Can global variable be passed into Python function?

Marko Rauhamaa marko at pacujo.net
Sat Mar 1 01:03:51 CET 2014


Steven D'Aprano <steve+comp.lang.python at pearwood.info>:

> You can't have it both ways: you cannot claim that switch or case is
> more readable than a chain of if...elif, and then propose a syntax
> which is effectively a chain of if...elif while still claiming it is
> an improvement. It's not. All you've done is introduce an element of
> implicit magic to the syntax.

You may be right that Python has too many practical problems for a truly
fitting switch syntax.

> Why (how?) would you want to use *multiple* classes for a single
> switch?

Well, I was just given a whole link on the very topic:

   <URL: http://c2.com/cgi/wiki?SwitchStatementsSmell>

Here's a shorter one:

   <URL: http://en.wikipedia.org/wiki/State_pattern>


Anyway, while implementing states as singleton inner class instances is
elegant and highly readable, the technique suffers in one significant
respect: it can be very slow. Often a state machine has numerous states
but the typical execution path only visits a few of them. However, the
pattern calls for declaring classes (or instances) for all states in the
state machine constructor. Switch statements make it possible to avoid
this creation overhead.

> Dict dispatch tables are elegant, attractive and efficient if you are 
> using pre-existing functions or functions you can create using lambda:

I beg to differ. The dict dispatch tables are very hard to read. The
fully blown-out if-else chain beats it in elegance hands down.

> There are all sorts of simple and neat ways to handle switches in
> Python. That's why the pressure on the language to grow special syntax
> is quite low. It simply isn't important enough.

Of course the threshold for new syntax should be exceptionally high.
Python as a language is already close to the sweet spot.


Marko



More information about the Python-list mailing list