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

Fredrik Lundh fredrik at pythonware.com
Sat Jun 24 19:04:50 CEST 2006

Guido van Rossum wrote:

just map

switch EXPR: case E1: ... case in E2: ... else: ... to VAR = EXPR if VAR == E1: ... elif VAR in E2: ... else: ... where VAR is a temporary variable, and case and case-in clauses can be freely mixed, and leave the rest to the code generator. (we could even allow "switch EXPR [as VAR]" to match a certain other sugar construct). This used to be my position. I switched after considering the alternatives for what should happen if either the switch expression or one or more of the case expressions is unhashable.

I don't see this as much of a problem, really: we can simply restrict the optimization to well-known data types ("homogenous" switches using integers or strings should cover 99.9% of all practical cases), and then add an opcode that checks uses a separate dispatch object to check if fast dispatch is possible, and place that before an ordinary if/elif sequence.

the dispatch object is created when the function object is created, along with default values and statics. if fast dispatch cannot be used for a function instance, the dispatch object is set to None, and the dispatch opcode turns into a NOP.

(each switch statement should of course have it's own dispatch object).

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