switch

[Asun Friere]

On Dec 9, 7:08 pm, Carl Banks <pavlovevide... at gmail.com> wrote:
> What if the object is a string you just read from a file?
>
> How do you dispatch using polymorphism in that case?

This would be a pertinent question, were I advocating that _all_
switch statements should, or even can, be replaced with "dispatch
using polymorphism."

What if, instead of reading strings from a file, you are parsing, say
xml, into an object framework isomorphic to the file's schema?  And
no, this is not a contrived example.  Now you want to print out the
structure, or a branch thereof.  To make matters interesting you want
to be able to print it out in a number of different formats.  So we
have:

  5 def print_out (element, fmnt) :
  6     if element.__class__ is schema.Title :
  7         if str(fmnt) == 'html' :
  8             print_out_spam_title(element)
  9         elif str(fmnt) == 'txt' :
 10             print_out_ham_title(element)
 11         elif ....
 12     elif element.__class__ is schema.Paragraph :
 13         if str(fmnt) == 'html' :
 14             print_out_spam_paragraph(element)
 15         elif str(fmnt) == 'txt' :
 16             print_out_ham_paragraph(element)
 17         elif ...
 18     elif element.__class__ is ...
 19         ...
 20

And so on for a dozen or so tags and 3 formats.  And imagine the joy
of adding the 4th or 5th format.

Now I guess you already realise that applying a dispatch mechanism
here will improve the design and result in code that is dryer, far
more easily extensible and arguably (but only arguably) more readible?