[Tutor] Are you allowed to shoot camels? [kinda OT]

Smith, Jeff jsmith at medplus.com
Fri Feb 4 22:55:53 CET 2005

Now who's joking?  Are you saying that 

switch var:
	case 'a':
		print 'a'
	case 'b' or 'c':
		print 'b or c'
	case 'd':
		print 'default case'

Is less clear and maintainable than

def do_this_function():
	print 'a'

def do_that_function():
	print 'b or c'

def do_pass_function():

def do_default_function():
	print 'default case'

ftable = { 'a' : do_this_function,
           'b' : do_that_function,
           'c' : do_that_function,
           'd' : do_pass_function }
ftable.get(var, do_default_function)()


-----Original Message-----
From: Alan Gauld [mailto:alan.gauld at freenet.co.uk] 
Sent: Friday, February 04, 2005 1:14 PM
To: Smith, Jeff; Jacob S.; Nicholas.Montpetit at deluxe.com;
tutor at python.org
Subject: Re: [Tutor] Are you allowed to shoot camels? [kinda OT]

> What you are try to do is "execute a block of code based on the
value of
> a single statement."  if/elif doesn't do that and thereby introduces
> possibility of errors.

In that case the best solution is a dictionary jump table.
That is more maintainable than either and much faster too.
And its also shorter to write.
[Especially with lambdas :-)]

> Note that the logic intended is that "on-this."  So why force the 
> programmer to rewrite it N times and thereby introduce the
> of N-1 typographical errors...

Thats a fair point although the dictionary solution avoids that. OTOH
such switches tend to proliferate thropugh code and become a big source
of maintenance headaches in their own right - multiple update syndrome
across multiple files potentially.

> Note that I've left out break.  I'm not convinced that fall-through is

> an important feature in switch and is usually the culpit in the cases 
> of abuse.

The problem is its so hard to tell when fall though is happening
intentionally  or by accident because someone forgot a break sytatement.

But when it comes to abuuse the fall through mechanism is one of the
worst offenders in C, its just too tempting to be "clever" with it.

> This is also true for the ternary operator.  The desired logic is to 
> assign the value of a variable based on the value of some other 
> variable.

But its not its based on the value of an *expression*. If the test could
be limited to a single valiable value it might be justified but its an
arbitrary expression. That makes it a conditional statement, which is
most clearly represented by an if/else... Well I think so :-)

> I also like Perl's unless statement but really prefer
> VBs DO/WHILE/UNTIL/LOOP constuct.  Nothing beats it for clarity of 
> expression.

I won't argue on either point, Python's minimalist
approach - there's only one way to do it - means a
paucity of looping constructs - something I point
out in my tutorial. But I can live with it for the
other niceties it brings.

Alan G.

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