[Python-Dev] Simple Switch statement (original) (raw)

Michael Urman murman at gmail.com
Tue Jun 27 01:26:45 CEST 2006

On 6/26/06, Raymond Hettinger <rhettinger at ewtllc.com> wrote:

With the simplified proposal, this would be coded with an inverse mapping:

for event in pygame.event.get(): switch eventmap[event.type]: case 'KEYDOWN': ... case 'KEYUP': ... case 'QUIT': ...

Ah, here's where it gets interesting. While that definitely works on the surface, it can run into some difficulties of scope. SDL (on which pygame is based) allows user-defined events, also integers, but without a predefined meaning. If pygame provides the inverse mapping, it won't contain the user-defined events. If you construct it, you have to choose between a local lookup and a global one, and then we start seeing a global store for an essentially local construct, or we risk differences when there's more than one locality for using it.

While you're right; it should be simple to ensure that the inverse map handles at least the set the switch handles, and it keeps evaluation simpler, I still find the limitation ugly. As mentioned, the early error checking is poor, and it just doesn't feel like the rest of python.

>I also would like to see a way to use 'is' [...]for the comparison

[If] the goal is having several distinct cases that are equal but not identical, then that's another story. I suggest leave the initial switch syntax as simple as possible and just switch on id(object).

Switching on id(object) only sounds palatable if we're not bound by the simple switch's limitations. Having a second (if inverted) mapping merely of identifier to object smells particularly rancid.

What if I want some cases done as identity, but some as equality? Since I'm having no luck finding a real use case for this, perhaps I should assume a nested switch would be adequate. Assuming static or capture it doesn't look too bad, so I think I'll go with Guido's hypothesis that it's a red herring.

switch id(value):
case id(object): ...
case id(None): ...
else:
    switch value:
    case 1: ...
    case 'orange':
    else: raise ValueError

Michael

Michael Urman http://www.tortall.net/mu/blog

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