> It's a bit out of scope to talk about this much here, but I just
> wanted to chime in here to say that you are not ridiculous.  :P  But
> Python does not have a robust pattern matching facility; the closest
> it has is a limited form of tuple matching:
> 
> ######################
> > def f((x,y), z):
> ...      print x, y, z
> ...
> >   f([1, 2], 3)
> 1 2 3
> ######################


That's not so much *matching tuples* as expanding any iterable object. 
It is equivalent to:

def f(_tmp, z):
    x, y = _tmp
    del _tmp
    print x, y, z


> with very limited applicability and rarely used.

In Python 2, it's unusual. In Python 3, it's impossible, as the facility 
is removed from the language.


-- 
Steve